Hyperoxic Rebreathing Research Articles

Effects of hyperventilation on repeated breath-holding while in a fasting state: do risks outweigh the benefits?

Breath-holding preceded by either an overnight fast or hyperventilation has been shown to potentiate the risk of a hypoxic blackout. However, no study has explored the combined effects of fasting and hyperventilation on apneic performance and associated physiological responses. Nine nondivers (8 males) attended the laboratory on two separate occasions (≥48 h apart), both after a 12-h overnight fast. During each visit, a hyperoxic rebreathing trial was performed followed by three repeated maximal static apneas preceded by either normal breathing (NORM) or a 30-s hyperventilation (HYPER). Splenic volume, hematology, cardiovascular, and respiratory variables were monitored. There were no interprotocol differences at rest or during hyperoxic rebreathing for any variable (P ≥ 0.09). On nine occasions (8 in HYPER), the subjects reached our safety threshold (oxygen saturation 65%) and were asked to abort their apneas, with the preponderance of these incidents (6 of 9) occurring during the third repetition. Across the sequential attempts, longer apneas were recorded in HYPER [median(range), 220(123-324) s vs. 185(78-296) s, P ≤ 0.001], with involuntary breathing movements occurring later [134(65-234) s vs. 97(42-200) s, P ≤ 0.001] and end-apneic partial end-tidal pressures of oxygen () being lower (P ≤ 0.02). During the final repetition, partial end-tidal pressure of carbon dioxide [(), 6.53 ± 0.46 kPa vs. 6.01 ± 0.45 kPa, P = 0.005] was lower in HYPER. Over the serial attempts, preapneic tidal volume was gradually elevated [from apnea 1 to 3, by 0.26 ± 0.24 L (HYPER) and 0.28 ± 0.30 L (NORM), P ≤ 0.025], with a correlation noted with preapneic (r = -0.57, P < 0.001) and (r = 0.76, P < 0.001), respectively. In a fasted state, preapnea hyperventilation compared with normal breathing leads to longer apneas but may increase the susceptibility to a hypoxic blackout.NEW & NOTEWORTHY This study shows that breath-holds (apneas) preceded by a 12-h overnight fast coupled with a 30-s hyperventilation as opposed to normal breathing may increase the likelihood of a hypoxic blackout through delaying the excitation of hypercapnic ventilatory sensory chemoreflexes. Evidently, this risk is exacerbated over a series of repeated maximal attempts, possibly due to a shift in preapneic gas tensions facilitated by an unintentional increase in tidal volume breathing.

Central and peripheral chemoreflexes in humans with treated hypertension.

Increased peripheral chemoreflex sensitivity is a pathogenic feature of human hypertension (HTN), while both central and peripheral chemoreflex sensitivities are reportedly augmented in animal models of HTN. Herein, we tested the hypothesis that both central and combined central and peripheral chemoreflex sensitivities are augmented in HTN. Fifteen HTN participants (68±5years; mean±SD) and 13 normotensives (NT; 65±6years) performed two modified rebreathing protocols in which the partial pressure of end-tidal carbon dioxide ( ) progressively increased while the partial pressure of end-tidal oxygen was clamped at either 150mmHg (isoxic hyperoxia; central chemoreflex activation) or 50mmHg (isoxic hypoxia; combined central and peripheral chemoreflex activation). Ventilation ( ; pneumotachometer) and muscle sympathetic nerve activity (MSNA; microneurography) were recorded, and ventilatory ( vs. slope) and sympathetic (MSNA vs. slope) chemoreflex sensitivities and recruitment thresholds (breakpoint) were calculated. Global cerebral blood flow (gCBF; duplex Doppler) was measured, and the association with chemoreflex responses was examined. Central ventilatory and sympathetic chemoreflex sensitivities were greater in HTN than NT (2.48±1.33 vs. 1.58±0.42Lmin-1 mmHg-1 , P=0.030: 3.32±1.90 vs. 1.77±0.62a.u.mmHg-1 , P=0.034, respectively), while recruitment thresholds were not different between groups. HTN and NT had similar combined central and peripheral ventilatory and sympathetic chemoreflex sensitivities and recruitment thresholds. A lower gCBF was associated with an earlier recruitment threshold for (R2 =0.666, P<0.0001) and MSNA (R2 =0.698, P=0.004) during isoxic hyperoxic rebreathing. These findings indicate that central ventilatory and sympathetic chemoreflex sensitivities are augmented in human HTN and perhaps suggest that targeting the central chemoreflex may help some forms of HTN. KEY POINTS: In human hypertension (HTN) increased peripheral chemoreflex sensitivity has been identified as a pathogenic feature, and in animal models of HTN, both central and peripheral chemoreflex sensitivities are reportedly augmented. In this study, the hypothesis was tested that both central and combined central and peripheral chemoreflex sensitivities are augmented in human HTN. We observed that both central ventilatory and sympathetic chemoreflex sensitivities were augmented in HTN compared to age-matched normotensive controls, but no difference was found in the combined central and peripheral ventilatory and sympathetic chemoreflex sensitivities. During central chemoreflex activation, the ventilatory and sympathetic recruitment thresholds were lower in those with lower total cerebral blood flow. These results indicate a potential contributory role of the central chemoreceptors in the pathogenesis of human HTN and support the possibility that therapeutic targeting of the central chemoreflex may help some forms of HTN.

The effect of dietary intake on apneic performance, cardiovascular and splenic responses during repeated breath holds.

Static apneas performed after an overnight fast as opposed to postprandially have been evinced to improve apneic performance. However, no study has explored the effect of dietary intake on apneic performance, cardiovascular or splenic responses over a series of repeated apneas. Ten healthy adults attended the laboratory on three separate occasions (≥48-h apart): after a 14-h fast (F14), 1 h postconsumption of a high-calorie, high-carbohydrate (HCHC) meal, or 1 h postconsumption of a low-calorie, low-carbohydrate (LCLC)-based meal. During each visit, the subjects performed a hyperoxic rebreathing trial and a series of three repeated maximal static apneas. Heart rate, peripheral oxyhemoglobin saturation ([Formula: see text]), and gas exchange were monitored continuously, whereas splenic volume (SV) and hematology were assessed after the rebreathing and apneas. At rest, after HCHC, the respiratory exchange ratio (0.87 ± 0.17, P ≤ 0.043), expired minute volume of carbon dioxide (CO2; HCHC, 0.35 ± 0.09 L/min, P ≤ 0.014), and SV (227 ± 45 mL, P ≤ 0.031) were higher compared with F14 (0.71 ± 0.08; 0.23 ± 0.04 L/min; 204 ± 49 mL) and LCLC (0.72 ± 0.07; 0.25 ± 0.03 L/min; 199 ± 49 mL). A faster CO2 accumulation was recorded during the HCHC (96 ± 35 s) rebreathing trial (F14, 162 ± 42 s, P = 0.001; LCLC, 151 ± 23 s, P = 0.002). Longer apneas were reported in F14 compared with HCHC (apneas 1-3, P ≤ 0.046) and LCLC (apneas 2-3, P ≤ 0.006). After the first apnea, SV was lower in F14 (141 ± 43 mL, P = 0.015) compared with HCHC (180 ± 34 mL). Moreover, after the third apnea, end-tidal partial pressure of oxygen and nadir [Formula: see text] were lower in F14 (8.6 ± 2.2 kPa, P = 0.028; 77 ± 13%, P = 0.009) compared with HCHC (10.1 ± 1.7 kPa; 84 ± 9%). No differences were measured in end-apneic end-tidal partial pressure of CO2, heart rate nor hematology across diets. Fasting improved apneic performance with apneas being terminated at lower oxygen levels through altering the rate of CO2 accumulation but without affecting the cardiovascular responses.

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.

Acute hyperglycemia does not affect central respiratory chemoreflex responsiveness to CO2 in healthy humans

The central respiratory chemoreceptor complex (CCRC) is comprised of brainstem neurons and surrounding interoceptors, which collectively increase ventilation in response to elevated brainstem tissue CO2/[H+] (i.e., central chemoreflex; CCR). The extent that the CCRC detects/responds to other metabolically related chemostimuli is unknown. We aimed to test the effects of acute oral glucose ingestion on CCR reactivity in heathy human participants (n = 38). We instrumented participants with a pneumotachometer (minute ventilation) and a gas sample line connected to a dual gas analyzer (pressure of end-tidal CO2). Following a baseline (BL) period and capillary blood [glucose] (BG) sample, fasted (F) participants underwent a modified hyperoxic rebreathing test to assess CCR reactivity. Participants then consumed a 75 g standard glucose beverage (glucose loaded; GL). Following 30-min, they underwent a second BL, BG sample and hyperoxic rebreathing test. BG and metabolic rate were higher in GL, confirming the metabolic stimulus. However, the ventilatory recruitment threshold and the CCR responses were unchanged between F and GL states.

Cerebrovascular carbon dioxide reactivity is influenced by sex despite comparable hemodynamic and respiratory chemosensitivity in young fit individuals

Cerebral blood flow (CBF) is generally higher in women compared to men at rest. Although accumulating evidence supports sex differences in CBF regulation, findings remain contradictory for dynamic cerebral autoregulation (dCA) and cerebrovascular reactivity to carbon dioxide (CVRCO2). Cardiorespiratory fitness represents an indicator of cardiovascular health. However, we have recently reported that dCA is reduced in young fit men compared to controls, and that dCA is attenuated in young fit women compared to fit men. However, the influence of sex on CVRCO2 in these young fit individuals remains misunderstood. The aim of this study was to examine the influence of sex on CVRCO2 in young fit individuals. Changes in middle cerebral artery mean blood velocity (MCAvmean; transcranial doppler), mean arterial pressure (MAP; photoplethysmography), heart rate (HR; ECG) and pulmonary ventilation (spirometer) were measured in 29 young fit individuals (women n=10, men n=19), in response to changes in end‐tidal partial pressure of carbon dioxide (PETCO2; gas analyzer) during the modified Duffin hyperoxic rebreathing test. Women were tested in the early follicular phase. CVRCO2, as well as hemodynamic and ventilatory chemosensitivity, were characterized using linear regressions on the rebreathing portion of the test after the determined ventilatory recruitment threshold. While age was not different between groups (26 ± 6 vs. 27 ± 5 years, P = 0.92), height: (1.64 ± 0.06 vs. 1.77 ± 0.08 m; P &lt; 0.0001), body weight (60 ± 6 vs. 72 ± 10 kg; P = 0.0009) and maximal oxygen uptake (48.4 ± 3.8 vs. 57.9 ± 6.8 mL−1kg−1min−1; P &lt; 0.0001) were lower in women. At rest, MCAvmean (72 ± 8 cm/s vs. 62 ± 11 cm/s; P = 0.02) and HR (73 ± 8 vs. 64 ± 11 bpm; P = 0.02) were higher in women, whereas MAP was not different between groups (100 ± 19 vs. 97 ± 10 mm Hg; P = 0.73). CVRCO2 was higher in women compared to men, both in absolute (1.59 ± 0.78 vs. 0.35 ± 1.51 cm/s/mm Hg, P = 0.01) and normalized terms (2.19 ± 0.97 vs. 0.54 ± 2.41 %/mm Hg, P = 0.02). Hemodynamic and respiratory chemosensitivity responses during CO2 rebreathing were not different between groups (all P &gt; 0.05). These results indicate that the cerebrovasculature of young fit women is more sensitive to CO2 than men despite comparable hemodynamic and ventilatory chemosensitivity responses.Support or Funding InformationS.I., L.L. and S.M. have been supported by a doctoral training scholarship from the Fonds de recherche du Québec ‐ Santé (FRQS)

Acute hyperglycemia does not alter central respiratory CO2 chemoreflex responsiveness

The control of breathing is modulated by two distinct but interacting respiratory chemoreceptor compartments: peripheral chemoreceptors (PCRs; carotid bodies) and central chemoreceptors (CCRs; brainstem). The PCRs respond to variety of chemostimuli, including arterial oxygen, arterial carbon dioxide, glucose, insulin and temperature, and are now thought to act as metabolic sensors. However, CCRs have only been characterized as carbon dioxide/acid sensors (PCO2/[H+]). PCRs respond to chemostimulation faster than CCRs, which may mask CCR responses to stimuli like acute hyperglycemia. The aim of this study was to characterize the relationship between alterations in blood glucose concentration and central respiratory chemoreflex responsiveness via hyperoxic rebreathing in healthy humans. We hypothesized that experimentally‐induced relative hyperglycemia following an oral glucose tolerance test (OGTT; 75g, 300 ml) would (a) increase the rate of CO2 accumulation during rebreathing and (b) augment central respiratory chemoreflex responsiveness due to a synergistic effect of hyperglycemia on the CO2 sensitivity of CCR neurons. In 10 healthy participants, we measured minute ventilation (VI; L/min; pneumotachometer) and end‐tidal carbon dioxide (PETCO2; Torr; gas analyzer). We used the modified hyperoxic rebreathing test to isolate CCRs and quantify the central chemoreflex responsiveness to increases in metabolically‐derived CO2. Blood glucose levels were higher 30‐min following the OGTT beverage (i.e., glucose‐loaded) compared to fasted (7.8±1.1 vs. 4.7±0.4 mmol/L; P&lt;0.0001). During rebreathing, the rate of end‐tidal (PET)CO2 accumulation was greater in the glucose loaded trial compared to the fasted trial (0.086±0.007 vs. 0.07±0.008 vs. Torr/sec; P=0.0002). These findings confirm the hyperglycemic stimulation between trials. However, the central respiratory chemoreflex responsiveness during rebreathing was not different between glucose loaded and fasted trials (2.3±1.4 vs. 2.5±1.8 vs. L/min/Torr CO2; P=0.59). These preliminary findings suggest that relative hyperglycemia does not affect the central respiratory chemoreflex responsive to hyperoxic rebreathing. The CCRs may be a more specialized respiratory chemoreceptor, with a phenotype to detect only metabolically‐derived accumulations in CO2.Support or Funding InformationNSERC Discovery and MRU Faculty of Science and TechnologyThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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.

Intra-individual variability in cerebrovascular and respiratory chemosensitivity: Can we characterize a chemoreflex “reactivity profile”?

Intra-individual variability in the magnitude of human cerebrovascular and respiratory chemoreflex responses is largely unexplored. By comparing response magnitudes of cerebrovascular CO2 reactivity (CVR; middle and posterior cerebral arteries; MCA, PCA), central (CCR; CO2) and peripheral respiratory chemoreflexes (PCR; CO2 and O2), we tested the hypothesis that a within-individual reactivity magnitude profile could be characterized. The magnitudes of CVR and CCR were tested with hyperoxic rebreathing and PCR magnitudes were tested through transient respiratory tests (TT-CO2, hypercapnia; TT-N2, hypoxia). No significant intra-individual relationships were found between CCR vs. CVR (MCA and PCA), CCR vs. PCR (TT-N2 or TT-CO2) (r<0.2, P>0.3) response magnitudes. Statistically significant relationships were found between MCA vs. PCA reactivity (r=0.45, P<0.01) and PCR TT-N2 vs. PCR TT-CO2 (r=0.79, P<0.001) responses. Using qualitative and quantitative comparisons, we conclude that an intra-individual chemoreflex reactivity magnitude profile cannot be characterized. These data highlight the considerable between- and within-individual variability that exists in human cerebrovascular and respiratory chemoreflexes.

Influence of prior hyperventilation duration on respiratory chemosensitivity and cerebrovascular reactivity during modified hyperoxic rebreathing

What is the central question of this study? We characterized and compared the cardiorespiratory and cerebrovascular responses to the 'Duffin' modified hyperoxic CO2 rebreathing test by randomly altering the prior hyperventilation duration. What is the main finding and its importance? Our main finding was that prior hyperventilation duration (1, 3 or 5min) had no effect on cardiorespiratory and cerebrovascular responses to the hyperoxic rebreathing test, within individuals. These findings suggest that the standard 5min prior hyperventilation duration used to clear body CO2 stores is unnecessary and can reasonably be shortened to 1min, reducing protocol times and improving participant comfort. The 'Duffin' modified hyperoxic rebreathing test allows investigators to characterize and quantify the ventilatory and cerebrovascular responses to CO2 across a large physiological range, allowing quantification of basal ventilation and the ventilatory recruitment threshold (VRT). Although the standard protocol includes 5min of prior hyperventilation to clear body CO2 stores, there is no experimental evidence that a full 5min is required. We hypothesized that there would be no within-individual differences in the cardiorespiratory or cerebrovascular responses to rebreathing with shortened hyperventilation duration prior to hyperoxic rebreathing. Using a rebreathing apparatus, transcranial Doppler ultrasound and beat-to-beat blood pressure monitoring, we tested 19 participants in the supine position using three randomly assigned hyperoxic rebreathing tests with 1, 3 or 5min of prior hyperventilation. We measured VRT (in Torr CO2 ), time to VRT (in seconds), central respiratory chemoreflex (breathing frequency, tidal volume and minute ventilation), cerebrovascular (middle and posterior cerebral artery velocity) and cardiovascular (heart rate and mean arterial pressure) responses to CO2 during hyperoxic rebreathing. Using linear regression and repeated-measures ANOVAs, we found no differences in any of the cardiorespiratory or cerebrovascular response magnitudes between trials (P>0.05). The only difference observed was in the time to VRT (in seconds), whereby 1min prior hyperventilation duration was shorter (135.4±19.7s) than with 3 or 5min prior hyperventilation (176.3±15.1 and 187.2±11.6s, respectively; P<0.001). Our findings indicate that 5min of prior hyperventilation is unnecessary during modified rebreathing when using it to quantify respiratory or cerebrovascular responses and can be reasonably shortened to 1min, reducing protocol times and improving participant comfort.

Central respiratory chemosensitivity and cerebrovascular CO2 reactivity: a rebreathing demonstration illustrating integrative human physiology.

One of the most effective ways of engaging students of physiology and medicine is through laboratory demonstrations and case studies that combine 1) the use of equipment, 2) problem solving, 3) visual representations, and 4) manipulation and interpretation of data. Depending on the measurements made and the type of test, laboratory demonstrations have the added benefit of being able to show multiple organ system integration. Many research techniques can also serve as effective demonstrations of integrative human physiology. The "Duffin" hyperoxic rebreathing test is often used in research settings as a test of central respiratory chemosensitivity and cerebrovascular reactivity to CO2. We aimed to demonstrate the utility of the hyperoxic rebreathing test for both respiratory and cerebrovascular responses to increases in CO2 and illustrate the integration of the respiratory and cerebrovascular systems. In the present article, methods such as spirometry, respiratory gas analysis, and transcranial Doppler ultrasound are described, and raw data traces can be adopted for discussion in a tutorial setting. If educators have these instruments available, instructions on how to carry out the test are provided so students can collect their own data. In either case, data analysis and quantification are discussed, including principles of linear regression, calculation of slope, the coefficient of determination (R(2)), and differences between plotting absolute versus normalized data. Using the hyperoxic rebreathing test as a demonstration of the complex interaction and integration between the respiratory and cerebrovascular systems provides senior undergraduate, graduate, and medical students with an advanced understanding of the integrative nature of human physiology.

Steady-state tilt has no effect on cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations.

What is the central question of this study? We investigated the effects of superimposed tilt and hypercapnia-induced cerebral arteriolar dilatation on anterior and posterior cerebrovascular CO2 reactivity using hyperoxic rebreathing in human participants. What is the main finding and its importance? The main findings are threefold: (i) cerebrovascular CO2 reactivity in the anterior and posterior cerebrovasculature is unchanged with tilt; (ii) cerebral autoregulation is unlikely responsible due to unchanging cerebrovascular resistance reactivity between positions; and (iii) cerebral blood flow is not pressure passive during tilt as it is with pharmacological or lower body negative pressure-induced changes in mean arterial pressure, suggesting that sympathetic activation or balanced transmural pressures during head-down tilt regulate cerebral blood flow. Cerebral autoregulation is a protective feature of the cerebrovasculature that maintains relatively constant cerebral perfusion in the face of static and dynamic fluctuations in mean arterial pressure (MAP). However, the extent that the cerebrovasculature can autoregulate in the face of superimposed steady-state orthostasis-induced changes in MAP (e.g. head-up and head-down tilt; HUT and HDT) and CO2 -mediated arteriolar dilatation is unknown. We tested the effects of steady-state tilt on cerebrovascular CO2 reactivity in the middle and and posterior cerebral artery in the following five body positions: 90 deg HUT, 45 deg HUT, supine, 45 deg HDT and 90 deg HDT on a tilt table during a modified hyperoxic rebreathing test. Absolute and relative cerebrovascular CO2 reactivity [cerebral blood velocity (CBV)/CO2 ], cerebrovascular resistance (CVR) reactivity (CVR/CO2 ) and MAP reactivity (MAP/CO2 ) were quantified using linear regression. Mean arterial pressure was significantly elevated in 90 deg HDT compared with other positions during baseline steady-state tilt (P < 0.01). Absolute CBV/CO2 and CVR/CO2 were greater in the middle cerebral artery than the posterior cerebral artery (P < 0.01) in all body positions, but relative measures were not different (P = 0.143 and P = 0.360, respectively), nor was there any interaction with tilt position. In addition, there was no difference in absolute (P = 0.556) and relative MAP/CO2 (P = 0.308) between positions. Our data demonstrate that cerebral blood flow remains well regulated during superimposed steady-state orthostatic stress and dynamic changes in the partial pressure of end-tidal CO2 during rebreathing. Cerebral autoregulation is likely not the mechanism responsible, but rather sympathetic nervous system activation or a balanced cerebrovascular transmural pressure with HDT maintains resting cerebral blood flow and cerebrovascular CO2 reactivity during rebreathing.

The effects of prior hyperventilation duration on central chemoreflex responses using the “Duffin” hyperoxic rebreathing test (1092.16)

The effects of prior hyperventilation duration on central chemoreflex responses using the “Duffin” hyperoxic rebreathing test The “Duffin” hyperoxic rebreathing test was developed from the “Read” rebreathing test by the addition of a five‐minute hyperventilation (HV) prior to switching to the rebreathing circuit. This HV eliminates CO2 stores, such that when the subject equilibrates with the rebreathing circuit, brainstem CO2 is below the ventilatory recruitment threshold (VRT) of the central respiratory chemoreceptors. This allows investigators to quantify the VRT prior to eliciting a central chemoreflex to increases in CO2 during rebreathing. An assumption of the test is that a full five minutes of prior HV is required. However, prolonged HV is uncomfortable for the subject, and there is no experimental evidence to validate this assumption. We aimed to test this assumption experimentally by comparing various durations of prior HV. We hypothesized that there would be no difference in central chemoreflex responses between randomized prior hyperventilation durations (HVD) of one, three and five minutes. Following instrumentation with a pneumotachometer and CO2 and O2 gas analyzers, 19 subjects underwent three hyperoxic rebreathing trials, each with randomized prior HV of one‐, three‐ and five‐minute durations, each coached to 18‐20 Torr PCO2. Rebreathing was carried out by switching to a circuit comprised of a 6L bag containing 7% CO2 and 93% O2. For all three trials, we quantified the VRT (PCO2; Torr), and respiratory frequency (min‐1/Torr CO2), inspired tidal volume (L/Torr CO2) and inspired ventilation (L/min/Torr CO2) with linear regression. We found no differences between HVD in any respiratory variable, within‐subject. Our data suggest that when using the “Duffin” rebreathing test, a one‐minute HV is sufficient to drive CO2 stores below the VRT, and the resulting central chemoreflex responses are no different than the standard five‐minute test.Grant Funding Source: NSERC

Chapter 9 - The effects of head-up and head-down tilt on central respiratory chemoreflex loop gain tested by hyperoxic rebreathing

Central respiratory chemosensitivity is mediated via chemoreceptor neurons located throughout brain stem tissue. These receptors detect proximal CO2/[H(+)] (i.e., controller gain) and modulate breathing in a classic negative feedback loop. Loop gain (responsiveness) is the theoretical product of controller (chemoreceptors), mixing/feedback (cardiovascular and cerebrovascular systems), and plant (pulmonary system) gains. The level of chemoreceptor stimulation is determined by interactions between mixing and plant gains. The extent to which steady-state changes in body position may affect central chemoreflex loop gain in response to CO2 is unclear. Because of the potential effects of tilt on pulmonary mechanics, we hypothesized that plant gain would be altered by head-up and head-down tilt (HUT, HDT) during hyperoxic rebreathing, which theoretically isolates plant gain by eliminating systemic arterial-tissue gradients. Sixteen subjects (eight females) underwent hyperoxic rebreathing tests on a tilt table to quantify central chemoreflex loop gain in five steady-state positions: 90° HUT, 45° HUT, supine, 45° HDT, and 90° HDT. Respiratory responses (tidal volume, VT; frequency, fR; minute ventilation, VE) were quantified during steady-state and increases in CO2 during rebreathing by linear regression above the ventilatory recruitment threshold (VRT). Using one-factor analysis of variance, we found that there were no differences in the respiratory responses between the five positions (VRT, P=0.711; VT, P=0.290; fR, P=0.748; VE, P=0.325). Our findings suggest that during steady-state orthostatic stress, the ability of subjects to mount a normal ventilatory response to increased CO2 was unaffected, despite any potential changes in pulmonary mechanics associated with positional challenges.

Differential cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations

The potential differences in cerebrovascular responses between the anterior and posterior circulations to changes in CO2 are unclear in humans. Using transcranial Doppler ultrasound, we compared the CO2 reactivity of the (1) BA and PCA and (2) MCA and PCA during hyperoxic rebreathing in supine position. The reactivity in the BA and PCA was similar in both absolute (1.27±0.5 and 1.27±0.6cm/s/Torr; P=0.992) and relative (3.98±1.3 and 3.66±1.5%/Torr CO2; P=0.581) measures, suggesting that the PCA is an adequate surrogate measure of reactivity for the BA. The MCA reactivity was greater than the PCA in absolute (2.09±0.7 and 1.22±0.5cm/s/Torr CO2; P<0.001), but not relative measures (3.25±1.0 and 3.56±1.6%/Torr CO2; P=0.629). Our findings (a) confirm regional differences in the absolute reactivity in the human brain and (b) suggest that in cerebrovascular studies investigating functions mediated by posterior brain structures (e.g., control of breathing), the posterior vasculature should also be insonated.

Carotid body tumors are not associated with an increased risk for sleep-disordered breathing

Tumors in the carotid bodies may interfere with their function as peripheral chemoreceptors. An altered control of ventilation may predispose to sleep-disordered breathing. This study aimed to assess whether patients with unilateral or bilateral carotid body tumors (uCBT or bCBT, respectively) or bilateral CBT resection (bCBR) display sleep-disordered breathing and to evaluate the global contribution of the peripheral chemoreceptor to the hypercapnic ventilatory response. Eight uCBT, eight bCBT, and nine bCBR patients and matched controls underwent polysomnography. The peripheral chemoreflex drive was assessed using euoxic and hyperoxic CO2 rebreathing tests. Daytime sleepiness and fatigue were assessed with the Epworth Sleepiness Scale and the Multidimensional Fatigue Index. All patient groups reported significant fatigue-related complaints, but no differences in excessive daytime sleepiness (EDS) were found. The apnea/hypopnea index (AHI) did not differ significantly between patient groups and controls. Only in bCBT patients, a trend towards a higher AHI was observed, but this did not reach significance (p=0.06). No differences in the peripheral chemoreflex drive were found between patients and controls. Patients with (resection of) CBTs have more complaints of fatigue but are not at risk for EDS. The presence or resection of CBTs is neither associated with an altered peripheral chemoreflex drive nor with sleep-disordered breathing.

The effects of head‐up and head‐down tilt on cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations

Cerebral autoregulation is a protective feature of the cerebrovasculature that maintains relatively constant cerebral perfusion in the face of static and dynamic fluctuations in mean arterial pressure (MAP). However, gravity‐dependent shifts in blood volume distribution during head‐up and head‐down tilt (HUT, HDT) have profound effects on venous return, cardiac output and MAP. Further, cerebral blood flow (CBF) is highly sensitive to the influence of changes in arterial CO2 (PaCO2) on arteriolar diameter. We tested the effects of steady‐state tilt on cerebral blood velocity (CBV) and CO2 reactivity in the middle and posterior cerebral arteries (MCA, PCA) using hyperoxic rebreathing and transcranial Doppler ultrasound in various positions on a tilt table. Following initial testing in supine position, subjects were positioned randomly and tested in each of four positions: 90° HDT, 45° HDT, 45° HUT and 90° HUT. There were no differences in steady‐state MCA or PCA CBV across positions. Absolute and relative CO2 reactivity slopes were calculated for the MCA and PCA using linear regression. Absolute reactivity was greater in the MCA than the PCA in all 5 positions, but there was no interaction between reactivity and tilt. Our data demonstrate that CBF is maintained through cerebral autoregulation in the face of superimposed steady‐state orthostatic stress and dynamic changes in PaCO2.

The increased ventilatory response to exercise in pregnancy reflects alterations in the respiratory control systems ventilatory recruitment threshold for CO2

We tested the hypothesis that the magnitude of the pregnancy-induced increase in exercise hyperpnea is predictable based on the level at which Pa CO2 is regulated at rest. We performed a detailed retrospective analysis of previous data from 25 healthy young women who performed exercise and rebreathing tests in the third trimester (TM(3); 36.5+/-0.2 weeks gestation; mean+/-SEM) and again 20.4+/-1.7 weeks post-partum (PP). At rest, arterialized venous blood was obtained for the estimation of Pa CO2, [H(+)] and [HCO(3)(-)]; and serum progesterone ([P(4)]) and 17beta-estradiol ([E(2)]) concentrations. Duffin's modified hyperoxic rebreathing procedure was used to evaluate changes in central ventilatory chemoreflex control characteristics at rest. Breath-by-breath ventilatory and gas exchange variables were measured at rest and during symptom-limited incremental cycle exercise tests. At rest in TM(3) compared with PP: Pa CO2, [H(+)], [HCO(3)(-)] and the central chemoreflex ventilatory recruitment threshold for Pa CO2 (VRT CO2) decreased, while ventilation (V E), [P(4)], [E(2)] and central chemoreflex sensitivity (V ES) increased (all p<or=0.001). The slope of the linear relation between V E and V CO2 during exercise was significantly higher in TM(3) vs. PP (31.2+/-0.6 vs. 27.5+/-0.5, p<0.001). The magnitude of this change in the VE-V CO2 slope correlated significantly with concurrent reductions in each of the VRT CO2 (R(2)=0.619, p<0.001), Pa CO2 (R(2)=0.203, p=0.024) and [HCO(3)(-)] (R(2)=0.189, p=0.030); and was independent (p>0.05) of changes in [P(4)], [E(2)] and V ES. In conclusion, the increased ventilatory response to exercise in pregnancy can be explained, in large part, by reductions in the respiratory control system's resting Pa CO2 equilibrium point as manifest primarily by reductions in the VRT CO2.

Influence of high altitude on cerebrovascular and ventilatory responsiveness to CO2

An altered acid-base balance following ascent to high altitude has been well established. Such changes in pH buffering could potentially account for the observed increase in ventilatory CO(2) sensitivity at high altitude. Likewise, if [H(+)] is the main determinant of cerebrovascular tone, then an alteration in pH buffering may also enhance the cerebral blood flow (CBF) responsiveness to CO(2) (termed cerebrovascular CO(2) reactivity). However, the effect altered acid-base balance associated with high altitude ascent on cerebrovascular and ventilatory responsiveness to CO(2) remains unclear. We measured ventilation , middle cerebral artery velocity (MCAv; index of CBF) and arterial blood gases at sea level and following ascent to 5050 m in 17 healthy participants during modified hyperoxic rebreathing. At 5050 m, resting , MCAv and pH were higher (P < 0.01), while bicarbonate concentration and partial pressures of arterial O(2) and CO(2) were lower (P < 0.01) compared to sea level. Ascent to 5050 m also increased the hypercapnic MCAv CO(2) reactivity (2.9 +/- 1.1 vs. 4.8 +/- 1.4% mmHg(1); P < 0.01) and CO(2) sensitivity (3.6 +/- 2.3 vs. 5.1 +/- 1.7 l min(1) mmHg(1); P < 0.01). Likewise, the hypocapnic MCAv CO(2) reactivity was increased at 5050 m (4.2 +/- 1.0 vs. 2.0 +/- 0.6% mmHg(1); P < 0.01). The hypercapnic MCAv CO(2) reactivity correlated with resting pH at high altitude (R(2) = 0.4; P < 0.01) while the central chemoreflex threshold correlated with bicarbonate concentration (R(2) = 0.7; P < 0.01). These findings indicate that (1) ascent to high altitude increases the ventilatory CO(2) sensitivity and elevates the cerebrovascular responsiveness to hypercapnia and hypocapnia, and (2) alterations in cerebrovascular CO(2) reactivity and central chemoreflex may be partly attributed to an acid-base balance associated with high altitude ascent. Collectively, our findings provide new insights into the influence of high altitude on cerebrovascular function and highlight the potential role of alterations in acid-base balance in the regulation in CBF and ventilatory control.