Rebreathing Research Articles

Prolonged sevoflurane, isoflurane and halothane anaesthesia in oxygen using rebreathing or non-rebreathing system in cats.

Effects of prolonged sevoflurane, isoflurane and halothane anaesthesia in oxygen on clinical, cardiopulmonary, haematologic, and serum biochemical findings were compared in healthy, premedicated cats breathing spontaneously during 6 h of anaesthesia using rebreathing (semi-closed circuit) or non-rebreathing (Bain coaxial circuit) system. Recovery from anaesthesia with sevoflurane was more rapid than that with halothane or isoflurane in both systems. Respiration and heart rates during sevoflurane anaesthesia were similar to those during isoflurane rather than halothane anaesthesia in both systems. The degree of respiratory acidosis during prolonged sevoflurane anaesthesia was similar to that during isoflurane anaesthesia, and was less than that during halothane anaesthesia in both rebreathing and non-rebreathing systems. Prolonged sevoflurane anaesthesia induced mean arterial pressure similar to isoflurane or halothane anaesthesia in the non-rebreathing system, but it depressed mean arterial pressure less than isoflurane or halothane anaesthesia in the rebreathing system. Time related increase in the arterial carbon dioxide partial pressure was observed during halothane anaesthesia especially in the rebreathing system, however, no significant time-related changes in cardiopulmonary variables were observed during either sevoflurane or isoflurane anaesthesia in both systems. There were no significant differences among sevoflurane, isoflurane and halothane anaesthesia in serum biochemical values in both systems.

Open heart surgery without blood transfusion for cyanotic congenital cardiac defects

Between November 1994 and January 1997, 42 cases of cyanotic congenital cardiac defects underwent definitive surgery at Matsudo Municipal Hospital. We evaluated 30 cases, each weighing from 7 to 20 kg. The procedures were performed at the age of 9 months to 6 years (mean age-2.4 years). The body weights were 7.7 to 20 kg (mean weight-11.4 kg). The preoperative diagnoses were Tetralogy of Fallot (TOF) in 19 cases, Fontan candidates in 6 and the others in 5. We classified them into 3 groups; Group A--15 cases were completed with non-blood transfusion, Group B--8 cases used only plasma protein fraction and Group C--7 cases used blood transfusion. Cardiopulmonary bypass (CPB) system is a semi-closed circuit and priming volume is 400 to 600 ml. There is no difference among the 3 groups in operative age, body weight, operation time, CPB time, aortic cross clamp time, bleeding and postoperative state. The same results were obtained in minimum base excess and urine output during CPB and the changes of hematocrit and total protein. In Groups A and B, CPB blood was returned to the patient as soon as possible after CPB was weaned, but in Group C, blood transfusion was performed without the return of CPB blood. In all groups, hemodynamics were stable. Retrospectively, it is thought that blood transfusion was not necessary in Group C and the use of the plasma protein fraction was not needed in Group B. In conclusion, the open heart surgery can be performed safely without blood transfusion for cyanotic congenital cardiac defects.

Use of maize as a food source for sea urchins in a recirculating rearing system

Six diets were tested on the somatic growth and mortality of the sea urchin Paracentrotus lividus reared in a semi-closed system. Three of the diets were algal-based (Palmaria palmata, Laminaria digitata and a mix of L. digitata (80%) plus P. palmata (20%)), the other three being based on maize (pure maize or a mix of maize (80%) plus P. palmata (20%) and maize (80%) plus L. digitata (20%)). Best growth rates were obtained with the mix maize plus P. palmata, the lowest conversion rates were obtained with the algal-based diets. Mortality rates were not significantly different between the diets. Sea urchins (3791) were reared in a semi-closed circuit and fed a mix of maize plus P. palmata. Growth, size distribution and mortality were studied to an age of 23 months, when their mean diameter was 21.4 mm (size distribution from 6 to 46 mm). The fastest growing urchins reached 40 mm in 15 months. Size frequency histograms were extrapolated to show that 25% of the urchin population reach a size >40 mm in 41 months. Forty-nine months would be necessary for 50% of the population to reach that size and 63 months for 75% of the population. © Rapid Science Ltd. 1998

Impedance Cardiography Accurately Measures Cardiac Output During Exercise in Children With Cystic Fibrosis

After validation of impedance cardiography (ICG) in healthy children, this same device was tested in children with cystic fibrosis (CF) to validate its capability of measuring cardiac output (Q) in this population. Comparative study of ICG vs the indirect Fick (CO2) method. Tertiary care children's teaching hospital. Twenty-one CF children with mean FEV1 of 77 +/- 21% predicted. ICG results were compared with CO2 rebreathing (RB) measurements of Q with sampling of capillary blood gases at two levels of exercise (0.5 and 1.5 W/kg). ICG measurements were made each minute, and duplicate RB measurements from 6 to 8 min at each workload. Q was regressed against oxygen uptake and results by each method were compared. Mean bias (QRB-QICG) was -0.09 +/- 0.94 L/min. The largest deviation of QICG from QRB was +33%, and 83% of corresponding QICG values were within +/-20% of QRB result. This device gives rapid, accurate, noninvasive Q measurements in children with CF.

The Favre System for Anorectal Manometry: Comparison with Other Manometry Systems in Vitro and in Healthy Volunteers

Background: The Favre system operates with airflow in a semi-closed circuit to transmit pressure. We aimed to evaluate the Favre system in comparison with other commonly used manometry systems. Methods: The Favre, Amdorfer, Arhan, and Synectics systems were evaluated in vitro and in 14 volunteers. Results: In vitro testing showed no relevant differences for latency, precision, or retest stability. In vivo, maximum resting pressure differed among all probes (P < 0.01) except for Arhan versus Arndorfer (NS). Maximum squeeze pressures differed among all probes (P < 0.01). Decrease of resting pressure during rectoanal inhibitory reflex was similar for Favre versus Arndorfer and Arhan versus Synectics (NS). Retest stability was higher with Favre than with Arndorfer (P < 0.05) and Synectics (P < 0.05) with regard to maximum resting pressure; it was higher with Favre than with Arhan (P < 0.01) and Arndorfer (P < 0.05) with regard to maximum squeeze pressure. Favre caused less discomfort than Synectics (P < 0.05) or Arndorfer (P < 0.05). Conclusion: The Favre probe is an excellent and cost-efficient system for routine anorectal manometry.

The Effects of Sevoflurane and Isoflurane Anesthesia on Renal Tubular Function in Patients with Moderately Impaired Renal Function

Increasing evidence indicates that sevoflurane anesthesia does not impair renal function in healthy patients despite higher concentrations of plasma inorganic fluoride. However, whether sevoflurane further affects renal tubular function in patients with impaired renal function is not known. We compared the effect of sevoflurane anesthesia with that of isoflurane anesthesia on renal tubular function in patients with moderately impaired renal function. Fourteen patients with creatinine clearance between 10 and 55 mL/min were anesthetized with either sevoflurane or isoflurane using a semiclosed circuit system. Plasma inorganic fluoride concentrations and urine N-acetyl-beta-D-glucosaminidase (NAG), gamma-glutamyltranspeptidase (gamma-GTP), and beta 2-microglobulin (beta 2MG) excretions were measured up to post-anesthetic day 14. Although both the peak plasma inorganic fluoride concentrations and the areas under the curve of plasma inorganic fluoride concentration versus time were significantly greater in the sevoflurane group than in the isoflurane group, urine NAG, gamma-GTP, and beta 2MG excretions per day did not differ between the two groups. These results indicate that sevoflurane and isoflurane may have similar effects on the renal tubules in patients with moderately impaired renal function.

The Effects of Sevoflurane and Isoflurane Anesthesia on Renal Tubular Function in Patients with Moderately Impaired Renal Function

Increasing evidence indicates that sevoflurane anesthesia does not impair renal function in healthy patients despite higher concentrations of plasma inorganic fluoride.However, whether sevoflurane further affects renal tubular function in patients with impaired renal function is not known. We compared the effect of sevoflurane anesthesia with that of isoflurane anesthesia on renal tubular function in patients with moderately impaired renal function. Fourteen patients with creatinine clearance between 10 and 55 mL/min were anesthetized with either sevoflurane or isoflurane using a semiclosed circuit system. Plasma inorganic fluoride concentrations and urine N-acetyl-beta-D-glucosaminidase (NAG), gamma-glutamyltranspeptidase (gamma-GTP), and beta2-microglobulin (beta2 MG) excretions were measured up to postanesthetic day 14. Although both the peak plasma inorganic fluoride concentrations and the areas under the curve of plasma inorganic fluoride concentration versus time were significantly greater in the sevoflurane group than in the isoflurane group, urine NAG, gamma-GTP, and beta2 MG excretions per day did not differ between the two groups. These results indicate that sevoflurane and isoflurane may have similar effects on the renal tubules in patients with moderately impaired renal function. (Anesth Analg 1996;82:909-13)

Arousal responses of the newborn piglet to airways obstruction and rebreathing during normothermia and hyperthermia.

Most experimental studies of arousal to respiratory stressors have been performed under neutral thermal conditions. The present study focussed on arousal responses with the added challenge of heat stress. The subjects were two groups of 10 sedated piglets aged 4-6 days and 2-6 days respectively. Respiratory stimuli (partial and total airways obstruction (AO) or rebreathing (RB) expired gases) were applied during non-rapid-eye-movement (NREM) sleep. Measurements of heart rate, SaO2, intra-pleural pressure or inspired CO2 and O2 were recorded during tests in normothermia and hyperthermia. Hyperthermia significantly shortened the time to arousal. Thus, arousal times from partial and total AO and from RB were: in normothermia, 40.3 +/- 5.8, 9.2 +/- 0.7 and 94.9 +/- 14.7 s respectively; in hyperthermia, 17.8 +/- 3.7, 7.6 +/- 0.9 and 68.5 +/- 9.1 s respectively. Cardio-respiratory variables at arousal were similar in both thermal states. Seven non-sedated piglets were challenged with RB stimuli during normothermia only to determine the influence of sleep state on arousal. Arousal in REM sleep was delayed approximately 2-3-fold (NREM, 41.2 +/- 8.2 s; REM, 88.5 +/- 18.1 s), occurring at a lower SaO2 and higher inspired CO2. It is concluded that arousal from NREM sleep under adverse conditions of hyperthermia shortens the arousal time from asphyxial stimuli induced by AO and RB with no change in the arousal threshold.

Free Communications (Section D)

In TCD studies, the pulsatility index (PI), which is based on flow waveform analysis, are widely used for assessment of cerebral vascular resistance (CVR). However, PI is not very accurate, because it has many sources of variability and error. We propose a ratio between mean arterial pressure (MAP) and mean velocity (Vm) for an index of CVR. If we assume that the lumen of basal cerebral vessels is constant than MAPNm should be directly proportional to CVR. The aim of our study was to evaluate validity of MAPNm. For this purpose different vasodilatatory stimuli, which affected cerebral resistance and conductive vessels, were used. Hyperventilation (HV) and rebreathing (RE) change blood pCO2, which has predominantly an influence on resistance vessels. Clonidine, alfa‐adrenegic blocker, produce vasodilation on perifery. L‐arginine (L‐arg), precursor of nitro oxide, supposed to be able to induce vasodilatation of cerebral vessels. Nitroglicerinc (NTG) acts primarily on conductive vessels. Ten heathy subjects, both gender, aged from 21 to 57 years (mean 33.4+−11.9 years) participated in our study. HV and RE were performed using capnograph (Oxicap, Ohmeda) We registered end‐tidel (ET) CO2. Clonidin was administrated intramusculary at dose? L‐arg was infused at rate 5mg/kg/min for 30 minutes. NTG was applied sublingualy at dose 0.6 mg. The response of cerebral circulation was continously monitored by Multi‐dop X (OWL, Germany), usmg bitemporal probes for recording of Vm in middle cerebral artenes. Simultaneously the MAP and heart rate (HR) were sampled by pletysmographic device (Finapress. Ohmeda, USA).During RE MAP increased (p=.035), HR did not change, PI (p=.0006) and MAP/Vm (p=.0002) decreased. Dunng HV MAP decreased (p=014), HR increased (p=.048), PI (p=.0001) and MAPNm (p&lt;0001) increased. We found good correlation between changes of PI and MAPNm (r=73, p&lt;01) as well as between changes of MAP/Vm and ET CO2 (r=78, p&lt;01) and moderate correlation between changes of PI and ET CO2 (r=53, p=016) After Clonidine administration MAP decreased (p&lt;0001). HR did not change significantly,PI (p=.04) as well as MAP/Vm (p=.017) decreased. There was no correlation between PI and MAV/Vm. After L=arg infusion MAP and HR did not change, while PI (p&lt;.001) and MAP/Vm (p&lt;.001) decreased. No correlation between PI and MAP/Vm was found. After NTG MAP decreased (p=0.019), HR did not change as well as PI, while MAP/Vm (p=0084) mcrcased There was no correlation between PI and MAP/Vm

Sevoflurane degradation product concentrations with soda lime during prolonged anesthesia

Study Objectives: To evaluate the decomposition of sevoflurane in soda lime during prolonged sevof lurane anesthesia in humans. To evaluate for evidence of renal or hepatotoxicity as a result of exposure to these sevoflurane degradation compounds. Design: Prospective evaluation in healthy volunteers. Setting: Clinical research unit and postanesthesia care unit of a university hospital. Patients: Six healthy male volunteers. Interventions: Subjects were anesthetized with sevoflurane 1 to 1.2 minimum alveolar concentration for greater than 9 hours with a semiclosed circuit anesthetic technique (5-liter total flow) with fresh soda lime as the absorbent. Measurements and Main Results: Laboratory tests of renal and hepatic function were performed before anesthesia and 1 and 5 days after anesthesia. During sevoflurane anesthesia, inhalation and exhalation circuit limb gas samples were obtained for degradation compound analysis. Only one degradation product, fluoromethyl-2,2-difluoro-l-(trifluoromethyl) vinyl ether (compound A), was detected. Inhalation concentration was maximal (7.6 ± 1.0 ppm) at 2 hours and did not increase further after this time point. There were no differences in preanesthesia and postanesthesia tests of hepatic and renal function. Conclusions:Levels of the degradation compound (compound A) produced in semiclosed circuit sevoflurane anesthesia with soda lime are well below potential toxic levels and thus appear safe. When sevoflurane is administered under these conditions for prolonged anesthesia, concentrations of compound A do not continue to increase throughout anesthesia.

Precision and accuracy of a noninvasive inert gas washin method for determination of cardiac output in men

Stout et al. (J. Appl. Physiol. 38:913-918, 1975) suggested an open-circuit multibreath (MB) inert gas method for determining pulmonary capillary blood flow (Qc) in anesthetized dogs receiving artificial ventilation. In the present work we investigated the accuracy and reproducibility of the MB method in nine healthy human subjects at spontaneous ventilation, and we compared the MB method with the inert gas rebreathing (RB) method. Qc was calculated at rest and during exercise at 50 or 100 W, and experimental errors were evaluated in computer simulations of a two-alveoli lung model. The calculated mean Qc values of the MB method were 5.56 +/- 1.23 (SD), 10.02 +/- 0.78, and 13.2 +/- 0.84 l/min, and the mean difference (MB Qc - RB Qc) was not significantly different (P > 0.05). The variation in Qc of the MB method was found to be significantly larger than that in Qc of the RB method (P < 0.01). Random measurement errors and uneven distribution of ventilation contributed to the experimental errors. We conclude that the MB method is inferior to the RB method but that the MB method may be useful under exercise conditions.

Effect of caffeine on the ventilatory response to inhaled carbon dioxide

We investigated the effect of caffeine on the hypercapnic ventilatory response (HCVR) using steady state (SS) and rebreathing (RB) methods in 6 subjects. They received caffeine (5 mg/kg) or saline intravenously in a randomized, double-blind, crossover manner, with measurement of serum caffeine levels. P et CO 2 and (V̇ e), normalized for vital capacity (VC), were measured continually during RB and during the last 5 min of SS runs. The slope of the V̇ e-P et CO 2 response increased from 0.21 ± 0.14 to 0.38 ± 0.14 and from 0.23 ± 0.12 to 0.59 ± 0.45 VC·min −1·mmHg −1, measured by RB and SS respectively ( P<0.05). Plotting V t vs P et CO 2 revealed a parallel shift (additive effect) in the response measured by RB after caffeine; but an increased slope (multiplicative effect) in the V t-P et CO 2 relation measured in SS. We conclude that caffeine acts as a respiratory stimulant and increases the HCVR, but that assessment of the caffeine-CO 2 interaction is dependent on the methodology employed.

Rebreathing TL&lt;sub&gt;CO&lt;/sub&gt; versus Single Breath TL&lt;sub&gt;CO&lt;/sub&gt; at Different Degrees of Unequal Ventilation

The influence of unequal ventilation on the differences between the rebreathing (RB) and the single breath (SB) measurement of the transfer factor of the lung for carbon monoxide (TLCO) was investigated. An apparatus was developed which measured both TLCORB and TLCOSB. Unequal ventilation increases with lung volume, caused by the combined effect of increasing ratio of total volume to RV from apex to basis, the fact that apex inspires before basis and asymmetric intra-acinar branching patterns. Therefore measurements were done at three different values of inspired volume (VI) in 10 normal subjects. To separate the effects of gas mixing and diffusion, residual volume (RV) was measured by He dilution in addition to TLCO. We found that RV and TLCO increase with increasing VI. The increase is larger with RB. We also found that RVRB is larger than RVSB at the three values of VI. TLCORB is smaller than TLCOSB except at vital capacity (VC). We concluded from the different behavior of RVRB and RVSB as a function of VI that the SB measurement increasingly underestimated RV with increasing unequal ventilation. This is also reflected in the measurement of TLCOSB. From the different behavior of TLCORB and TLCOSB as a function of VI we concluded that TLCORB was smaller than TLCOSB when unequal ventilation was minimal. This is caused by the smaller mean alveolar volume during RB. But increasing unequal ventilation causes an increasing underestimation with the SB method, resulting in TLCOSB being equal to TLCORB at VC.

Effets de la nifédipine en prémédication sur l'hypothermie peropératoire

The intraoperative timecourse of core temperature in patients premedicated with nifedipine (n = 30) was compared to that of control patients (n = 30). Distal oesophageal temperature (T CORE) was recorded every five minutes during total hip replacement in 60 adults ranked ASA 1 to 2. Patients in the control group were only premedicated with 100 mg of oral hydroxyzine. The treatment group consisted of 30 patients taking nifedipine for blood pressure control or coronary insufficiency. They were given 10 mg sublingual nifedipine as well as the hydroxyzine premedication. Anaesthesia was induced with thiopentone, fentanyl and vecuronium, and maintained with nitrous oxide in oxygen and halothane in a semiclosed circuit. The slopes of the timecourse for T CORE were established for each patient, using two linear regressions, between 0 and 0.5 h and from 1 to 2 h. The two groups did not differ in age, weight, ambient temperature, blood pressure, heart rate, and volume of unwarmed blood transfused. T CORE differed significantly from the 25th minute on until the end of the study period. Contrary to all expectation the T CORE at 2 h was higher in the nifedipine group (34.85 ± 0.09 °C) than in the control group (34.01 ± 0.14 °C, p < 0.001). T CORE decreased more rapidly in the control group during the first study interval (0 to 0.5 h), − 1.50 ± 0.60 °C · h −1 vs − 2.34 ± 1.02 °C · h −1 (p < 0.001). The second slopes did not differ particulary (− 0.96 ± 1.32 °C · h −1 vs − 0.90 ± 0.42 °C · h − respectively). The most important cause of hypothermia during the first half-hour of anaesthesia was probably an internal redistribution of heat. Nifedipine apparently minimised this as patients treated with nifedipine were already vasodilated and had a low central to peripheral temperature gradient. The rate of cooling in the second study period (1 to 2 h) was similar in both groups as halothane produced a similar degree of vasodilation in both groups.

Characteristics of Linear Actuator with Semi-Closed Magnetic Circuit Excited by Discharge Current of Capacitor.

The simple structured linear actuator considered in this paper is based on the model, which was called the single-phase oscillating motor, proposed by Jayawant. We have already reported about the static and the kinetic characteristics of the actuator of our model about two cases considered and not considered the magnetic saturation when it was driven by an AC source. Although the theoretical procedue to calculate the characteristics is very simple, the calculated results, in both cases, have shown close agreement with the experimental ones. To improve the actuator for the practical use, some methods will be considered and one of them is to use a DC source instead of an AC source. Using the charged capacitor as a DC supply, a instantenious large current flows easily in the exciting coil in comparison with the case of an AC supply.In this paper, qualitative explanation of the ring motion and calculated results of the ring speed which have good agreement with experimental results are reported.

REBREATHING DUE TO INCOMPETENT UNIDIRECTIONAL VALVES IN THE CIRCLE ABSORBER SYSTEM

REBREATHING DUE TO INCOMPETENT UNIDIRECTIONAL VALVES IN THE CIRCLE ABSORBER SYSTEM

Flows and budgets of nutrient salts and organic carbon in relation to a red tide in the Ria of Vigo (NW Spain)

In late September 1986, a major red tide occurred in the Ria of Vigo Hydrographic conditions led to water retention, with circulation in a semi-closed circuit In this study nutrient salts and carbon fluxes were calculated, and their consumption quantified, by use of a box model for the ria. The dinoflagellate Gymnodinium catenaturn, scarcely present on 4 September, increased in concentration to lo4 cell d m 3 by 21 September. This was favoured by chemical causes, such as the high availability of ammonium (6.2 mol s 1 were delivered to the upper water layer) and organic carbon, and by physical causes, such as the rapid circulation (862 X lo3 kg s l ) with reducing mixing, almost half that of the preceding date. The nutrient salts came mostly from the remineralisation of organic matter The amounts contributed by freshwater, 5 % of the total, did not have an appreciable influence on the formation of the red tide. Vertical migration of the dinoflagellate played an important role It is estimated that this organism was responsible for 22 % of the synthesis of nitrogenous organic matter The red tide came to an end with the November rains, which created rapid circulation which washed out the ria.

レジャー用半閉式潜水呼吸器ｅＯＢＡの評価

Semi-closed underwater breathing apparatus has commonly been used among military and commercial divers, but never available for recreational divers because of complicated operations, difficulties of maintenance, and expensiveness. Nippon Sanso K.K. has newly deployed a semi-closed circuit underwater breathing apparatus called the "eOBA". It is especially designed for divers to enjoy shallow (max. 5 m) and short (10 min.) dives. This eOBA was evaluated from manned testing based on physiological requirements for the apparatus. Four male and four female subjects participated in the three test trials. Subjects maintained their position at the depths of 2 m and 4 m for 10 min. and e exercised on the underwater ergometer at the depth of 2.4 m. The pressure at the mouthpiece, tidal volume, breathing rate, the inspired O2 level, CO2 level were monitored on breath-by-breath basis. The inspired O2 level ranged from 45% (at exercise) to 60% (at rest) and the inspired CO2 level were kept less than 1.0% in the most cases. The P-V loop indicated relatively little external work of breathing (less than 0.1 kg.m/l) and allowable peak pressures (less than 25cmH2O). Results show that the eOBA meets the standards sufficiently under the test conditions and is a safe apparatus for recreational divers if it is properly used.

The effect of sleep deprivation on activity of the genioglossus muscle.

Sleep deprivation appears to increase the severity of obstructive sleep apnea, and inadequate activation of the genioglossus muscle may play an important role in the pathogenesis of obstructive sleep apnea. Therefore, we investigated the effect of sleep deprivation on genioglossal electromyographic (EMG) activity. Eleven men were studied during room air breathing and CO2 rebreathing before sleep deprivation (control), after 1 night of sleep deprivation, and the day after sleep recovery. We measured inspired minute ventilation, tidal volume, respiratory frequency, and peak integrated inspiratory genioglossal EMG activity. After sleep deprivation, no significant changes in inspiratory minute ventilation or tidal volume occurred during room air breathing or CO2 rebreathing, but the breathing frequency during CO2 rebreathing increased significantly after sleep deprivation. Genioglossal EMG activity was diminished during CO2 rebreathing after sleep deprivation, but this was significant only in subjects 30 yr of age and older. The fall in EMG activity was independent of changes in tidal volume. All variables returned towards control levels after sleep recovery. We conclude that sleep deprivation selectively decreases genioglossal EMG activity during CO2 rebreathing in awake older subjects. This influence of sleep deprivation may play a role in the pathogenesis or severity of obstructive sleep apnea.

Abnormal cerebrovascular responses to CO2 in sleep apnea patients.

The responses of common carotid blood flow (CCF), pressure (BP), and resistance (R) to variations in respiratory gases were compared during waking periods in 10 sleep apnea patients (SA) and 10 healthy controls (N) of similar age. Respiratory gases were altered by 3-min CO2 rebreathing (RB), 3-min hyperventilation (HV), and 4-min hypoxia (HYP) procedures. CCF was measured continuously by a 5-MHz pulsed Doppler duplex scanner and R was calculated using brachial BP. During RB, which increased end-tidal PCO2 (PACO2) by 15 mm Hg, SA had a lower CCF and greater BP response and therefore a significantly different (positive) change in R compared with N. The ventilatory responses to CO2 were not significantly different. With HV the PACO2 fell by 13 mm Hg in both groups and CCF fell more markedly in SA than N with the same change in BP; therefore, R was increased significantly more in SA. The HYP results did not demonstrate a difference between groups. These results suggest that abnormal cerebrovascular responses to PACO2, initiated either by unusual vasoactive properties of cerebral resistance vessels or peculiar venous outlow patterns, may initiate or potentiate periodic breathing in SA by prolonging lung-to-brain circulation time.