Post-hyperventilation apnoea in conscious humans

Inspiratory muscle fatigue at the swimming tumble turns: its occurrence and effects on kinematic parameters of the turns

Introduction: The present study had two objectives: 1) to investigate the effects of tumble turns on the development of inspiratory muscle fatigue (IMF) and compare this to whole swimming, and 2) to evaluate the effects of pre-induced IMF on the kinematic parameters of tumble turns. Fourteen young club-level swimmers (13 ± 2 years of ages) completed three swim trials. Methods: The first trial was used to determine the 400-m front crawl swim time at maximal effort (400FC). The other two trials consisted of a series of 15 tumble turns at the 400FC pace. In one of the turn-only trials, IMF was pre-induced (TURNS-IMF), whereas in the other turn-only trial it was not (TURNS-C). Results: Compared with baseline values, the values for maximal inspiratory mouth pressure (PImax) at the end of the swim were significantly lower at all trials. However, the magnitude of inspiratory muscle fatigue was less after TURNS-C (PImax decreased by 12%) than after 400FC (PImax decreased by 28%). The tumble turns were slower during 400FC than during TURNS-C and TURNS-IMF. In addition, compared to TURNS-C, turns in the TURNS-IMF were performed with higher rotation times and shorter apnea and swim-out times. Discussion: The results of the present study suggest that tumble turns put a strain on the inspiratory muscles and directly contribute to the IMF observed during 400FC swimming. Furthermore, pre-induced IMF resulted in significantly shorter apneas and slower rotations during tumble turns. IMF therefore has the potential to negatively affect overall swimming performance, and strategies should be sought to reduce its effects.

Are thyrotropin-releasing hormone (TRH) and analog taltirelin viable reversal agents of opioid-induced respiratory depression?

Opioid-induced respiratory depression (OIRD) is a potentially life-threatening complication of opioid consumption. Apart from naloxone, an opioid antagonist that has various disadvantages, a possible reversal strategy is treatment of OIRD with the hypothalamic hormone and neuromodulator thyrotropin-releasing hormone (TRH). In this review, we performed a search in electronic databases and retrieved 52 papers on the effect of TRH and TRH-analogs on respiration and their efficacy in the reversal of OIRD in awake and anesthetized mammals, including humans. Animal studies show that TRH and its analog taltirelin stimulate breathing via an effect at the preBötzinger complex, an important respiratory rhythm generator within the brainstem respiratory network. An additional respiratory excitatory effect may be related to TRH's analeptic effect. In awake and anesthetized rodents, TRH and taltirelin improved morphine- and sufentanil-induced respiratory depression, by causing rapid shallow breathing. This pattern of breathing increases the work of breathing, dead space ventilation, atelectasis, and hypoxia. In awake and anesthetized humans, a continuous infusion of intravenous TRH with doses up to 8 mg, did not reverse sufentanil- or remifentanil-induced respiratory depression. This is related to poor penetration of TRH into the brain compartment but also other causes are discussed. No human data on taltirelin are available. In conclusion, data from animals and human indicate that TRH is not a viable reversal agent of OIRD in awake or anesthetized humans. Further human studies on the efficacy and safety of TRH's more potent and longer lasting analog taltirelin are needed as this agent seems to be a more promising reversal drug.

Incidence and Risk Factors for Hypoxia in Deep Sedation of Propofol for Artificial Abortion Patients

Background

Respiratory depression is a life-threatening adverse effect of deep sedation. This study aimed to investigate the factors related to hypoxia caused by propofol during intravenous anesthesia.

Methods

Three hundred and eight patients who underwent painless artificial abortion in the outpatient department of Shanghai Tenth People’s Hospital between November 1, 2019 and June 30, 2020 were divided into two groups according to whether the patients experienced hypoxia (SpO₂ < 95%). Preoperative anxiety assessments, anesthesia process, and operation-related information of the two groups were analyzed. The univariate analysis results were further incorporated into logistic regression analysis for multivariate analysis to determine the independent risk factors affecting hypoxia.

Results

Univariate analysis revealed that body mass index (BMI) (21.80 ± 2.94 vs. 21.01 ± 2.39; P = 0.038, 95% confidence interval (CI) = [−1.54, −0.04]), propofol dose (15.83 ± 3.21 vs. 14.39 ± 3.01; P = 0.002, CI = [−2.34, −0.53]), menopausal days (49.64 ± 6.03 vs. 52.14 ± 5.73; P = 0.004, CI = [0.79, 4.21]), State Anxiety Inventory score (51.19 ± 7.55 vs. 44.49 ± 8.96; P < 0.001, CI = [−9.26, −4.15]), and Self-rating Anxiety Scale score (45.86 ± 9.48 vs. 42.45 ± 9.88; P = 0.021, CI = [−6.30, −0.53]) were statistically significant risk factors for hypoxia during the operation. Logistic regression analysis showed that propofol dosage, menopausal days, and State Anxiety Inventory score were independent risk factors for hypoxia.

Conclusion

Patient anxiety affects the incidence of hypoxia when undergoing deep intravenous anesthesia with propofol. We can further speculate that alleviating patient anxiety can reduce the incidence of hypoxia.

Clinical Trial Registration

[http://www.chictr.org.cn], identifier [ChiCTR2000032167].

Amygdala-driven apnea and the chemoreceptive origin of anxiety

Although the amygdala plays an important part in the pathogenesis of anxiety and generation of exteroceptive fear, recent discoveries have challenged the directionality of this brain-behavior relationship with respect to interoceptive fear. Here we highlight several paradoxical findings including: (1) amygdala lesion patients who experience excessive fear and panic following inhalation of carbon dioxide (CO₂), (2) clinically anxious patients who have significantly smaller (rather than larger) amygdalae and a pronounced hypersensitivity toward CO₂, and (3) epilepsy patients who exhibit apnea immediately following stimulation of their amygdala yet have no awareness that their breathing has stopped. The above findings elucidate an entirely novel role for the amygdala in the induction of apnea and inhibition of CO₂-induced fear. Such a role is plausible given the strong inhibitory connections linking the central nucleus of the amygdala with respiratory and chemoreceptive centers in the brainstem. Based on this anatomical arrangement, we propose a model of Apnea-induced Anxiety (AiA) which predicts that recurring episodes of apnea are being unconsciously elicited by amygdala activation, resulting in transient spikes in CO₂ that provoke fear and anxiety, and lead to characteristic patterns of escape and avoidance behavior in patients spanning the spectrum of anxiety. If this new conception of AiA proves to be true, and activation of the amygdala can repeatedly trigger states of apnea outside of one's awareness, then it remains possible that the chronicity of anxiety disorders is being interoceptively driven by a chemoreceptive system struggling to maintain homeostasis in the midst of these breathless states.

Prolonged post-hyperventilation apnea in two young adults with hyperventilation syndrome

BACKGROUND

The prognosis of hyperventilation syndrome (HVS) is generally good. However, it is important to proceed with care when treating HVS because cases of death following hyperventilation have been reported. This paper was done to demonstrate the clinical risk of post-hyperventilation apnea (PHA) in patients with HVS.

CASE PRESENTATION

We treated two patients with HVS who suffered from PHA. The first, a 21-year-old woman, had a maximum duration of PHA of about 3.5 minutes and an oxygen saturation (SpO2) level of 60%. The second patient, a 22-year-old woman, had a maximum duration of PHA of about 3 minutes and an SpO2 level of 66%. Both patients had loss of consciousness and cyanosis. Because there is no widely accepted regimen for treating patients with prolonged PHA related to HVS, we administered artificial ventilation to both patients using a bag mask and both recovered without any after effects.

CONCLUSION

These cases show that some patients with HVS develop prolonged PHA or severe hypoxia, which has been shown to lead to death in some cases. Proper treatment must be given to patients with HVS who develop PHA to protect against this possibility. If prolonged PHA or severe hypoxemia arises, respiratory assistance using a bag mask must be done immediately.

Hypoxic ventilatory response in Tac1-/- neonatal mice following exposure to opioids

Morphine is the dominating analgetic drug used in neonates, but opioid-induced respiratory depression limits its therapeutic use. In this study, we examined acute morphine effects on respiration during intermittent hypoxia in newborn Tac1 gene knockout mice (Tac1-/-) lacking substance P and neurokinin A. In vivo, plethysmography revealed a blunted hypoxic ventilatory response (HVR) in Tac1-/- mice. Morphine (10 mg/kg) depressed the HVR in wild-type animals through an effect on respiratory frequency, whereas it increased tidal volumes in Tac1-/- during hypoxia, resulting in increased minute ventilation. Apneas were reduced during the first hypoxic episode in both morphine-exposed groups, but were restored subsequently in Tac1-/- mice. Morphine did not affect ventilation or apnea prevalence during baseline conditions. In vitro, morphine (50 nM) had no impact on anoxic response of brain stem preparations of either strain. In contrast, it suppressed the inspiratory rhythm during normoxia and potentiated development of posthypoxic neuronal arrest, especially in Tac1-/-. Thus this phenotype has a higher sensitivity to the depressive effects of morphine on inspiratory rhythm generation, but morphine does not modify the reactivity to oxygen deprivation. In conclusion, although Tac1-/- mice are similar to wild-type animals during normoxia, they differed by displaying a reversed pattern with an improved HVR during intermittent hypoxia both in vivo and in vitro. These data suggest that opioids and the substance P-ergic system interact in the HVR, and that reducing the activity in the tachykinin system may alter the respiratory effects of opioid treatment in newborns.

The psychophysiology of blood-injection-injury phobia: looking beyond the diphasic response paradigm

Blood-injection-injury (BII) phobia is an anxiety disorder that may be accompanied by vasovagal fainting during confrontation with the feared stimuli. The underlying pattern of autonomic regulation has been characterized as a diphasic response, with initial increases in heart rate and blood pressure that are typical of a fight-flight response, and subsequent drops in blood pressure and/or heart rate that may precipitate vasovagal fainting. Tensing skeletal muscles of the arms, legs, and trunk (applied tension) has been proposed as a technique to cope with this dysregulation. This review critically examines the empirical basis for the diphasic response and its treatment by applied tension in BII phobia. An alternative perspective on the psychophysiology of BII phobia and vasovagal fainting is offered by focusing on hypocapnia that leads to cerebral blood flow reductions, a perspective supported by research on neurocardiogenic and orthostatically-induced syncope. The evidence may indicate a role for respiration-focused coping techniques in BII phobia.

Mechanisms of periodic breathing during exercise in patients with chronic heart failure

BACKGROUND

Periodic breathing (PB) in heart failure (HF) is attributed to many factors, including low cardiac output delaying the time it takes pulmonary venous blood to reach the central and peripheral chemoreceptors, low lung volume, lung congestion, augmented chemoreceptor sensitivity, and the narrow difference between eupneic carbon dioxide tension and apneic/hypoventilatory threshold.

METHODS AND RESULTS

We measured expired gases, ventilation, amplitude, and duration of PB in 23 patients with PB during progressive exercise tests done with 0 mL, 250 mL, or 500 mL of added dead space. Periodicity of PB remained constant despite heart rate, oxygen consumption, and minute ventilation increasing. Within each PB cycle, starting from the beginning of exercise, the largest (peak) tidal volume approached maximum observed tidal volume, while the smallest (nadir) tidal volume increased as exercise power output increased. PB ceased when nadir tidal volume reached peak tidal volume. End-tidal carbon dioxide increased with added dead space, and PB ceased progressively earlier during the exercise done with increased dead space.

CONCLUSION

Circulatory delay does not contribute to the PB observed in exercising HF patients. The pattern of gradually increasing nadir tidal volume during exercise and the effect of dead space on both PB ceasing and end-tidal carbon dioxide suggest that low tidal volume and carbon dioxide apnea threshold are important contributors to PB that occurs during exercise in HF.

Mice with Blunted Hypoxic Ventilatory Response are Susceptible to Respiratory Disturbance during Hypoxia

Hypoxia causes a life-threatening situation, and the ventilatory response to hypoxia plays an important role in preventing death. We have hypothesized that persons with a blunted hypoxic ventilatory response may have a weak defense response to hypoxic episodes and be susceptible to fatal respiratory disturbances. However, precise correlations between the hypoxic ventilatory response and respiratory disturbances are not well understood. In the present study we examined the hypoxic and hypercapnic ventilatory responses in nine inbred mouse strains (A/J, AKR/N, BALB/c, C3H/He, C57BL/6, DBA/2, NZW, SWR/J, and 129Sv). Breathing frequency, tidal volume and minute ventilation of unanesthetized and unrestrained mice were assessed by whole body plethysmography. Age-matched mice were exposed for 3 min to 10% O(2) in N(2) gas or 10% CO(2) in hyperoxic gas to determine the acute ventilatory response to chemical stimuli. Basal respiratory variables and hypoxic ventilatory responses differed among the strains, but the hypercapnic ventilatory response did not differ. The hypoxic ventilatory response was the highest in AKR/N mice and the lowest in SWR/J mice. These findings suggest that genetic factors may have influenced the hypoxic ventilatory response but not the hypercapnic ventilatory response. To examine the effects of severe hypoxic stress on the respiratory cycle, we exposed the strain with the highest or lowest hypoxic ventilatory response to 6% O(2) in N(2) until the onset of apnea. The "appearance time of apnea", which is defined as the time from the hypoxic loading to the onset of apnea, was shorter in the SWR/J strain than in the AKR/N strain. We suggest that a lower hypoxic ventilatory response may be a risk factor for apnea under hypoxia.

Cardio-respiratory measures following isocapnic voluntary hyperventilation

In some individuals, breathing is greater than at rest following voluntary hyperventilation. Most previous investigations have employed short hyperventilation periods; here we examine the time course of cardio-respiratory measures before, during, and after a 5-min voluntary hyperventilation, maintaining isocapnia throughout. We examined the possible co-involvement of the cardiovascular system; hypothesising that post-hyperventilation hyperpnoea results from an increase in autonomic arousal. In four subjects (two males, two females) of 18 (nine males, nine females) we observed a post-hyperventilation hyperpnoea, characterised by a slow decline of ventilation toward resting levels with a time constant of 109.0 +/- 16.1s. By contrast, heart rate, and systolic and diastolic blood pressure were unchanged from rest during and after voluntary hyperventilation for all subjects. We concluded that males and females were equally likely to exhibit post-hyperventilation hyperpnoea, and suggest that they may be characterised by an increased resting heart rate and the choice of breathing frequency to increase ventilation during the voluntary hyperventilation. We further concluded that post-hyperventilation hyperpnoea is rare, but when present is a strong and lasting phenomenon, and that it is not the result of an increased autonomic arousal.

Contribution of the respiratory rhythm to sinus arrhythmia in normal unanesthetized subjects during positive-pressure mechanical hyperventilation

The precise contribution of the CO2-dependent respiratory rhythm to sinus arrhythmia in eupnea is unclear. The respiratory rhythm and sinus arrhythmia were measured in 12 normal, unanesthetized subjects in normocapnia and hypocapnia during mechanical hyperventilation with positive pressure. In normocapnia (41 +/- 1 mmHg), the respiratory rhythm was always detectable from airway pressure and inspiratory electromyogram activity. The amplitude of sinus arrhythmia (138 +/- 21 ms) during mechanical hyperventilation with positive pressure was not significantly different from that in eupnea. During the same mechanical hyperventilation pattern but in hypocapnia (24 +/- 1 mmHg), the respiratory rhythm was undetectable and the amplitude of sinus arrhythmia was significantly reduced (to 40 +/- 5 ms). These results show a greater contribution to sinus arrhythmia from the respiratory rhythm during hypocapnia caused by mechanical hyperventilation than previously indicated in normal subjects during hypocapnia caused by voluntary hyperventilation. We discuss whether the respiratory rhythm provides the principal contribution to sinus arrhythmia in eupnea.

Chemoreflex control of ventilation is altered during wakefulness in humans with OSA

We hypothesized that patients with obstructive sleep apnea (OSA) have a different awake ventilatory response to carbon dioxide above and below eupnea compared with normal. Eight male subjects with OSA and control subjects matched for gender, race, age, height and weight voluntarily hyperventilated during wakefulness to reduce the partial pressure of carbon dioxide (PET(CO2)) below 25 mmHg. Subjects were then switched into a rebreathing bag containing a normocapnic (42 mmHg) hypoxic [partial pressure of end tidal oxygen (PET(O2))=50 mmHg (H50)] or hyperoxic [PET(O2)=140 mmHg (H140)] gas mixture. During the trial PET(CO2) increased while PET(O2) was maintained at a constant level. The point at which ventilation and PET(CO2) increased linearly was considered to be the carbon dioxide ventilatory recruitment threshold (VRT(CO2)). Measurements of ventilation and its components (i.e. tidal volume and breathing frequency) were made below this threshold and the slope of the minute ventilation; tidal volume or breathing frequency response above the threshold was determined. Four trials for a given oxygen level were completed. The PET(CO2) that demarcated the VRT(CO2) was increased (H(50)=43.43+/-0.92 vs. 41.05+/-0.67; H(140)=47.65+/-0.80 vs. 45.28+/-0.75), as were measures of ventilation below the threshold (H(50)=18.50+/-2.11 vs. 13.44+/-1.43; H(140)=19.66+/-2.71 vs. 10.83+/-1.24) in the OSA subjects compared with control. In contrast the OSA and control subjects did not respond differently to changes in PET(CO2) above the threshold. We conclude that the PET(CO2) that delineates the VRT(CO2) and ventilation below this threshold is elevated in subjects with OSA.

CO2-dependent components of sinus arrhythmia from the start of breath holding in humans

A substantial portion of sinus arrhythmia in conscious humans appears to be caused by the CO2-dependent central respiratory rhythm. Under some circumstances, therefore, sinus arrhythmia might indicate the presence of the central respiratory rhythm. Humans can voluntarily modify their central respiratory rhythm (e.g., by pacing breathing or by delaying or advancing breaths), but it is not clear what happens to it from the start of breath holding. In this study, we show that sinus arrhythmia persists from the start of breath holds prolonged by preoxygenation. We also show that some of the frequency components of sinus arrhythmia start within each subject's eupneic frequency range and change when end-tidal Pco2 is lowered or raised, as we would expect if the central respiratory rhythm continues from the start of breath holding. We discuss whether sinus arrhythmia can indicate if the central respiratory rhythm continues from the start of breath holding.

Effect of ventilator mode on sleep quality in critically ill patients

To determine whether sleep quality is influenced by the mode of mechanical ventilation, we performed polysomnography on 11 critically ill patients. Because pressure support predisposes to central apneas in healthy subjects, we examined whether the presence of a backup rate on assist-control ventilation would decrease apnea-related arousals and improve sleep quality. Sleep fragmentation, measured as the number of arousals and awakenings, was greater during pressure support than during assist-control ventilation: 79 +/- 7 versus 54 +/- 7 events per hour (p = 0.02). Central apneas occurred during pressure support in six patients; heart failure was more common in these six patients than in the five patients without apneas: 83 versus 20% (p = 0.04). Among patients with central apneas, adding dead space decreased sleep fragmentation: 44 +/- 6 versus 83 +/- 12 arousals and awakenings per hour (p = 0.02). Changes in sleep-wakefulness state caused greater changes in breath components and end-tidal CO2 during pressure support than during assist-control ventilation. In conclusion, inspiratory assistance from pressure support causes hypocapnia, which combined with the lack of a backup rate and wakefulness drive can lead to central apneas and sleep fragmentation, especially in patients with heart failure.

Effects of neural drives on breathing in the awake state in humans

We have developed a mathematical model of the regulation of ventilation that successfully simulates breathing in the awake as well as in sleeping states. In previous models, which were used to simulate Cheyne-Stokes breathing and respiration during sleep, the controller was only responsive to chemical stimuli, and allowed no ventilation at sub-normal carbon dioxide levels. The current model includes several new features. The chemical controller responds continuously to changes in P(CO(2)) with a lower sensitivity during hypocapnia than in the hypercapnic ranges. Hypoxia interacts multiplicatively with P(CO(2)) over the entire range of activity. The controller in the current model, besides the chemical drive, includes also a neural component. This neural drive increases and decreases as the level of alertness changes, and adds or subtracts from ventilation levels demanded by the chemical controller. The model also includes the effects of post-stimulus potentiation (PSP) and hypoxic ventilatory depression (HVD). While PSP eliminates apneas after a disturbance and also dampens the subsequent dynamics of the respiration, it is not a major factor in the damping of the response. Another finding is that HVD is destabilizing. The model is the first to reproduce results reported in conscious humans after hyperventilation and after acute and longer-term hypoxia. It also reproduces the effects of NREM sleep.

Influence of continuous speaking on ventilation

This study was conducted to explore the influence of speaking on ventilation. Twenty healthy young men were studied during periods of quiet breathing and prolonged speaking using noninvasive methods to measure chest wall surface motions and expired gas composition. Results indicated that all subjects ventilated more during speaking than during quiet breathing, usually by augmenting both tidal volume and breathing frequency. Ventilation did not change across repeated speaking trials. Quiet breathing was altered from its usual behavior following speaking, often for several minutes. Speaking-related increases in ventilation were found to be strongly correlated with lung volume expenditures per syllable. These findings have clinical implications for the respiratory care practitioner and the speech-language pathologist.

Hyperventilation and attention: effects of hypocapnia on performance in a stroop task

This study aimed to investigate the effect of hypocapnia on attentional performance. Hyperventilation, producing hypocapnia, is associated with physiological changes in the brain and with subjective symptoms of dizziness, concentration problems and derealization. In this study (N=42), we examined cognitive performance on a Stroop-like task, following either 3 min of hypocapnic or normocapnic overbreathing. Both overbreathing trials were run on separate days, each preceded by a baseline trial with the same task during normal breathing. More and other symptoms were reported after hypocapnia compared to normocapnia. Also, more errors were made and progressively slower reaction times (RT's) were observed during recovery from hypocapnia. These performance deficits were only found in participants characterized by apneas. The number of symptoms did not correlate with RT's or errors. The pattern of data suggested that hypoxia, as a result of apneas during recovery from hypocapnia, caused the cognitive performance deficit.

The duff, no duff casualty

A patient who presented with "hyperventilation syndrome" was initially mis-treated as severe crush injury, illustrating the need for thorough assessment of all casualties whilst on exercise prior to arranging casualty treatment and evacuation.

Apneic threshold for CO2 in the anesthetized rat: fundamental properties under steady-state conditions

Experiments were performed to measure the apneic threshold for CO2 and its fundamental properties in anesthetized rats under steady-state conditions. Breathing was detected from diaphragmatic electromyogram activity. Mechanical hyperventilation resulted in apnea once arterial PCO2 (PaCO2) had fallen far enough. Apnea was not a reflex response to lung inflation because it did not occur immediately, was not prevented by vagotomy, and was reversed by raising PaCO2 without changing mechanical hyperventilation. The apneic threshold was measured by hyperventilating rats mechanically with O2 until apnea had occurred and then raising PaCO2 at constant hyperventilation until breathing reappeared. The mean PaCO2 level of the apneic threshold in 42 rats was 32.8 +/- 0.4 Torr. The level of the threshold did not depend on the volume at which the lungs were inflated. The level of the threshold, under steady-state conditions, was the same when approached from hypocapnia as from eupnea. The level of the threshold could be raised by 9 Torr by chronic elevation of the eupneic PaCO2 level by 18 Torr.

Hypoxic ventilatory response and breathlessness following hypocapnic and isocapnic hyperventilation

STUDY OBJECTIVES

To investigate the etiology of posthyperventilation (post-HV) hypoxemia following voluntary hyperventilation (VHV), we examined the effects of hypocapnic (hypo-CO2) and isocapnic (iso-CO2) VHV on the hypoxic ventilatory response (O2-response) and on the sensation of breathlessness during the O2-response.

METHODS

O2-responses and visual analog scale (VAS) scores for estimating breathlessness in 10 normal subjects during the O2-response under iso-CO2 conditions and under hypo-CO2 conditions immediately following voluntary maximal HV of 3 min duration were examined.

RESULTS

Although there was no significant difference in the post-HV ventilation levels following hypo-CO2 vs iso-CO2 VHV, the VAS scores at the start of the O2-response following hypo-CO2 VHV (30.2+/-24.2 mm) were significantly higher (p<0.05) than the VAS scores at the start of the O2-response following iso-CO2 VHV (13.7+/-8.4 mm). However, VHV did not have a significant effect on the O2-response at 2 min after the VHV when the arterial O2 saturation (SaO2) was below 90%. The nonsteady-state hypo-CO2 induced by VHV greatly attenuated the O2-response below 90% SaO2 and VAS scores at 70% SaO2.

CONCLUSIONS

Elevated VAS scores immediately following the hypo-CO2 VHV, which might be independent of actual breathing levels, and the attenuation of the O2-response following the hypo-CO2 VHV were not due to input from lung and chest wall mechanoreceptors induced by the hyperpnea itself, but rather to the hypo-CO2 induced by hyperpnea.

The effect of hypoxia on the ventilatory response to carbon dioxide in man

We used rebreathing with prior hyperventilation to measure ventilatory responses to CO2 at iso-oxic PO2's of 100, 80, 60 and 40 mmHg in seven subjects. The mean sub-threshold ventilation (S.E.) of 7.60 (1.31) L min-1 did not vary with iso-oxic PO2. The mean peripheral-chemoreflex threshold of 41 (0.6)) mmHg PCO2 at an iso-oxic PO2 of 100 was greater than 39 (1.2) and 39 (0.6) at 60 and 40, respectively. The mean peripheral-chemoreflex sensitivity of 11.5 (5.2) L min-1 mmHg-1 at an iso-oxic PO2 of 40 was significantly greater than 3.0 (1.3), 2.7 (1.2) and 2.4 (1.2) at 60, 80 and 100, respectively. The mean central-chemoreflex threshold of 45 (1.5) mmHg PCO2 at an iso-oxic PO2 of 40 was significantly less than 48 (0.4) and 48 (0.7) at 80 and 100, respectively. The mean central-chemoreflex sensitivity of 5.0 (1.1) L min-1 mmHg-1 did not vary with iso-oxic PO2. These findings provide insights into the control of breathing in humans, including the implication that CO2 must exceed its peripheral-chemoreflex threshold before hypoxia can effectively increase ventilation.

Cardiovascular periodicities in healthy humans in the absence of breathing and under reduced chemical drive of respiration

In 20 healthy human subjects (aged 20-24 years) power density spectra in 1-min time series were computed by modified MATLAB procedures from inter-beat heart intervals and systolic and diastolic finger blood pressure. High- (HF), mid- (MF) and low-frequency (LF) peaks (0.16-0.37 Hz, 0.08-0.12 Hz and 0.02-0.05 Hz, respectively) were distinguished in power density spectra (PDS). During 1-min voluntary apneas MF and LF power increased and in 18 out of 20 subjects HF peaks disappeared in PDS. Breath holding performed in hyperoxia or in hyperoxia accompanied by 1-min hyperventilation preceding apnea augmented power at MF and LF range and abolished marginal HF peaks present in the spectrum in two subjects. It is concluded that apneic PDS is characterized by augmented power of MF and LF rhythms, and represents endogenous free running cardiovascular periodicities released from feed-back mechanical, reflex and feed-forward central entrainment to respiratory rhythm.

Influence of hypoxic duration and posthypoxic inspired O2 concentration on short term potentiation of breathing in humans

1. Short term potentiation (STP) of breathing refers to respiratory activity at a higher level than expected just from the dynamics of the peripheral and central chemoreceptors. In humans STP is activated by hypoxic stimulation. 2. To investigate the effects of the duration of hypoxia and the posthypoxic inspired O2 concentration on STP, the ventilatory responses to 30 s and 1, 3 and 5 min of hypoxia (end-tidal PO2, P(ET.O2) approximately 6.5 kPa) followed by normoxia (P(ET.O2) approximately 14.5 kPa) and hyperoxia (P(ET.O2) approximately 70 kPa) were studied in ten healthy subjects. End-tidal PCO2 (P(ET.CO2)) was clamped during hypoxic and recovery periods at 5.7 kPa. 3. Steady-state ventilation (VE) was 13.7 +/- 0.6 l min-1 during normoxia and increased to 15.5 +/- 0.3 l min-1 during hyperoxia (P < 0.05) due to the reduced Haldane effect and some decrease in cerebral blood flow (CBF). 4. The mean responses following hypoxia reached normoxic baseline after 69, 54, 12 and 12 s when 30 s and 1, 3 and 5 min of hypoxia, respectively, were followed by normoxia. An undershoot of 10 and 20% below hyperoxic baseline was observed when 3 and 5 min of hypoxia, respectively, were followed by hyperoxia. Hyperoxic VE reached hyperoxic baseline after 9, 15, 12 and 9 s at the termination of 30 s and 1, 3 and 5 min of hypoxia, respectively. 5. Normoxic recovery from 30 s and 1 min of hypoxia displayed a fast and subsequent slow decrease towards normoxic baseline. The fast component was attributed to the loss of the hypoxic drive at the site of the peripheral chemoreceptors, and the slow component to the decay of the STP that had been activated centrally by the stimulus. A slow decrease at the termination of 30 s and 1 min of hypoxia by hyperoxia was not observed since this component was cancelled by the increase in ventilatory output due to the reduced Haldane effect and some decrease of CBF. 6. Decay of the STP was not apparent in the normoxic recovery from 3 and 5 min of hypoxia as a slow component since it cancelled against the slow ventilatory increase related to the increase of brain tissue PCO2 due to the reduction of CBF at the relief of hypoxia. The undershoot observed when hyperoxia followed 3 and 5 min of hypoxia reflects the stimulatory effects of hyperoxia on VE. 7. The manifestation of the STP as a slow ventilatory decrease depends on the duration of hypoxia and the subsequent inspired oxygen concentration. We argue that STP is not abolished by the central depressive effects of hypoxia, although the manifestation of the STP may be overridden or counteracted by other mechanisms.

The nature of breathing during hypocapnia in awake man

We have studied post-hyperventilation breathing pattern in eight, awake, healthy, naive volunteers after 5 min voluntary or mechanical hyperventilation during normocapnia (PETCO2 = 38 mmHg) and 'hypocapnia (24 mmHg). Breathing was monitored for 10 min post-hyperventilation, 'non-invasively', using calibrated respiratory inductance plethysmography; wakefulness was confirmed with electroencephalography. Comparison of breathing following hypocapnic voluntary hyperventilation with that following hypocapnic mechanical hyperventilation indicated that ventilation was elevated following voluntary hyperventilation; this would suggest that 'after-discharge' exists in man following active hyperventilation, even during hypocapnia. In the absence of 'after-discharge' (i.e. following mechanical hyperventilation), hypocapnia was clearly associated with hypoventilation. Apnoeas (increased TE) were present during hypocapnia; but neither the duration nor the occurrence of apnoea was related to the absolute level of PETCO2. Most notable, was the marked increase in breath-by-breath variability of TI, TE and VT during hypocapnia. The increased variability of breathing during hypocapnia may reflect fluctuations in behavioural drives associated with wakefulness.

A review of the control of breathing during exercise

During the past 100 years many experimental investigations have been carried out in an attempt to determine the control mechanisms responsible for generating the respiratory responses observed during incremental and constant-load exercise tests. As a result of these investigations a number of different and contradictory control mechanisms have been proposed to be the sole mediators of exercise hyperpnea. However, it is now becoming evident that none of the proposed mechanisms are solely responsible for eliciting the exercise respiratory response. The present-day challenge appears to be one of synthesizing the proposed mechanisms, in order to determine the role that each mechanism has in controlling ventilation during exercise. This review, which has been divided into three primary sections, has been designed to meet this challenge. The aim of the first section is to describe the changes in respiration that occur during constant-load and incremental exercise. The second section briefly introduces the reader to traditional and contemporary control mechanisms that might be responsible for eliciting at least a portion of the exercise ventilatory response during these types of exercise. The third section describes how the traditional and contemporary control mechanisms may interact in a complex fashion to produce the changes in breathing associated with constant-load exercise, and incorporates recent experimental evidence from our laboratory.