Hypoxic Rebreathing Research Articles

Peripheral chemoreflex restrains skeletal muscle blood flow during exercise in participants with treated hypertension.

We tested the hypothesis that in human hypertension, an increased tonicity/sensitivity of the peripheral chemoreflex causes a sympathetically mediated restraint of nutritive blood flow to the exercising muscles. Fourteen patients with treated hypertension (age 69±11 years, 136±12/80±11mmHg; mean±SD) were studied under conditions of intravenous 0.9% saline (control) and low-dose dopamine (2µgkg-1min-1) to inhibit the peripheral chemoreflex, at baseline, during isocapnic hypoxic rebreathing and during rhythmic handgrip exercise (3min, 50% maximum voluntary contraction). At baseline, dopamine did not change mean blood pressure (95±10 vs. 98±10mmHg, P=0.155) but increased brachial artery blood flow (59±20 vs. 48±16mlmin-1, P=0.030) and vascular conductance (0.565±0.246 vs. 0.483±0.160mlmin-1mmHg-1; P=0.039). Dopamine attenuated the increase in mean blood pressure (∆3±4 vs. ∆8 ±6mmHg, P=0.007) to isocapnic hypoxic rebreathing and reduced peripheral chemoreflex sensitivity by 28±37% (P=0.044). Rhythmic handgrip exercise induced increases in brachial artery blood flow and vascular conductance (both P<0.05 vs. rest after 45s) that were greater with dopamine than saline (e.g. Δ76±54 vs. Δ60±43mlmin-1 and Δ0.730±0.440 vs. Δ0.570±0.424mlmin-1mmHg-1, respectively, at 60s; main effect of condition both P<0.0001). Our results indicate that the peripheral chemoreflex is tonically active at rest and restrains the blood flow and vascular conductance increases to exercise in treated human hypertension. KEY POINTS: It was hypothesised that in human hypertension, an increased tonicity/sensitivity of the peripheral chemoreflex causes a sympathetically mediated restraint of nutritive blood flow to the exercising muscles. Treated patients with hypertension (n=14) were studied under conditions of intravenous 0.9% saline (control) and low-dose dopamine (2 µgkg-1min-1) to inhibit the peripheral chemoreflex. Low-dose dopamine reduced resting ventilation and peripheral chemoreflex sensitivity, and while mean blood pressure was unchanged, brachial artery blood flow and vascular conductance were increased. Low-dose dopamine augmented the brachial artery blood flow and vascular conductance responses to rhythmic handgrip. These findings indicate that the peripheral chemoreflex is tonically active at rest and restrains the blood flow, and vascular conductance increases to exercise in treated human hypertension.

Circulatory Effects of Peripheral Chemoreflex Inhibition During Exercise in Treated Hypertension

The sensitization of the peripheral (carotid) chemoreflex can cause a sympathetically mediated restraint of blood flow to the exercising muscles in some disease states (e.g., heart failure, COPD). Such impairments in nutritive blood flow have the potential to impair exercise tolerance and raise exercising blood pressure (BP). Increased peripheral chemoreflex sensitivity and pronounced exercise-induced surges in BP have been reported in hypertension. Herein, we tested the hypothesis that suppressing peripheral chemoreflex sensitivity improves skeletal muscle blood flow during exercise in hypertension. Fourteen patients with treated hypertension (age 69±11 years, systolic BP 136±12 mmHg, diastolic BP 80±11 mmHg; mean ± SD) performed a resting baseline (3 min), an isocapnic hypoxia rebreathing test and rhythmic handgrip exercise (3 min at 50% of maximal voluntary contraction, 1 s contraction and 2 s relaxation) with either intravenous 0.9% saline (control) or low-dose dopamine (2 mcg·kg−1·min−1) to inhibit the peripheral chemoreflex. BP (finger plethysmography), heart rate (HR; electrocardiogram) and ventilation (V̇E; heated pneumotachometer) were recorded throughout. Brachial artery blood flow was monitored (duplex Doppler ultrasound) at rest (1 min average) and during handgrip (15 s averages), and vascular conductance calculated as brachial blood flow/mean BP. Dopamine blunted the peak (last 15 s) increase in V̇E during isocapnic hypoxic rebreathing compared to saline infusion (Δ6±6 vs. Δ9±5 L·min−1, P=0.026), while also attenuating peak mean BP (Δ3±4 vs. Δ8±6 mmHg, P=0.007). Dopamine did not change resting mean BP (95±10 vs. 98±10 mmHg, P=0.155) and HR (68±7 vs. 66±8 beats·min−1, P=0.100). However, dopamine increased resting brachial blood flow (59±20 vs. 48±16 mL·min−1, P=0.030) and vascular conductance (0.565±0.246 vs. 0.483±0.160 mL·min−1·mmHg−1, P=0.039) and decreased resting V̇E (10.5±3.2 vs. 11.5±3.1 L·min−1, P=0.036) versus saline. Handgrip increased brachial artery blood flow (p&lt;0.05 vs. rest after 45 s), vascular conductance (p&lt;0.05 vs. rest after 45 s) in both saline and dopamine trials, while mean BP, HR and V̇E were unchanged (main effect of time P=0.829, P=0.954 and P=0.255, respectively). Notably, brachial artery blood flow (e.g., Δ76±54 vs. Δ60±43 mL·min−1 at 60 s) and vascular conductance (e.g., Δ0.730±0.440 vs. Δ0.570±0.424 mL·min−1·mmHg−1 at 60 s) responses to handgrip were greater with dopamine than saline (main effect of condition both p&lt;0.0001). Collectively, these findings indicate that the use of low-dose dopamine to suppress peripheral chemoreflex sensitivity improves skeletal muscle blood flow during rhythmic handgrip exercise in patients with treated hypertension. Funding support provided by the Health Research Council of New Zealand (19/687) and the Sidney Taylor Trust. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The effect of acute oral pyridoxine supplementation on peripheral chemoreflex sensitivity in human hypertension

ATP has been identified as an important signalling molecule in the carotid body. Animal models of hypertension have uncovered that ATP transmission via purinergic (P2) receptors increases carotid body tonicity and sensitivity, which drives sympathetic outflow and elevates blood pressure. A naturally occurring metabolite of pyridoxine (Vitamin B6), namely pyridoxal-5-phosphate, is a non-selective P2X receptor antagonist. In this study we tested the hypothesis that oral administration of pyridoxine reduces peripheral chemoreflex sensitivity and blood pressure in human hypertension. 14 treated hypertensive patients (4 men, 71±5 yr, 27±6 kg·m -2 , 156±19/83±8 mmHg) completed a double-blind placebo-controlled crossover study with either oral pyridoxine (600 mg) or placebo. Two-hours later, minute ventilation (V̇ E ), end-tidal partial pressures of oxygen and carbon dioxide (P ET O 2 and P ET CO 2 ), mean arterial pressure (MAP), and heart rate (HR) were recorded during an isocapnic hypoxic rebreathing protocol (target P ET O 2 45mmHg). Cardiorespiratory responses were taken as the change from baseline to peak rebreathing (final 15s). Arterial oxygen saturation (S a O 2 ) was calculated from P ET O 2 using the Severinghaus equation, and peripheral chemoreflex sensitivity taken as ΔV̇ E divided by ΔS a O 2 . Baseline V̇ E (13.02±3.26 vs. 12.68±4.13 L·min -1 , P=0.616), MAP (109±11 vs. 104±10 mmHg, P=0.058), and HR (63±11 vs. 62±8 BPM, P=0.463), were not different between pyridoxine and placebo conditions, respectively. The ΔV̇ E response to isocapnic hypoxia tended to be blunted with pyridoxine compared to placebo (6.88±3.94 vs. 9.87±7.69 L·min -1 ; P=0.188), but this did not reach statistical significance. Similarly, peripheral chemoreflex sensitivity was not different with pyridoxine compared to placebo (-0.46±0.31 vs. -0.64±0.49 L·min -1 ·% -1 , P=0.148). However, individuals with a higher peripheral chemoreflex sensitivity in the placebo condition, displayed a more marked reduction in peripheral chemoreflex sensitivity following pyridoxine (R 2 =0.63, P=0.005). While HR and MAP increased during isocapnic hypoxia (both P&lt;0.001), the magnitude of this response did not differ with pyridoxine or placebo administration. In this preliminary study, oral pyridoxine lowered neither blood pressure nor peripheral chemoreflex sensitivity in human hypertension. However, oral pyridoxine did lower peripheral chemoreflex sensitivity in those hypertension patients in whom baseline peripheral chemoreflex sensitivity (i.e., placebo condition) was highest. It is possible that oral pyridoxine supplementation may be beneficial for those hypertensive patients who exhibit overactivation of their peripheral chemoreflex. Health Research Council of New Zealand (Ref# 19/687), the University of Auckland Faculty Research Development Fund, and the New Zealand Lottery Grants Board (R-LHR-2021-153114). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Cerebrovascular Carbon Dioxide Reactivity with Hyperoxia and Hypoxia in Humans with Treated Hypertension

Hypertension increases the risk of cerebrovascular diseases such as stroke and dementia, but the underlying mechanisms are currently unclear. While chronic hypertension can lead to anatomical and functional adaptations within the cerebral circulation, its effect on cerebral blood flow (CBF) control remains poorly understood. We tested the hypothesis that the CBF responses to CO2 with either high or low oxygen levels are impaired in treated hypertensive patients compared to normotensive aged‐matched controls.In 14 treated hypertensive patients (age: 68±5 years, mean±SD) and 8 normotensive age‐matched controls (age: 66±6 years), we assessed cardiorespiratory and cerebrovascular variables at rest, during voluntary hyperventilation (partial pressure of end‐tidal CO2 [PETCO2] ~25 mmHg), and hyperoxic and hypoxic rebreathing. Using transcranial Doppler ultrasound and finger photoplethysmography, we measured beat‐to‐beat middle cerebral artery blood velocity (MCAv: index of CBF) and BP, respectively, while PETCO2 was assessed using a gas analyzer. Linear regression was used to determine the MCAv‐CO2 slope (i.e., cerebrovascular CO2 reactivity).At rest, mean BP was higher in the hypertensive patients (108±9 mmHg) compared to normotensive controls (93±10 mmHg, p=0.001), while MCAv (46.8±11.4 vs. 53.3±10.4 cm/s) and PETCO2 (41±4 vs. 42±3 mmHg) were not different between groups (p&lt;0.05). Hyperventilation‐induced hypocapnia elicited similar reductions in MCAv in the hypertensive and normotensive groups (Δ14.8±4.7 vs. Δ16.5±5.7 cm/s, p=0.484). Similarly, the MCAv‐CO2slope was not different between the hypertensive and normotensive groups during either hyperoxic rebreathing (1.53±1.05 vs. 1.98±0.56 cm/s/mmHg, p=0.320) or hypoxic rebreathing (1.94±1.04 vs. 2.21±1.13 cm/s/mmHg, p=0.815).These preliminary findings indicate that in treated hypertensive patients, the MCAv responses to hypocapnia, and hypercapnia administrated with either high or low oxygen levels, do not differ from that observed in age‐matched normotensive controls.

Reproducibility of Respiratory Chemoreflex Characterization by Modified Rebreathing

Modified hypoxic and hyperoxic rebreathing is used to characterize central and peripheral respiratory chemoreflexes in humans. This study examined within‐ and between‐day reproducibility of the key parameters that are identified by this method: ventilatory recruitment threshold (VRT) and chemoreflex sensitivity. Seven healthy males (mean ± SD; age: 27±5 years) visited the laboratory on 4 occasions to performrepetitions of modified rebreathing tests in isoxic‐hypoxic (end‐tidal PO2 (PETO2) =50 mmHg) and ‐hyperoxic conditions (PETO2=150 mmHg). Days 1 and 2 consisted of three repeatedhyperoxic and three hypoxic trials, respectively; Days 3 and 4 included both a hyperoxic and a hypoxic trial completed in a counterbalanced order. Ventilation, end‐tidal PCO2 (PETCO2) and PETO2 were measured breath‐by‐breath by pneumotach and dual gas analyser. For each rebreathing trial, the PETCO2 at which ventilation began to rise was identified as the VRT (mmHg) and the slope of the response above VRT provided respiratory chemoreflex sensitivity (L∙min‐1∙mmHg‐1). Within‐ and between‐day reproducibility was assessed by one‐way ANOVA and intraclass correlation (ICC) analyses. The VRT was not different between trials performed on a single day for bothhyperoxic (44±2, 45±3, vs 44±3 mmHg; for Trials 1, 2, vs 3, respectively; p=0.50; ICC=0.926) and hypoxic rebreathing (40±3, 40±4, vs 40±4 mmHg; p=0.66; ICC=0.994). Chemoreflex sensitivity also was not different between trials performed on the same day in hyperoxic (4.3±2.2, 5.0±3.1, vs4.8±2.3 L∙min‐1∙mmHg‐1; p=0.33; ICC=0.955) or hypoxic conditions (5.6±2.2, 6.2±2.7, 5.7±2.0 L∙min‐1∙mmHg‐1; p=0.33; ICC=0.953). Between days, the VRT was not different for hyperoxic rebreathing (44±3 vs 44±2 vs 45±2 mmHg for Days 1, 3, vs 4, respectively; p=0.10; ICC=0.813) but was different for hypoxic rebreathing (40±3 vs 40±2 vs 41±3 mmHg for Days 2‐4, respectively; p=0.01; ICC=0.931). Chemoreflex sensitivity was not different between days for either hyperoxic (4.8±2.5,4.4±2.7, vs 4.9±2.0 L∙min‐1∙mmHg‐1; p=0.01; ICC=0.906) or hypoxic rebreathing (5.8±2.2, 5.8±2.6, vs 7.8±2.8 L∙min‐1∙mmHg‐1; p=0.01; ICC=0.750). Collectively, these data indicate that model parameters derived from hyperoxic and hypoxic modified rebreathing have good‐to‐excellent test‐retest reproducibility when repeated on the same or different days.

Hypercapnia Attenuates Ventricular Ectopy during Wakefulness in a Young Man with Heart Failure

Ventricular premature beats (VPBs) are common in heart failure (HF). Patients with HF often exhibit oscillations in breathing pattern (or Cheyne‐Stokes Respiration, CSR) that lead to increased VPBs during the hyperpneic phase. In such patients, CO2 inhalation during sleep eliminates CSR and reduces VPB prevalence. Whether this VPB‐lowering effect of CO2 occurs in awake HF patients without CSR remains unexplored. In a young CSR‐free HF patient (26 years; LVEF=20%; BMI=30 kg·m−2) with mild obstructive sleep apnea (AHI=17 events/h) and frequent ectopy, we compared the incidence of VPBs during: 1) resting breathing; 2) volitional hyperventilation; 3) and at standardized intervals (quartiles) during two CO2 rebreathing tests with end‐tidal O2 tension (PETO2) maintained at either 50 mmHg (hypercapnic‐hypoxia) or 150 mmHg (hypercapnic‐hyperoxia). Breath‐by‐breath ventilation, PETO2, and PETCO2 were measured by flowmeter and rapid gas analyzer and ECG and O2 saturation were monitored continuously. At rest (PETCO2=39 mmHg) and during hyperventilation (PETCO2=28 mmHg), VPBs occurred with 15% and 12% of all heart beats, respectively. With hyperoxic CO2 rebreathing (PETO2=148mmHg), as PETCO2 increased from 40, 46, 52, 57 mmHg in the first through fourth quartile, VPB incidence decreased from 11% to 6%, 3%, and 0%, respectively. Hypoxic rebreathing (PETO2=51mmHg) showed a similar trend: VPB incidence fell from 9% at a PETCO2 of 40 mmHg to 0% at 46 mmHg and remained at 0% thereafter. The hypercapnia‐induced reduction in VPB frequency was not dependent on heightened ventilation: VPBs were evident during volitional hyperventilation (12%) but not during the CO2 rebreathing quartiles with matched ventilation (0% for both hypoxia and hyperoxia). Furthermore, blood O2 also did not appear to modulate this response: VPB incidence at rest (15%) was similar to both hypoxic (11%; O2 saturation=86%) and hyperoxic (13%; O2 saturation=99%) CO2 rebreathing quartiles with matched ventilation (~9 L·min−1) and PETCO2 (~39 mmHg). These observations suggest that in HF, acute hypercapnia attenuates ventricular ectopy. The mechanism appears to be unrelated to increased respiratory or sympathetic drive, myocardial stretch accompanying large tidal volumes, or altered O2 availability. How hypercapnia exerts anti‐arrhythmic effects remain uncertain; we hypothesize that CO2 mediated increases in coronary flow may be involved.Support or Funding InformationExperiments were funded by a Project Grant from the Canadian Institutes of Health Research (PJT 159491).

PARTICIPATION OF INOS IN THE EFFECT OF IL-1β ON LUNG VENTILATION AND HYPOXIC CHEMORECEPTION

With systemic inflammation, hypoxia, and an increase in the respiratory, a significant increase in the systemic level of pro-inflammatory cytokines is observed. Recently we demonstrated that elevated IL-1β level in blood reduces the ventilatory response to hypoxia. The aim of the present study was to examine the hypothesis that the respiratory effect of IL-1β may be mediating the NO-dependent mechanisms.The experiments were performed on anaesthetized rats. We studied the effects of intravenous administration of cytokine during inhibition of iNO-synthase. In order to we used aminoguanidine bicarbonate - specific inhibitor of iNOS, which was injected in the tail vein for 30 minutes before the administration of cytokine. During the hypoxic rebreathing experiments, was found that the increase of IL-1β level in blood weakens the respiratory response to hypoxia. The ventilatory, tidal volume and mean inspiratory flow responses decreased by 29%, 31% and 53% respectively. INO-synthase inhibitor pretreatment eliminated these respiratory effects of IL-1β. Thus the data indicate that the ability of IL-1β to reduce the ventilatory hypoxic response is mediated by the NO-dependent pathway.

Comparative Assessment of Central and Peripheral Chemoreceptor Reflex Regulation of Muscle Sympathetic Nerve Activity and Ventilation

We tested the hypothesis that central and peripheral chemoreceptor‐mediated sympathetic activation elicited by increases in CO2 tension (PCO2) mirror concurrent ventilatory responses. Twelve healthy young men (mean±SD; age: 24±4 yr; BMI: 24.5±1.3 kg·m−2) performed a modified rebreathing protocol designed to equilibrate central and peripheral chemoreceptor PCO2 tensions with end‐tidal PCO2 (PETCO2) at two isoxic end‐tidal PO2 (PETO2) tensions such that central chemoreceptor reflex responses (hyperoxia) can be isolated from additive peripheral responses (i.e., hypoxia minus hyperoxia). Subject's ventilation (volume turbine) and muscle sympathetic nerve activity (MSNA; microneurography, right fibular nerve) responses to rebreathing at isoxic PETO2 tensions of 150 mmHg (hyperoxia) and 50 mmHg (hypoxia) were recorded continuously. During rebreathing, the PETCO2 (mmHg) at which ventilation (L·min−1) and total MSNA (au) began to rise were identified as the PETCO2 recruitment thresholds (RT). Slopes of these responses above RT also were determined. In hyperoxic rebreathing (i.e., the central chemoreflex response), the mean RT for ventilation (46±3 mmHg) and MSNA (45±4 mmHg) did not differ (p&gt;0.05). Mean slopes were 2.3±0.9 L·min−1·mmHg−1 and 2.1±1.5 au·mmHg−1, respectively. In hypoxic rebreathing (i.e., the summed central and peripheral chemoreflex response), the RT for both ventilation and MSNA were not different (41±3 mmHg vs 41±3 mmHg, p&gt;0.05) but were lower (p&lt;0.05) compared to hyperoxia. Mean slopes for ventilation (4.0±1.8 L·min−1·mmHg−1) and MSNA (5.1±3.4 au·mmHg−1) in hypoxia were greater than in hyperoxia yielding a mean peripheral chemoreflex sensitivity (hypoxic slope minus hyperoxic slope) of 1.7±0.1 L·min−1·mmHg−1 for ventilation and 3.0±2.6 au·mmHg−1 for MSNA. The 95% limits of agreement of RT between ventilation and MSNA for the central chemoreflex were −7 to 6 mmHg (bias=−1 mmHg, p&gt;0.05) and −5 to 5 mmHg (bias=0 mmHg, p&gt;0.05) for the peripheral chemoreflex. There was no relationship (p&gt;0.05) between the ventilatory and MSNA sensitivity for either the central (r2=0.16) or peripheral (r2=0.01) chemoreceptor reflexes. These data highlight two novel findings: 1) ventilatory and sympathetic arms of the central and peripheral chemoreceptor reflexes are initiated at similar PETCO2 recruitment thresholds and 2) sympathetic responsiveness cannot be predicted from the sensitivities of the ventilatory central and peripheral chemoreceptor reflexes.Support or Funding InformationExperiments were funded by a Project Grant from the Canadian Institutes of Health Research (PJT 159491). Dr. Floras holds the Canada Research Chair in Integrative Cardiovascular Biology.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Reply from J.‐L. Fan, K. R. Burgess and P. N. Ainslie

We thank Drs Teppema and Berendsen for their interest in our recent paper (Fan et al. 2012). If we interpret their letter correctly, they have raised three noteworthy discussion points. First, why greater elevations in middle cerebral artery blood flow velocity (MCAv) and ventilatory CO2 sensitivities were evident during initial arrival to 5050 m (Fan et al. 2010) compared to those measured 5–12 days at this elevation. Second, they expressed concerns over curve fitting during isocapnic hypoxic rebreathing for the comparison of ventilatory O2 sensitivity between sea level and following ascent to 5050 m. Finally, they expressed skepticism over our use of intravenous acetazolamide to examine its effect on breathing control. Although we feel many of these points are discussed in our paper (Fan et al. 2012), the salient points are outlined below. The differences from during initial arrival to 5050 m (Fan et al. 2010) to those measured 5–12 days at this elevation (Fan et al. 2012) probably reflect partial acclimatization over this time frame. In our more recent study, we observed normalized pH and MCAv to sea level values, whereas our earlier report found these variables to be significantly elevated during initial arrival to 5050 m. Moreover, we found the changes in resting ventilation and with ascent to 5050 m to be greater following 5–12 days compared with initial arrival. This difference in the state of acclimatization would certainly account for the differences in the CO2 sensitivities between our two papers. Another possibility for these apparent discrepant findings may be due to the difference in sample sizes between the two studies. In our more recent report, only 11 subjects were included in the final statistical analysis, resulting in a tendency for the ventilatory CO2 sensitivity to be elevated at 5050 m (P= 0.085). In contrast, our earlier study reported ventilatory CO2 sensitivity in 17 subjects. Therefore, we believe that the lack of change in ventilatory CO2 sensitivity observed in Fan et al. (2012) could be attributed to insufficient statistical power to detect the changes in central chemoreflex following ascent to high altitude. Finally, the transition we used from room air at both altitudes to the end hyperventilation state resulted in a similar degree of mild hyperoxic exposure at both altitudes. We agree that testing peripheral chemoreflex sensitivity is extremely difficult, especially in the field, and additional O2 may be needed inside the rebreathing bag to ensure similar ranges. However, since the ranges assessed pre- and post-acetazolamide were similar at each altitude, we believe that the soda lime rebreathing method was sufficient to identify any potential changes in peripheral chemoreflex associated with our intervention. Moreover, we argue that the ranges assessed were physiologically more relevant at each altitude. We acknowledge that these points raised by Teppema and Berendsen are important considerations when assessing changes in hypoxic ventilatory sensitivities at high altitude. Nevertheless, acute gas control using end-tidal forcing is technologically challenging, especially in real field conditions as opposed to the safe confines of the laboratory (Teppema & Dahan, 2010). Also, although the authors make some interesting points about chemoreflex testing, based on their elegant review paper (Teppema & Dahan, 2010), comparisons with high altitude is almost impossible because of marked acid–base changes and the interactions between the peripheral and central chemoreflexes. Although we would suggest that no approach is perfect, we attempted to target end-tidal CO2 levels for the rebreathing at high altitude so the degree of CO2 washout of all tissues would be similar at the two altitudes. As discussed in our paper, Teppema and Berendsen mention that metabolic acidosis associated with chronic oral acetazolamide ingestion leads to a rise in ventilation and drop in , which independently stabilizes breathing – presumably via an increased plant gain (Dempsey, 2005). However, since the majority of the work on acetazolamide has looked at oral ingestions, our reasoning for using acute intravenous administration was to examine whether acetazolamide may alter ventilatory control and breathing stability by mechanisms other than metabolic acidosis. Oral and i.v. acetazolamide adminstration is clearly physiologically very different and no similarities can be drawn between the two. In summary, whilst thanking Drs Teppema and Berendsen for their interest, we feel their comments do not detract from our salient findings. We do agree, however, that the cerebrovascular response to CO2 may indeed be initiated at the arterial side of the blood–brain barrier.

Integrated human physiology: breathing, blood pressure and blood flow to the brain

The cerebral vasculature rapidly adapts to changes in perfusion pressure (cerebral autoregulation), regional metabolic requirements of the brain (neurovascular coupling), autonomic neural activity, and humoral factors (cerebrovascular reactivity). Regulation of cerebral blood flow (CBF) is therefore highly controlled and involves a wide spectrum of regulatory mechanisms that together work to maintain optimum oxygen and nutrient supply. It is well-established that the cerebral vasculature is highly sensitive to changes in arterial blood gases, in particular the partial pressure of arterial carbon dioxide (). The teleological relevance of this unique feature of the brain is likely to lie in the need to tightly control brain pH and its related impact on ventilatory control at the level of the central chemoreceptors. Changes in arterial blood gases, in particular those that cause hypoxaemia and hypercapnia, also lead to widespread effects on the systemic vasculature often leading to sympathoexcitation and related blood pressure (BP) elevations via vasoconstriction (Ainslie et al. 2005). In this issue of The Journal of Physiology an elegant study by Battisti-Charbonney and co-workers provide a relevant example of integrative human physiology (Battisti-Charbonney et al. 2011). Using continuous bilateral measurements of blood flow velocity in the middle cerebral arteries (as a surrogate index of CBF) and BP, the authors gauged the CBF responses to CO2 changes under the background condition of either hyperoxia or hypoxia. The key findings indicate that the relationship between CBF velocity over a wide range of end-tidal () values during hypocapnia (: ∼25 mmHg) and hyperoxic or hypoxic rebreathing (: 55–60 mmHg and 45–50 mmHg, respectively) are optimally fitted using a sigmoid (logistic) curve rather than a linear curve. Above the upper limits of CO2 reactivity (i.e. near the threshold (∼55 to 60 mmHg) where CBF velocity has plateaued despite further elevations in ) linear elevations in BP then progressed, presumably via chemoreflex-induced elevations in sympathetic nerve activity (SNA). Notably, the authors are the first to integrate this logistic and linear fitting approach to document the influence of and related changes in mean arterial pressure (MAP) on CBF. Collectively, these experiments demonstrate that rebreathing tests – when analysed as described – may provide an estimate of the cerebrovascular response to CO2 (and O2) at a constant BP, as well as an estimate of the cerebrovascular passive response to both BP and CO2. In the broader context of integrative physiology, these findings are noteworthy on many levels. For example, impairment in cerebrovascular reactivity to CO2 and failure to effectively counter-regulate (or autoregulate) against systemic BP fluctuations could lead to a predisposition to adverse cerebrovascular events such as stroke, infarct extension and haemorrhagic transformation of existing strokes (Aries et al. 2010). However, the critical physiological and methodological consideration is that traditional tests to assess cerebrovascular reactivity to CO2 or cerebrovascular autoregulation treat these factors as separate identities. Clearly they are not: elevations in will lead to sympathoexcitation and increases in BP via vasoconstriction (Ainslie et al. 2005). The latter, as exampled by Battisti-Charbonney and co-workers, will have independent effects on CBF from those of (Lucas et al. 2010). Conversely, emerging evidence indicates that acute changes in BP may then impact on alveolar ventilation and thus , in part via the aptly named ‘ventilatory baroreflex’ (Stewart et al. 2011). Moreover, because the brain is relatively pressure-passive (Lucas et al. 2010) and since elevations in also ‘impair’ the brain's capability to defend against BP changes (Panerai et al. 1999), considerations of BP as a critical determinant of CBF is warranted in these conditions. An example of these integrated changes in and BP occur in a myriad everyday activities: postural change, coughing, laughing, defecation, exercise, sexual activity, to name but a few. The merit of the newly proposed method as a useful clinical tool to explore the separate and combined quantification of the cerebrovascular reactivity to CO2 and BP needs to be established. However, consideration of the combined influence of both and BP on the brain would seem meritorious from a systems physiology viewpoint. In summary, in view of the article by Battisti-Charbonney et al., we have attempted to highlight some of the common factors that independently, synergistically and often antagonistically participate in the regulation of CBF. Research exploring these complex interactions is currently lacking. Future studies with particular focus on these integrative physiological mechanisms are clearly warranted in both health and disease states.

Differences in the control of breathing between Andean highlanders and lowlanders after 10 days acclimatization at 3850 m

We used Duffin's isoxic hyperoxic ( mmHg) and hypoxic ( mmHg) rebreathing tests to compare the control of breathing in eight (7 male) Andean highlanders and six (4 male) acclimatizing Caucasian lowlanders after 10 days at 3850 m. Compared to lowlanders, highlanders had an increased non-chemoreflex drive to breathe, characterized by higher basal ventilation at both hyperoxia (10.5 +/- 0.7 vs. 4.9 +/- 0.5 l min(1), P = 0.002) and hypoxia (13.8 +/- 1.4 vs. 5.7 +/- 0.9 l min(1), P < 0.001). Highlanders had a single ventilatory sensitivity to CO(2) that was lower than that of the lowlanders (P < 0.001), whose response was characterized by two ventilatory sensitivities (VeS1 and VeS2) separated by a patterning threshold. There was no difference in ventilatory recruitment thresholds (VRTs) between populations (P = 0.209). Hypoxia decreased VRT within both populations (highlanders: 36.4 +/- 1.3 to 31.7 +/- 0.7 mmHg, P < 0.001; lowlanders: 35.3 +/- 1.3 to 28.8 +/- 0.9 mmHg, P < 0.001), but it had no effect on basal ventilation (P = 0.12) or on ventilatory sensitivities in either population (P = 0.684). Within lowlanders, VeS2 was substantially greater than VeS1 at both isoxic tensions (hyperoxic: 9.9 +/- 1.7 vs. 2.8 +/- 0.2, P = 0.005; hypoxic: 13.2 +/- 1.9 vs. 2.8 +/- 0.5, P < 0.001), although hypoxia had no effect on either of the sensitivities (P = 0.192). We conclude that the control of breathing in Andean highlanders is different from that in acclimatizing lowlanders, although there are some similarities. Specifically, acclimatizing lowlanders have relatively lower non-chemoreflex drives to breathe, increased ventilatory sensitivities to CO(2), and an altered pattern of ventilatory response to CO(2) with two ventilatory sensitivities separated by a patterning threshold. Similar to highlanders and unlike lowlanders at sea-level, acclimatizing lowlanders respond to hypobaric hypoxia by decreasing their VRT instead of changing their ventilatory sensitivity to CO(2).

Variability of the ventilatory response to Duffin's modified hyperoxic and hypoxic rebreathing procedure in healthy awake humans

We quantified the magnitude of within- and between-day, within-subject variability of the ventilatory response to Duffin's modified rebreathing procedure in 20 healthy humans. The P(ETCO2) at which ventilation increased with progressive increases in P(ETCO2) during rebreathing was identified as the ventilatory recruitment threshold (VRT(CO2)); the ventilatory response below and above the VRT(CO2) was taken as an estimate of non-chemoreflex drives to breathe (V(EB)) and chemoreflex sensitivity (V(ES)), respectively. Within- and between-day intraclass correlation coefficients for each of these parameters were >0.60 (range: 0.62-0.93), indicating good-to-excellent test-retest reliability. Within- and between-day, within-subject coefficients of variation for hyperoxic and hypoxic V(EB) (range: 24.6-30.7%) and V(ES) (range: 18.5-32.7%) were relatively high but acceptable, while those for the VRT(CO2) were very small (range: 3.0-3.8%). In conclusion, Duffin's modified rebreathing procedure, in both its hyperoxic and hypoxic form, is a highly reliable tool for measurement of chemoreflex and non-chemoreflex ventilatory control characteristics over short and long periods of time in healthy humans.

Systemic and Cerebrovascular Function following Short Duration Intermittent and Continuous Hypoxia in Humans

Patients with obstructive sleep apnoea (OSA) have impairment in cerebral and systemic endothelial function. To examine to what extent these initial changes are caused by the repetitive hypoxic apnoeas, we randomly gave 10 healthy volunteers, two conditions: 1) short duration intermittent hypoxia (SDIH), which was designed to mimic moderate to severe OSA (i.e. 20s voluntary hypoxic breath‐holds, SpO2 = 85%, 30 times per h for 2 h); 2) short duration continuous hypoxia (SDCH) of similar intensity and duration (i.e. 20 min, SpO2 = 85%). Before (pre) and immediately after (post) each intervention we measured ventilation (VE), cerebral blood flow velocity (CBFV) and arterial blood pressure (BP) in conditions of eucapnia and during hyperoxic (PETO2 "d 150 mmHg) and hypoxic (PETO2 = 50 mmHg) rebreathing. Systemic endothelial function was assessed using flow‐mediated dilatation (FMD). All baseline variables (end‐tidal gases, VE , CBFV, BP, and FMD) were unchanged following both SDIH and SDCH. Whilst the slope of the CBFV‐CO2 relationship was higher during hypoxic rebreathing, exposure to SDIH and SDCH did not alter these relationships. These data indicate that the SDIH and SDCH protocols used in this study were not of sufficient duration to elicit any changes in cerebral or systemic vascular function. The duration of SDIH or SDCH required to impact on vascular health warrants further research.

Cardiorespiratory and cerebrovascular responses to hyperoxic and hypoxic rebreathing: Effects of acclimatization to high altitude

We examined the cardiorespiratory and cerebrovascular responses to hyperoxic and hypoxic rebreathing at low attitude and high altitude. We measured ventilation, middle cerebral artery blood flow velocity (MCAv) and arterial blood pressure in conditions of eucapnia, hypocapnia (voluntary hyperventilation) and during hyperoxic and hypoxic rebreathing firstly at low altitude (1400 m) then again at high altitude (3840 m) in five individuals, following 9 days at altitude >5000 m. High altitude was associated with elevations in the blood pressure response to hyperoxic rebreathing, whilst cerebrovascular reactivity was reduced and ventilatory sensitivity was unchanged. During hypoxic rebreathing, whilst ventilatory and blood pressure reactivity were increased (vs. low altitude and hyperoxic rebreathing), cerebrovascular reactivity was preserved. In conclusion, at high altitude there was an enhancement in peripheral but not central chemosensitivity to CO(2); and during hypoxic rebreathing, marked elevations in blood pressure may restore some of the reduction in cerebrovascular CO(2) reactivity, potentially reflecting a pressure-passive relationship in the brain offsetting sympathetic-induced cerebral vasoconstriction.

Anxiety sensitivity as a predictor of panic attacks

Anxiety sensitivity (AS) is the fear of physical symptoms of anxiety and related sensations believed to have harmful consequences. AS may play a central role in the nature and etiology of panic disorder (PD) and the genesis of panic attacks. We collected Anxiety Sensitivity Index (ASI) scores from PD patients and controls to determine if AS accurately predicts panic. ASIs were completed prior to panic induction using the modified Read rebreathing test in both hypoxic and hyperoxic conditions. Total scores first-order factors, and individual item ASI scores were correlated with panic presence (Spearman correlation) for each of the hypoxic and hyperoxic rebreathing tests for both study populations. Control subjects' data correlated significantly for items 4, 8, and 11 of the ASI for the hyperoxic ( n=9; r S=0.63, 0.70, and 0.63, respectively) and items 4 and 8 for the hypoxic rebreathing tests ( n=9; r S=0.63 and 0.70, respectively). Panic patients' data correlated significantly for item 1 of the ASI for hyperoxic tests ( n=8; r S=0.76) and item 5 for the hypoxic tests ( n=8; r S=0.95). Total ASI scores or first-order factors (physical, social concerns, and mental incapacitation) scores of either study group did not correlate significantly with panic presence. AS may not be a reliable predictor of panicogenic responses to CO 2-induced panic in either PD or normal control populations. AS may not be an ultimate causal element in eliciting panic attacks.

Reliability of a Rebreathing Procedure to Evaluate Respiratory Chemoreflex Control

0858 PURPOSE: To examine the reliability and reproducibility of a modified rebreathing procedure and assess its utility as a research/diagnostic tool. METHODS: The rebreathing technique used was modified to include 5 minutes of prior hyperventilation and maintenance of iso-oxia at hyperoxic (150 mmHg) or hypoxic (50 mmHg) oxygen tensions during rebreathing (Duffin et al., Respir. Physiol., 2000). Twentyeight healthy subjects (14 men, 14 women) performed both hyperoxic and hypoxic rebreathing experiments in the morning (AM) and evening (PM) of the same day, with 12–13 hours separating AM and PM tests. Reliability was examined using the Pearson product-moment correlation coefficient. A three-way ANOVA was used to evaluate the reproducibility. Within and between-subject coefficients of variation (CVs) were calculated. RESULTS: Mean hyperoxic and hypoxic ventilatory responses to carbon dioxide (CO2) did not differ between the AM and PM. Significant reliability coefficients were observed for hyperoxic (r = 0.45 – 0.75) and hypoxic (r = 0.67 – 0.78) rebreathing responses. Bland-Altman plots confirmed no systematic bias between AM and PM tests. Between-subject CVs for basal (41.0 and 39.1%) and sensitivity (34.3 and 33.7%) responses, for hypoxic and hyperoxic tests were similar, but the CVs for the chemoreflex threshold for CO2 were much less (4.8 and 4.9%) for both hyperoxic and hypoxic tests. Within-subject CVs for basal (23.9 ± 4.1 and 20.7 ± 3.7%), threshold (2.6 ± 0.4 and 2.6 ± 0.4%) and sensitivity (17.5 ± 3.1 and 19.9 ± 2.6%) responses to CO2, for hyperoxic and hypoxic tests, were reasonably low. CONCLUSIONS: The results support the within-day reliability and reproducibility of the modified rebreathing procedure. These data support the use of the modified rebreathing procedure to investigate the effects of interventions/physiological perturbations on the chemoreflex control of breathing. However, its use to diagnose disorders in respiratory chemoreflex control in individual patients is not supported. Supported by the William M. Spear Foundation and Ontario Thoracic Society.

Reliability of a Rebreathing Procedure to Evaluate Respiratory Chemoreflex Control

0858 PURPOSE: To examine the reliability and reproducibility of a modified rebreathing procedure and assess its utility as a research/diagnostic tool. METHODS: The rebreathing technique used was modified to include 5 minutes of prior hyperventilation and maintenance of iso-oxia at hyperoxic (150 mmHg) or hypoxic (50 mmHg) oxygen tensions during rebreathing (Duffin et al., Respir. Physiol., 2000). Twentyeight healthy subjects (14 men, 14 women) performed both hyperoxic and hypoxic rebreathing experiments in the morning (AM) and evening (PM) of the same day, with 12–13 hours separating AM and PM tests. Reliability was examined using the Pearson product-moment correlation coefficient. A three-way ANOVA was used to evaluate the reproducibility. Within and between-subject coefficients of variation (CVs) were calculated. RESULTS: Mean hyperoxic and hypoxic ventilatory responses to carbon dioxide (CO2) did not differ between the AM and PM. Significant reliability coefficients were observed for hyperoxic (r = 0.45 – 0.75) and hypoxic (r = 0.67 – 0.78) rebreathing responses. Bland-Altman plots confirmed no systematic bias between AM and PM tests. Between-subject CVs for basal (41.0 and 39.1%) and sensitivity (34.3 and 33.7%) responses, for hypoxic and hyperoxic tests were similar, but the CVs for the chemoreflex threshold for CO2 were much less (4.8 and 4.9%) for both hyperoxic and hypoxic tests. Within-subject CVs for basal (23.9 ± 4.1 and 20.7 ± 3.7%), threshold (2.6 ± 0.4 and 2.6 ± 0.4%) and sensitivity (17.5 ± 3.1 and 19.9 ± 2.6%) responses to CO2, for hyperoxic and hypoxic tests, were reasonably low. CONCLUSIONS: The results support the within-day reliability and reproducibility of the modified rebreathing procedure. These data support the use of the modified rebreathing procedure to investigate the effects of interventions/physiological perturbations on the chemoreflex control of breathing. However, its use to diagnose disorders in respiratory chemoreflex control in individual patients is not supported. Supported by the William M. Spear Foundation and Ontario Thoracic Society.

Human hypoxic ventilatory response with blood dopamine content under intermittent hypoxic training

Adaptation to intermittent hypoxia can enhance a hypoxic ventilatory response (HVR) in healthy humans. Naturally occurring oscillations in blood dopamine (DA) level may modulate these responses. We have measured ventilatory response to hypoxia relative to blood DA concentration and its precursor DOPA before and after a 2-week course of intermittent hypoxic training (IHT). Eighteen healthy male subjects (mean 22.8+/-2.1 years old) participated in the study. HVRs to isocapnic, progressive, hypoxic rebreathing were recorded and analyzed using piecewise linear approximation. Rebreathing lasted for 5-6 min until inspired O2 reached 8 to 7%. IHT consisted of three identical daily rebreathing sessions separated by 5-min breaks for 14 consecutive days. Before and after the 2-week course of IHT, blood was sampled from the antecubital vein to measure DA and DOPA content. The investigation associated pretraining high blood DA and DOPA values with low HVR (r = -0.66 and -0.75, respectively), elevated tidal volume (r = 0.58 and 0.37) and vital capacity (r = 0.69 and 0.58), and reduced respiratory frequency (r = -0.89 and -0.82). IHT produced no significant change in ventilatory responses to mild hypoxic challenge (Peto2 from 110 to 70-80 mm Hg; 1 mm Hg = 133.3 Pa) but elicited a 96% increase in ventilatory response to severe hypoxia (from 70-80 to 45 mm Hg). Changes in HVRs were not accompanied by statistically significant shifts in blood DA content (24% change), although a twofold increase in DOPA concentration was observed. Individual subject's changes in DA and DOPA content were not correlated with HVR changes when these two parameters were evaluated in relation to the IHT. We hypothesize that DA flowing to the carotid body through the blood may provoke DA autoreceptor-mediated inhibition of endogenous DA synthesis-release, as shown in our baseline data.

Respiratory Muscle Function and Hypoxic Ventilatory Control in Patients With Type I Diabetes

The interaction among pulmonary mechanics, respiratory muscle performance, and ventilatory control in subjects with insulin-dependent diabetes mellitus has so far received little attention. We therefore decided to assess the role of central factors and peripheral factors on the ventilatory response to a hypoxic stimulus in type I diabetic patients. Eight patients in stable condition aged 19 to 48 years old, with insulin-dependent diabetes mellitus (duration of the disease, 36 to 240 months) and no history of smoking, cardiopulmonary involvement, or autonomic neuropathy; and an age- and gender-matched control group. In each patient, we measured the following: pulmonary volumes; diffusing capacity of the lung for carbon monoxide (D(LCO)); time and volume components of ventilation (tidal volume [V(T)] and respiratory frequency); static compliance (Clstat) and dynamic compliance (Cldyn); swings in pleural pressure (Pes) and gastric pressure (Pg); and transdiaphragmatic pressure (Pdi), obtained by subtracting Pes from Pg. Maximal inspiratory Pes and Pdi during a maximal sniff maneuver were also measured. Swings in Pes and Pdi during V(T) as a percentage of Pes and Pdi during the maximal sniff maneuver [Pessw(%Pessn) and Pdisw(%Pdisn), respectively] were both considered as a measure of central respiratory output, and the Pessw(%Pessn)/V(T) ratio was considered as an index of neuroventilatory dissociation (NVD) of the inspiratory pump. Subjects were studied at baseline and during hypoxic rebreathing. Pulmonary volumes and D(LCO) were normal or slightly reduced. A lower Cldyn, higher central respiratory output, and NVD were found. During hypoxic rebreathing, patients had lower V(T), similar central respiratory output, and greater NVD per unit change in arterial oxygen saturation compared with values in control subjects. An increase in dynamic elastance, computed as 1/Cldyn, during hypoxia was found in patients, but not in normal subjects, and was directly related to concurrent changes in NVD. We have shown that the assessment of a normal Clstat and normal routine parameters of airway obstruction does not permit the definite exclusion of the role of peripheral airway involvement in insulin-dependent diabetes mellitus. Peripheral airway involvement is likely to influence indices of hypoxic ventilator) drive by modulating a normal central motor output into a rapid and shallow pattern of ventilatory response.

Dyspnoea, peripheral airway involvement and respiratory muscle effort in patients with Type I diabetes mellitus under good metabolic control

Dyspnoea and pulmonary dysfunction have recently been associated with Type I (insulin-dependent) diabetes mellitus. The putative role of altered pulmonary mechanics and of performance of inspiratory muscles in inducing dyspnoea has not been yet assessed in Type I diabetes. To better focus on this topic we evaluated nine patients with Type I diabetes mellitus, aged 19 to 48 years with good and stable metabolic control, without a history of smoking and microvascular complications, alongside a group of 14 healthy control subjects. In each subject, pulmonary volumes, static and dynamic compliance, pleural pressure swings (Pplsw), maximal inspiratory pressures (Pplsn), Pplsw(%Pplsn), a measure of respiratory muscle effort, and tension-time index [TTI=TI/TTOTxPplsw(%Pplsn)] were measured (TI=inspiratory time;TTOT=total time of the respiratory cycle). All subjects were studied at baseline and during hypoxic rebreathing. Patients had normal pulmonary volumes. During hypoxic rebreathing, a normal change in respiratory muscle effort [DeltaPplsw(%Pplsn)/DeltaSaO2] and DeltaTTI/DeltaSaO2, and a lower change in tidal volume versus change in oxygen saturation [DeltaVT(% vital capacity)/DeltaSaO2], resulted in a higher ratio of respiratory effort to tidal volume [Pplsw(%Pplsn)/VT(% vital capacity)], a measure of neuroventilatory dissociation of the respiratory pump. Hypoxic dyspnoea, assessed by a modified Borg scale, showed a greater rate of rise (DeltaBorg/DeltaSaO2) and a greater increase for a given level of respiratory effort in patients. Moreover, neuroventilatory dissociation related to the expression of peripheral airway involvement, as assessed in terms of low dynamic compliance, and to concurrent change in dyspnoea sensation. Patients with Type I diabetes mellitus under good metabolic control and with normal lung volumes may have abnormal peripheral airway function. The latter is thought to be responsible for the association between dyspnoea sensation and neuroventilatory dissociation.