The pattern of breathing following step changes of alveolar partial pressures of carbon dioxide and oxygen in man

Coordination of Respiration, Swallowing, and Chewing in Healthy Young Adults

Examining the coordination of respiration and swallowing is important for elucidating the mechanisms underlying these functions and assessing how respiration is linked to swallowing impairment in dysphagic patients. In this study, we assessed the coordination of respiration and swallowing to clarify how voluntary swallowing is coordinated with respiration and how mastication modulates the coordination of respiration and swallowing in healthy humans. Twenty-one healthy volunteers participated in three experiments. The participants were asked to swallow 3 ml of water with or without a cue, to drink 100 ml of water using a cup without breathing between swallows, and to eat a 4-g portion of corned beef. The major coordination pattern of respiration and swallowing was expiration–swallow–expiration (EE type) while swallowing 3 ml of water either with or without a cue, swallowing 100 ml of water, and chewing. Although cueing did not affect swallowing movements, the expiratory time was lengthened with the cue. During 100-ml water swallowing, the respiratory cycle time and expiratory time immediately before swallowing were significantly shorter compared with during and after swallowing, whereas the inspiratory time did not differ throughout the recording period. During chewing, the respiratory cycle time was decreased in a time-dependent manner, probably because of metabolic demand. The coordination of the two functions is maintained not only in voluntary swallowing but also in involuntary swallowing during chewing. Understanding the mechanisms underlying respiration and swallowing is important for evaluating how coordination affects physiological swallowing in dysphagic patients.

Reprint of "Drawing breath without the command of effectors: the control of respiration following spinal cord injury"

The maintenance of blood gas and pH homeostasis is essential to life. As such breathing, and the mechanisms which control ventilation, must be tightly regulated yet highly plastic and dynamic. However, injury to the spinal cord prevents the medullary areas which control respiration from connecting to respiratory effectors and feedback mechanisms below the level of the lesion. This trauma typically leads to severe and permanent functional deficits in the respiratory motor system. However, endogenous mechanisms of plasticity occur following spinal cord injury to facilitate respiration and help recover pulmonary ventilation. These mechanisms include the activation of spared or latent pathways, endogenous sprouting or synaptogenesis, and the possible formation of new respiratory control centres. Acting in combination, these processes provide a means to facilitate respiratory support following spinal cord trauma. However, they are by no means sufficient to return pulmonary function to pre-injury levels. A major challenge in the study of spinal cord injury is to understand and enhance the systems of endogenous plasticity which arise to facilitate respiration to mediate effective treatments for pulmonary dysfunction.

AltitudeOmics: enhanced cerebrovascular reactivity and ventilatory response to CO2 with high-altitude acclimatization and reexposure

The present study is the first to examine the effect of high-altitude acclimatization and reexposure on the responses of cerebral blood flow and ventilation to CO2. We also compared the steady-state estimates of these parameters during acclimatization with the modified rebreathing method. We assessed changes in steady-state responses of middle cerebral artery velocity (MCAv), cerebrovascular conductance index (CVCi), and ventilation (V(E)) to varied levels of CO2 in 21 lowlanders (9 women; 21 ± 1 years of age) at sea level (SL), during initial exposure to 5,260 m (ALT1), after 16 days of acclimatization (ALT16), and upon reexposure to altitude following either 7 (POST7) or 21 days (POST21) at low altitude (1,525 m). In the nonacclimatized state (ALT1), MCAv and V(E) responses to CO2 were elevated compared with those at SL (by 79 ± 75% and 14.8 ± 12.3 l/min, respectively; P = 0.004 and P = 0.011). Acclimatization at ALT16 further elevated both MCAv and Ve responses to CO2 compared with ALT1 (by 89 ± 70% and 48.3 ± 32.0 l/min, respectively; P < 0.001). The acclimatization gained for V(E) responses to CO2 at ALT16 was retained by 38% upon reexposure to altitude at POST7 (P = 0.004 vs. ALT1), whereas no retention was observed for the MCAv responses (P > 0.05). We found good agreement between steady-state and modified rebreathing estimates of MCAv and V(E) responses to CO2 across all three time points (P < 0.001, pooled data). Regardless of the method of assessment, altitude acclimatization elevates both the cerebrovascular and ventilatory responsiveness to CO2. Our data further demonstrate that this enhanced ventilatory CO2 response is partly retained after 7 days at low altitude.

Ventilatory failure, ventilator support, and ventilator weaning

The development of acute ventilatory failure represents an inability of the respiratory control system to maintain a level of respiratory motor output to cope with the metabolic demands of the body. The level of respiratory motor output is also the main determinant of the degree of respiratory distress experienced by such patients. As ventilatory failure progresses and patient distress increases, mechanical ventilation is instituted to help the respiratory muscles cope with the heightened workload. While a patient is connected to a ventilator, a physician's ability to align the rhythm of the machine with the rhythm of the patient's respiratory centers becomes the primary determinant of the level of rest accorded to the respiratory muscles. Problems of alignment are manifested as failure to trigger, double triggering, an inflationary gas-flow that fails to match inspiratory demands, and an inflation phase that persists after a patient's respiratory centers have switched to expiration. With recovery from disorders that precipitated the initial bout of acute ventilatory failure, attempts are made to discontinue the ventilator (weaning). About 20% of weaning attempts fail, ultimately, because the respiratory controller is unable to sustain ventilation and this failure is signaled by development of rapid shallow breathing. Substantial advances in the medical management of acute ventilatory failure that requires ventilator assistance are most likely to result from research yielding novel insights into the operation of the respiratory control system.

Worsening of central sleep apnea at high altitude--a role for cerebrovascular function

Although periodic breathing during sleep at high altitude occurs almost universally, the likely mechanisms and independent effects of altitude and acclimatization have not been clearly reported. Data from 2005 demonstrated a significant relationship between decline in cerebral blood flow (CBF) at sleep onset and subsequent severity of central sleep apnea that night. We suspected that CBF would decline during partial acclimatization. We hypothesized therefore that reductions in CBF and its reactivity would worsen periodic breathing during sleep following partial acclimatization. Repeated measures of awake ventilatory and CBF responsiveness, arterial blood gases during wakefulness. and overnight polysomnography at sea level, upon arrival (days 2-4), and following partial acclimatization (days 12-15) to 5,050 m were made on 12 subjects. The apnea-hypopnea index (AHI) increased from to 77 ± 49 on days 2-4 to 116 ± 21 on days 12-15 (P = 0.01). The AHI upon initial arrival was associated with marked elevations in CBF (+28%, 68 ± 11 to 87 ± 17 cm/s; P < 0.05) and its reactivity to changes in PaCO2 [>90%, 2.0 ± 0.6 to 3.8 ± 1.5 cm·s(-1)·mmHg(-1) hypercapnia and 1.9 ± 0.4 to 4.1 ± 0.9 cm·s(-1)·mmHg(-1) for hypocapnia (P < 0.05)]. Over 10 days, the increases resolved and AHI worsened. During sleep at high altitude large oscillations in mean CBF velocity (CBFv) occurred, which were 35% higher initially (peak CBFv = 96 cm/s vs. peak CBFv = 71 cm/s) than at days 12-15. Our novel findings suggest that elevations in CBF and its reactivity to CO(2) upon initial ascent to high altitude may provide a protective effect on the development of periodic breathing during sleep (likely via moderating changes in central Pco2).

Understanding the rhythm of breathing: so near, yet so far

Breathing is an essential behavior that presents a unique opportunity to understand how the nervous system functions normally, how it balances inherent robustness with a highly regulated lability, how it adapts to both rapidly and slowly changing conditions, and how particular dysfunctions result in disease. We focus on recent advancements related to two essential sites for respiratory rhythmogenesis: (a) the preBötzinger Complex (preBötC) as the site for the generation of inspiratory rhythm and (b) the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) as the site for the generation of active expiration.

Effects of acetazolamide on cerebrovascular function and breathing stability at 5050 m

One of the many actions of the carbonic anhydrase inhibitor, acetazolamide (ACZ), is to accelerate acclimatisation and reduce periodic breathing during sleep. The mechanism(s) by which ACZ may improve breathing stability, especially at high altitude, remain unclear. We tested the hypothesis that acute I.V. ACZ would enhance cerebrovascular reactivity to CO₂ at altitude, and thereby lower ventilatory drive and improve breathing stability during wakefulness. We measured arterial blood gases, minute ventilation (˙VE) and middle cerebral artery blood flow velocity (MCAv) before and 30 min following ACZ administration (I.V. 10 mg kg⁻¹) in 12 healthy participants at sea level and following partial acclimatisation to altitude (5050 m).Measures were made at rest and during changes in end-tidal PCO₂ and PO₂ (isocapnic hypoxia). At sea level, ACZ increased resting MCAv and its reactivity to both hypocapnia and hypercapnia (P < 0.05), and lowered resting VE, arterial O₂ saturation (Sa,O₂ ) and arterial PO₂ (Pa,O₂) (P < 0.05); arterial PCO₂ (Pa,CO₂ ) was unaltered (P > 0.05). At altitude, ACZ also increased resting MCAv and its reactivity to both hypocapnia and hypercapnia (resting MCAv and hypocapnia reactivity to a greater extent than at sea level). Moreover, ACZ at altitude elevated Pa,CO₂ and again lowered resting Pa,O₂ and Sa,O₂ (P <0.05). Although the ˙VE sensitivity to hypercapnia or isocapnic hypoxia was unaltered following ACZ at both sea level and altitude (P > 0.05), breathing stability at altitude was improved (e.g. lower incidence of ventilatory oscillations and variability of tidal volume; P < 0.05). Our data indicate that I.V. ACZ elevates cerebrovascular reactivity and improves breathing stability at altitude, independent of changes in peripheral or central chemoreflex sensitivities. We speculate that Pa,CO₂-mediated elevations in cerebral perfusion and an enhanced cerebrovascular reactivity may partly account for the improved breathing stability following ACZ at high altitude.

Influence of indomethacin on ventilatory and cerebrovascular responsiveness to CO2 and breathing stability: the influence of PCO2 gradients

Indomethacin (INDO), a reversible cyclooxygenase inhibitor, is a useful tool for assessing the role of cerebrovascular reactivity on ventilatory control. Despite this, the effect of INDO on breathing stability during wakefulness has yet to be examined. Although the effect of reductions in cerebrovascular CO(2) reactivity on ventilatory CO(2) sensitivity is likely dependent upon the method used, no studies have compared the effect of INDO on steady-state and modified rebreathing estimates of ventilatory CO(2) sensitivity. The latter method includes the influence of PCO(2) gradients and cerebral perfusion, whereas the former does not. We examined the hypothesis that INDO-induced reduction in cerebrovascular CO(2) reactivity would 1) cause unstable breathing in conscious humans and 2) increase ventilatory CO(2) sensitivity during the steady-state method but not during rebreathing methods. We measured arterial blood gases, ventilation (VE), and middle cerebral artery velocity (MCAv) before and 90 min following INDO ingestion (100 mg) or placebo in 12 healthy participants. There were no changes in resting arterial blood gases or Ve following either intervention. INDO increased the magnitude of Ve variability (index of breathing stability) during spontaneous air breathing (+4.3 +/- 5.2 Deltal/min, P = 0.01) and reduced MCAv (-25 +/- 19%, P < 0.01) and MCAv-CO(2) reactivity during steady-state (-47 +/- 27%, P < 0.01) and rebreathing (-32 +/- 25%, P < 0.01). The Ve-CO(2) sensitivity during the steady-state method was increased with INDO (+0.5 +/- 0.5 l x min(-1) x mmHg(-1), P < 0.01), while no changes were observed during rebreathing (P > 0.05). These data indicate that the net effect of INDO on ventilatory control is an enhanced ventilatory loop gain resulting in increased breathing instability. Our findings also highlight important methodological and physiological considerations when assessing the effect of INDO on ventilatory CO(2) sensitivity, whereby the effect of INDO-induced reduction of cerebrovascular CO(2) reactivity on ventilatory CO(2) sensitivity is unmasked with the rebreathing method.

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.

Postural change alters autonomic responses to breath-holding

OBJECTIVE

We used breath-holding during inspiration as a model to study the effect of pulmonary stretch on sympathetic nerve activity.

METHODS

Twelve healthy subjects (7 females, 5 males; 19-27 years) were tested while they performed an inspiratory breath-hold, both supine and during a 60 degrees head-up tilt (HUT 60). Heart rate (HR), mean arterial blood pressure (MAP), respiration, muscle sympathetic nerve activity (MSNA), oxygen saturation (SaO(2)) and end tidal carbon dioxide (ETCO(2)) were recorded. Cardiac output (CO) and total peripheral resistance (TPR) were calculated.

RESULTS

While breath-holding, ETCO(2) increased significantly from 41 +/- 2 to 60 +/- 2 Torr during supine (p < 0.05) and 38 +/- 2 Torr to 58 +/- 2 during HUT60 (p < 0.05); SaO(2) decreased from 98 +/- 1.5% to 95 +/- 1.4% supine, and from 97 +/- 1.5% to 94 +/- 1.7% during HUT60 (p = NS). MSNA showed three distinctive phases, a quiescent phase due to pulmonary stretch associated with decreased MAP, HR, CO, and TPR; a second phase of baroreflex-mediated elevated MSNA which was associated with recovery of MAP and HR only during HUT60; CO and peripheral resistance returned to baseline while supine and HUT60; a third phase of further increased MSNA activity related to hypercapnia and associated with increased TPR.

INTERPRETATION

Breath-holding results in initial reductions of MSNA, MAP, and HR by the pulmonary stretch reflex followed by increased sympathetic activity related to the arterial baroreflex and chemoreflex.

Asymmetric control of inspiratory and expiratory phases by excitability in the respiratory network of neonatal mice in vitro

Rhythmic motor behaviours consist of alternating movements, e.g. swing-stance in stepping, jaw opening and closing during chewing, and inspiration-expiration in breathing, which must be labile in frequency, and in some cases, in the duration of individual phases, to adjust to physiological demands. These movements are the expression of underlying neural circuits whose organization governs the properties of the motor behaviour. To determine if the ability to operate over a broad range of frequencies in respiration is expressed in the rhythm generator, we isolated the kernel of essential respiratory circuits using rhythmically active in vitro slices from neonatal mice. We show respiratory motor output in these slices at very low frequencies (0.008 Hz), well below the typical frequency in vitro (approximately 0.2 Hz) and in most intact normothermic mammals. Across this broad range of frequencies, inspiratory motor output bursts remained remarkably constant in pattern, i.e. duration, peak amplitude and area. The change in frequency was instead attributable to increased interburst interval, and was largely unaffected by removal of fast inhibitory transmission. Modulation of the frequency was primarily achieved by manipulating extracellular potassium, which significantly affects neuronal excitability. When excitability was lowered to slow down, or in some cases stop, spontaneous rhythm, brief stimulation of the respiratory network with a glutamatergic agonist could evoke (rhythmic) motor output. In slices with slow (<0.02 Hz) spontaneous rhythms, evoked motor output could follow a spontaneous burst at short (<or=1 s) or long (approximately 60 s) intervals. The intensity or timing of stimulation determined the latency to the first evoked burst, with no evidence for a refractory period greater than approximately 1 s, even with interburst intervals >60 s. We observed during inspiration a large magnitude (approximately 0.6 nA) outward current generated by Na(+)/K(+) ATPase that deactivated in 25-100 ms and thus could contribute to burst termination and the latency of evoked bursts but is unlikely to control the interburst interval. We propose that the respiratory network functions over a broad range of frequencies by engaging distinct mechanisms from those controlling inspiratory duration and pattern that specifically govern the interburst interval.

Lateral parabrachial nucleus mediates shortening of expiration and increase of inspiratory drive during hypercapnia

We have previously shown that unilateral or bilateral lesions of the lateral parabrachial nucleus (LPBN) in anesthetized, vagotomized rats markedly and selectively attenuate the shortening of expiratory duration (T(E)) during hypoxia without appreciably affecting all other hypoxic response components. Here, we report that unilateral LPBN lesion by kainic acid in the same group of animals not only abolished normal T(E)-shortening during central chemoreceptors activation by hyperoxic hypercapnia, but led to paradoxical T(E)-prolongation and corresponding decrease of respiratory frequency. Furthermore, LPBN lesion significantly attenuated the increase in phrenic activity during hyperoxic hypercapnia, without appreciably affecting the corresponding shortening of inspiratory duration (T(I)). These findings provide the first evidence indicating that central chemoafferent inputs are organized in parallel and segregated pathways that separately modulate inspiratory drive, T(I), and T(E) in conjunction with similar parallel and segregated central processing of peripheral chemoafferent inputs reported previously [Young, D.L., Eldridge, F.L., Poon, C.S., 2003. Integration-differentiation and gating of carotid afferent traffic that shapes the respiratory pattern. J. Appl. Physiol. 94, 1213-1229].

Lateral parabrachial nucleus mediates shortening of expiration during hypoxia

Acute hypoxia elicits complex time-dependent responses including rapid augmentation of inspiratory drive, shortening of inspiratory and expiratory durations (T(I), T(E)), and short-term potentiation and depression. The central pathways mediating these varied effects are largely unknown. Here, we show that the lateral parabrachial nucleus (LPBN) of the dorsolateral pons specifically mediates T(E)-shortening during hypoxia and not other hypoxic response components. Twelve urethane-anesthetized and vagotomized adult Sprague-Dawley rats were exposed to 1-min poikilocapnic hypoxia before and after unilateral kainic acid or bilateral electrolytic lesioning of the LPBN. Bilateral lesions resulted in a significant increase in baseline T(E) under hyperoxia. After unilateral or bilateral lesions, the decrease in T(E) during hypoxia was markedly attenuated without appreciable changes in all other hypoxic response components. These findings add to the mounting evidence that the central processing of peripheral chemoafferent inputs is segregated into parallel integrator and differentiator (low-pass and high-pass filter) pathways that separately modulate inspiratory drive, T(I), T(E) and resultant short-term potentiation and depression.

Effect of spinal cord injury on the respiratory system: basic research and current clinical treatment options

Spinal cord injury (SCI) often leads to an impairment of the respiratory system. The more rostral the level of injury, the more likely the injury will affect ventilation. In fact, respiratory insufficiency is the number one cause of mortality and morbidity after SCI. This review highlights the progress that has been made in basic and clinical research, while noting the gaps in our knowledge. Basic research has focused on a hemisection injury model to examine methods aimed at improving respiratory function after SCI, but contusion injury models have also been used. Increasing synaptic plasticity, strengthening spared axonal pathways, and the disinhibition of phrenic motor neurons all result in the activation of a latent respiratory motor pathway that restores function to a previously paralyzed hemidiaphragm in animal models. Human clinical studies have revealed that respiratory function is negatively impacted by SCI. Respiratory muscle training regimens may improve inspiratory function after SCI, but more thorough and carefully designed studies are needed to adequately address this issue. Phrenic nerve and diaphragm pacing are options available to wean patients from standard mechanical ventilation. The techniques aimed at improving respiratory function in humans with SCI have both pros and cons, but having more options available to the clinician allows for more individualized treatment, resulting in better patient care. Despite significant progress in both basic and clinical research, there is still a significant gap in our understanding of the effect of SCI on the respiratory system.

Ventilatory responses to isocapnic and poikilocapnic hypoxia in humans

We examined the hypoxic ventilatory response (HVR) including breathing frequency (f(R)) and tidal volume (V(T)) responses during 20 min of step isocapnic (IH) and poikilocapnic (PH) hypoxia (45 Torr). We hypothesized an index related to [Formula: see text] (pHPR) may be more robust during PH. Peak HVR was suppressed during PH (P<0.001), and mediated by V(T) during PH and both V(T) and f(R) during IH. The relative magnitude of HVD remained similar between conditions indicating a suppressive role of hypocapnia in development of the HVR unrelated to the degree of subsequent HVD, implying a primarily O(2) dependant mechanism. Post-hypoxic frequency decline was observed following both IH (3.4+/-3.7 bpm, P<0.05) and PH (3.6+/-3.1 bpm, P<0.01), despite no f(R) response during exposure to PH. Use of pHPR improved the signal to noise ratio during PH, though failed to detect the peak ventilatory response, and therefore may not be appropriate when describing peak responses.

Influence of arterial O2 on the susceptibility to posthyperventilation apnea during sleep

To investigate the contribution of the peripheral chemoreceptors to the susceptibility to posthyperventilation apnea, we evaluated the time course and magnitude of hypocapnia required to produce apnea at different levels of peripheral chemoreceptor activation produced by exposure to three levels of inspired P(O2). We measured the apneic threshold and the apnea latency in nine normal sleeping subjects in response to augmented breaths during normoxia (room air), hypoxia (arterial O2 saturation = 78-80%), and hyperoxia (inspired O2 fraction = 50-52%). Pressure support mechanical ventilation in the assist mode was employed to introduce a single or multiple numbers of consecutive, sigh-like breaths to cause apnea. The apnea latency was measured from the end inspiration of the first augmented breath to the onset of apnea. It was 12.2 +/- 1.1 s during normoxia, which was similar to the lung-to-ear circulation delay of 11.7 s in these subjects. Hypoxia shortened the apnea latency (6.3 +/- 0.8 s; P < 0.05), whereas hyperoxia prolonged it (71.5 +/- 13.8 s; P < 0.01). The apneic threshold end-tidal P(CO2) (Pet(CO2)) was defined as the Pet(CO2)) at the onset of apnea. During hypoxia, the apneic threshold Pet(CO2) was higher (38.9 +/- 1.7 Torr; P < 0.01) compared with normoxia (35.8 +/- 1.1; Torr); during hyperoxia, it was lower (33.0 +/- 0.8 Torr; P < 0.05). Furthermore, the difference between the eupneic Pet(CO2) and apneic threshold Pet(CO2) was smaller during hypoxia (3.0 +/- 1.0 Torr P < 001) and greater during hyperoxia (10.6 +/- 0.8 Torr; P < 0.05) compared with normoxia (8.0 +/- 0.6 Torr). Correspondingly, the hypocapnic ventilatory response to CO2 below the eupneic Pet(CO2) was increased by hypoxia (3.44 +/- 0.63 l.min(-1).Torr(-1); P < 0.05) and decreased by hyperoxia (0.63 +/- 0.04 l.min(-1).Torr(-1); P < 0.05) compared with normoxia (0.79 +/- 0.05 l.min(-1).Torr(-1)). These findings indicate that posthyperventilation apnea is initiated by the peripheral chemoreceptors and that the varying susceptibility to apnea during hypoxia vs. hyperoxia is influenced by the relative activity of these receptors.

The respiratory response to carbon dioxide in humans with unilateral and bilateral resections of the carotid bodies

The acute hypercapnic ventilatory response (AHCVR) arises from both peripheral and central chemoreflexes. In humans, one technique for identifying the separate contributions of these chemoreflexes to AHCVR has been to associate the rapid component of AHCVR with the peripheral chemoreflex and the slow component with the central chemoreflex. Our first aim was to validate this technique further by determining whether a single slow component was sufficient to describe AHCVR in patients with bilateral carotid body resections (BR) for glomus cell tumours. Our second aim was to determine whether the slow component of AHCVR was diminished following carotid body resection as has been suggested by studies in experimental animals. Seven BR subjects were studied together with seven subjects with unilateral resections (UR) and seven healthy controls. A multifrequency binary sequence in end-tidal PCO2 was employed to stimulate ventilation dynamically under conditions of both euoxia and mild hypoxia. Both two- and one-compartment models of AHCVR were fitted to the data. For BR subjects, the two-compartment model fitted significantly better on 1 out of 13 occasions compared with 22 out of 28 occasions for the other subjects. Average values for the chemoreflex sensitivity of the slow component of AHCVR differed significantly (P < 0.05) between the groups and were 0.95, 1.38 and 1.50 l min-1 Torr-1 for BR, UR and control subjects, respectively. We conclude that, without the peripheral chemoreflex, AHCVR is adequately described by a single slow component and that BR subjects have sensitivities for the slow component that are lower than those of control subjects.

Dynamic ventilatory response to CO(2) in congestive heart failure patients with and without central sleep apnea

Nonobstructive (i.e., central) sleep apnea is a major cause of sleep-disordered breathing in patients with stable congestive heart failure (CHF). Although central sleep apnea (CSA) is prevalent in this population, occurring in 40-50% of patients, its pathogenesis is poorly understood. Dynamic loop gain and delay of the chemoreflex response to CO(2) was measured during wakefulness in CHF patients with and without CSA by use of a pseudorandom binary CO(2) stimulus method. Use of a hyperoxic background minimized responses derived from peripheral chemoreceptors. The closed-loop and open-loop gain, estimated from the impulse response, was three times greater in patients with nocturnal CSA (n = 9) than in non-CSA patients (n = 9). Loop dynamics, estimated by the 95% response duration time, did not differ between the two groups of patients. We speculate that an increase in dynamic gain of the central chemoreflex response to CO(2) contributes to the genesis of CSA in patients with CHF.

An integrated model of the human ventilatory control system: the response to hypercapnia

This work presents a mathematical model of the human respiratory control system, based on physiological knowledge. It includes three compartments for gas storage and exchange (lungs, brain tissue and other body tissues), and various kinds of feedback mechanisms. These comprehend peripheral chemoreceptors in the carotid body, central chemoreceptors in the medulla and a central ventilatory depression. The latter acts by reducing the response of the central neural system to the afferent peripheral chemoreceptor activity during prolonged hypoxia of the brain tissue. Furthermore, the model considers local blood flow adjustments in response to O2 and CO2 arterial pressure changes. In this study, the model has been validated by simulating the response to square changes in alveolar PCO2, performed at different constant levels of alveolar PO2. A good agreement with data reported in the literature has been checked. Subsequently, a sensitivity analysis on the role of the main feedback mechanisms on ventilation response to CO2 has been performed. The results suggest that the ventilatory response to CO2 challenges during hyperoxia can be almost completely ascribed to the central chemoreflex, while, during normoxia, the peripheral chemoreceptors provide a modest contribution too. By contrast, the response to hypercapnic stimuli during hypoxia involves a complex superimposition among different factors with disparate dynamics. Hence, results suggest that the ventilatory response to hypercapnia during hypoxia is more complex than that provided by simple empirical models, and that discrimination between the central and peripheral components based on time constants may be misleading.

Methodological and physiological variability within the ventilatory response to hypoxia in humans

Measurement of the acute hypoxic ventilatory response (AHVR) requires careful choice of the hypoxic stimulus. If the stimulus is too brief, the response may be incomplete; if the stimulus is too long, hypoxic ventilatory depression may ensue. The purpose of this study was to compare three different techniques for assessing AHVR, using different hypoxic stimuli, and also to examine the between-day variability in AHVR. Ten subjects were studied, each on six different occasions, which were >/=1 wk apart. On each occasion, AHVR was assessed using three different protocols: 1) protocol SW, which uses square waves of hypoxia; 2) protocol IS, which uses incremental steps of hypoxia; and 3) protocol RB, which simulates an isocapnic rebreathing test. Mean values for hypoxic sensitivity were 1.02 +/- 0.48, 1.15 +/- 0.55, and 0.93 +/- 0.60 (SD) l. min(-1). %(-1) for protocols SW, IS, and RB, respectively. These differed significantly (P < 0.01). The coefficients of variation for measurement of AHVR were 20, 23, and 36% for the three protocols, respectively. These were not significantly different. There was a significant physiological variation in AHVR (F (50,100) = 3.9, P < 0. 001), with a coefficient of variation of 26%. We conclude that there was relatively little systematic variation between the three protocols but that AHVR varies physiologically over time.

A model of the chemoreflex control of breathing in humans: model parameters measurement

We reviewed the ventilatory responses obtained from rebreathing experiments on a population of 22 subjects. Our aim was to derive parameter estimates for an 'average subject' so as to model the respiratory chemoreflex control system. The rebreathing technique used was modified to include a prior hyperventilation, so that rebreathing started at a hypocapnic P(CO2) and ended at a hypercapnic P(CO2). In addition, oxygen was added to the rebreathing bag in a controlled manner to maintain iso-oxia during rebreathing, which allowed determination of the response at several iso-oxic P(O2) levels. The breath-by-breath responses were analysed in terms of tidal volume, breathing frequency and ventilation. As P(CO2) rose, ventilation was first steady at a basal value, then increased as P(CO2) exceeded a breakpoint. We interpreted this first breakpoint as the threshold of the combined central and peripheral chemoreflex responses. Above, ventilation increased linearly with P(CO2), with tidal volume usually contributing more than frequency to the increase. When breathing was driven strongly, such as in hypoxia, a second breakpoint P(CO2) was often observed. Beyond the second breakpoint, ventilation continued to increase linearly with P(CO2) at a different slope, with frequency usually contributing more than tidal volume to the increase. We defined the parameters of the variation of tidal volume, frequency and ventilation with P(O2) and P(CO2) for an average subject based on a three-segment linear fit of the individual responses. These were incorporated into a model of the respiratory chemoreflex control system based on the general scheme of the 'Oxford' model. However, instead of considering ventilatory responses alone, the model also incorporates tidal volume and frequency responses.

Identification of fast and slow ventilatory responses to carbon dioxide under hypoxic and hyperoxic conditions in humans

1. Under conditions of both euoxia and hypoxia, it is generally accepted that the ventilatory response to CO2 has both rapid (peripheral chemoreflex) and slow (central chemoreflex) components. However, under conditions of hyperoxia, it is unclear in humans whether the fast component is completely abolished or merely attenuated in magnitude. 2. The present study develops a technique to determine whether or not a two-compartment model fits the ventilatory response to CO2 significantly better than a one-compartment model. Data were collected under both hypoxic (end-tidal PO2 = 50 Torr) conditions, when two components would be expected, and under hyperoxic (end-tidal PO2 = 200 Torr) conditions, when the presence of the fast compartment is under question. 3. Ten subjects were recruited, of whom nine completed the study. The end-tidal PCO2 of each subject was varied according to a multi-frequency binary sequence that involved 13 steps into and 13 steps out of hypercapnia lasting altogether 1408 s. 4. In four out of nine subjects in hypoxia, and six out of nine subjects in hyperoxia, the two-compartment model fitted the data significantly better than the one-compartment model (F ratio test on residuals). This improvement in fit was significant for the pooled data in both hypoxia (P < 0.05) and hyperoxia (P < 0.005). Mean ventilatory sensitivities for the central chemoreflex were (mean +/- s.e.m.) 1. 69 +/- 0.39 l min-1 Torr-1 in hypoxia and 2.00 +/- 0.32 l min-1 Torr-1 in hyperoxia. Mean ventilatory sensitivities for the peripheral chemoreflex were 2.42 +/- 0.36 l min-1 Torr-1 in hypoxia and 0.75 +/- 0.16 l min-1 Torr-1 in hyperoxia. 5. It is concluded that the rapid and slow components of the ventilatory response to CO2 can be separately identified, and that a rapid component persists under conditions of hyperoxia.

Effect of hyperoxic hypercapnia on variational activity of breathing

Dysrhythmias of breathing occur in several clinical disorders, but their mechanistic basis is obscure. To understand their pathophysiology, factors responsible for the variability of breathing need to be defined. We studied the effect of hyperoxic hypercapnia (CO2) on the variational activity of breathing in 14 volunteers before and after delivering CO2 nonobstrusively via a plastic hood. Compared with air, CO2 increased the gross variability of minute ventilation (VI) and tidal volume (VT), and decreased that of inspiratory time (TI) and expiratory time (TE) (all p < 0.03). CO2 increased the autocorrelation coefficient at a lag of one breath for VI (p < 0.05), the number of consecutive breath lags having significant autocorrelation coefficients for VI and VT (both p < 0.01), and the cycle time of oscillations in VI (p = 0.03) and VT (p = 0.04). Uncorrelated random behavior constituted > or = 80% of the variance of each breath component, correlated behavior represented 9 to 20%, and oscillatory behavior represented < 1% during both air and CO2. CO2 increased the correlated behavior of volume components, which was accompanied by development of low-frequency oscillations with a cycle time consistent with central chemoreceptor activation.

Transitional effects of inspired CO2 on lung mechanics in anesthetized rabbits

In eight anesthetized, spontaneously breathing New Zealand rabbits airflow, tidal volume, tracheal and esophageal pressures were measured. Frequency, inspiratory and expiratory durations, respiratory system and lung resistances, and elastances, total postinspiratory muscle pressure and its timing parameters were determined. The measurements were performed: (a) under control condition; (b) continuously, during the first 90 sec of the inhalation of a mixture of 5% CO2 + 95% O2, (carbogen, CI); and (c) 6 min after the start of CI. During CI both lung resistance and elastance decreased, and inspiratory driving pressure and postinspiratory muscle pressure increased. No modification could be detected in either the pattern or the duration of postinspiratory muscle activity. In conclusion, mechanical changes precede and facilitate the ventilatory adjustment to CO2 in rabbits.

Control of the respiratory cycle in conscious humans

We studied in conscious humans the relative strength of mechanisms controlling timing and drive components of the respiratory cycle around their resting set points. A system of auditory feedback with end-tidal PCO2 held constant in mild hyperoxia via an open circuit was used to induce subjects independently to change inspiratory time (TI) and tidal volume (VTI) over a wide range above and below the resting values for every breath for up to 1 h. Four protocols were studied in various levels of hypercapnia (1-5% inspired CO2). We found that TI (and expiratory time) could be changed over a wide range (1.17 - 2.86 s, P < 0.01 for TI) and VTI increased by > or = 500 ml (P < 0.01) without difficulty. However, in no protocol was it possible to decrease VTI below the free-breathing resting value in response to reduction of auditory feedback thresholds by up to 600 ml. This applied at all levels of chemical drive studied, with resting VTI values varying from 1.06 to 1.74 liters. When reduction in VTI was forced by the more "programmed" procedure of isocapnic panting, end-expiratory of volume was sacrificed to ensure that peak tidal volume reached a fixed absolute lung volume. These results suggest that the imperative for control of resting breathing is to prevent reduction of VTI below the level dictated by the prevailing chemical drive, presumably to sustain metabolic requirements of the body, whereas respiratory timing is weakly controlled consistent with the needs for speech and other nonmetabolic functions of breathing.

Dynamic response characteristics of CO2-induced air hunger

The time course of change in 'air hunger', the uncomfortable urge to breathe, was assessed following sudden increases and decreases in PETCO2. Healthy normal men and women were mechanically ventilated at constant tidal volume and frequency, and were required to rate the perceived intensity of air hunger every 10-15 sec. PETCO2 was changed by altering FICO2 unbeknownst to the subject. Air hunger changed to its new level following steps with a median time constant of about 50 sec during hyperoxia. Changes in air hunger following PETCO2 steps were slightly faster when background gas was slightly hypoxic. Although the present results are consistent with the hypothesis that air hunger and ventilatory drive share the same receptors and central neural processes, analysis of dynamic response is probably not sensitive enough to disprove the hypothesis.

Differential ventilatory control during constant work rate and incremental exercise

The purpose of this study was to determine whether the tachypneic breathing pattern of constant work rate, heavy exercise (CWE) is unique to CWE or whether it represents the usual pattern of the respiratory control system at high levels of ventilation (VI). We compared breathing pattern in ten healthy subjects (age 20-29 years) during CWE and maximal incremental exercise (MIE) on a bicycle ergometer. Work rate was constant at 76% of maximum work rate in CWE and progressively increased by 25 watts/minute until exhaustion during MIE. Breathing pattern was examined at matched levels of VI equivalent to 80% and about 100% of maximum VI during CWE (97.1 and 121.4 L.min-1, respectively). Exercise duration (mean+standard deviation) was 13 +/- 6 and 12 +/- 1 min during CWE and MIE, respectively (P = NS). Tidal volume (VT) fell by an average of 0.20 L towards the end of CWE, but was maintained relatively high and constant towards the end of MIE. At high, but not lower, matched levels of VI breathing pattern during CWE was significantly more rapid and shallow than that during MIE. The tachypnoea of CWE did not correlate with the progressive rise in VI, oxygen uptake or cardiac frequency during CWE. We conclude that (1) CWE is associated with a tachypneic influence that is absent or less during incremental exercise; this tachypnea is most marked at the end of CWE. (2) The tachypnoea of CWE is not part of a generalized rate accelerating process during CWE. The mechanism(s) underlying the tachypnoea are unclear but it may be related to inspiratory muscle fatigue, pulmonary oedema, and/or altered respiratory mechanics.

Post-hyperventilation apnoea in conscious humans

1. In nine normal subjects, analysis was performed of the number, length and location of apnoeic pauses during 20 min of recovery following voluntary overbreathing (VHV). Four different rates of recovery of end-tidal PCO2 (PET,CO2), studied in randomized order, were induced by overbreathing to 15 or 25 mmHg, each for 3 or 6 min. Subjects breathed mildly hyperoxic gas mixtures (inspired PO2 approximately 250 mmHg) to and fro into an open circuit via a mouthpiece and pneumotachograph. 2. Apnoeic pauses rarely occurred immediately after the end of VHV but gradually increased in number and length. When averaged across all subjects and protocols, the largest pauses occurred 2.0 +/- 0.3 min (S.D.; range 1.6-2.4 min) after the end of VHV. Based on a definition of apnoea as expiratory time greater than 6 s, apnoeas occurred between mean times of 0.8 and 5.6 min after the end of VHV, the end of this period being associated with a mean PET,CO2 value of 36.4 mmHg, which was below the initial mean resting value of 39.8 mmHg. 3. Within this apnoeic period, 80% of experiments produced apnoeas of less than 10 s duration, 61% of between 10 and 20 s duration and 42% of between 20 and 30 s duration. Only one out of nine subjects consistently failed to show apnoeas. 4. The range of lengths of individual apnoeas and the number per minute were independent of the length and level of VHV and were not significantly different between the four protocols. 5. The number and length of apnoeas did not change in repeated runs in each subject. We were not able to confirm previous reports that apnoeas occurred more frequently in subjects familiar with the experiment. 6. These results reconciled previous studies showing either apnoea or hyperpnoea following voluntary overbreathing in conscious humans. They showed an initial period of heightened breathing lasting about a minute with few apnoeas, consistent with 'after-discharge'. Beyond that, apnoeas occurred as an 'all-or-nothing' phenomenon as long as PET,CO2 was on average less than 3.4 mmHg below resting PET,CO2. The occurrence and length of apnoeas was consistent in individual subjects with no evidence of a learning effect.

Latency of the ventilatory chemoreflex response to hypoxia in humans

Latencies for the ventilatory response to hypoxia have been estimated from data from experiments in which square waves of isocapnic hypoxia (periods 30 sec and 60 sec) were presented to 5 subjects. Distorted steps were excluded from the analysis, and the remaining steps were time-aligned relative to the step and then averaged. For the 30 sec data, the median latency for the response to the step into hypoxia was 1 breath or 5.1 sec (time to mid-point of first significantly different breath) and for the step out of hypoxia was 1 breath or 4.7 sec. The number of transients analyzed averaged 87 per subject per transition type. For the 60 sec data, the median latency for the step into hypoxia was 2 breaths or 6.8 sec, and for the step out of hypoxia was 2 breaths or 6.0 sec. The number of transients analyzed averaged 40 per subject per transition type. These latencies are generally shorter than those reported previously and suggest that the ventilatory variability may have served to lengthen the measured latency of response in previous studies.

Dynamics of the ventilatory response to hypoxia in humans

Dynamic responses of the ventilatory system to rapid variations in isocapnic hypoxia were studied in five subjects. Sawtooth-shaped inputs were presented at constant amplitude with periods of 120, 90, 60, 45 and 30 sec, and square-wave inputs at different amplitudes with periods of 120, 60 and 30 sec. A breath-by-breath model fitting technique was used to assess whether any of a number of first order models of hypoxic ventilatory dynamics could fit the data adequately. The following was found: 1) An equation for the desaturation of haemoglobin provided a better expression for hypoxia in the model than did a hyperbolic function of PO2. 2) The gain and/or offset model parameters varied significantly between experiments, but the time constant and pure delay terms did not. 3) The time constants, and to a lesser extent the pure delays, were found to vary significantly between sawtooth experiments of different frequencies. The failure of a single set of dynamic parameters to describe all the responses suggests that the model is incomplete. 4) There was significant asymmetry in the hypoxic response with the on-transient dynamics faster than the off-transient dynamics. The results of the model fitting study suggest that a first order model cannot fully describe the hypoxic ventilatory dynamics.

Dynamics of the ventilatory response in man to step changes of end-tidal carbon dioxide and of hypoxia during exercise

1. Four human subjects exercised in hypoxia (end-tidal partial pressure of O2 (P(ET),O2) ca 55 Torr; heart rate ca 100-130 beats min-1), and the contribution to the respiratory drive of the peripheral and central chemoreflex pathways have been separated on the basis of the latencies and the time courses of the responses to sudden changes of stimulus. 2. The subjects were exposed to repeated end-tidal step changes in PCO2 of ca 3-3.5 Torr (at nearly constant P(ET),O2) and PO2 (between ca 55 and 230 Torr) at three regions along the expiratory ventilation VE-P(ET),CO2 response line (hypocapnia, eucapnia, hypercapnia). The dynamics of the ventilatory responses were calculated using a two-compartment non-linear least-squares optimization method. 3. The component of the response attributable to the peripheral chemoreflex loop may in some subjects contribute up to 75% of the ventilatory drive during mild hypocapnic hypoxic exercise and ca 72% of the total gain following steps of P(ET),CO2 during hypoxic exercise. These data support the notion that the effectiveness of the peripheral chemoreceptor pathway is enhanced in moderate exercise. 4. During hypoxic exercise, the time delays and time constants attributed to the peripheral chemoreflex pathways (ca 3.5 and 9 s respectively) and to the central chemoreflex pathways (ca 9.5 and 47 s respectively) are some of the shortest reported. 5. The dynamics of the peripheral and central chemoreflex pathways appeared to be largely independent of each other. 6. There was a notable absence of systematic change of inspiratory and expiratory durations during the step-induced transients.

Effects of different levels of end-tidal PO2 on ventilation during isocapnia in humans

The purpose of this investigation was to examine how the ventilatory decline observed during sustained, eucapnic hypoxia (HVD) is affected by different levels of hypoxia. Six subjects were each studied 3-6 times at each of 5 different levels of isocapnic hypoxia (end-tidal PO2 equal to 45, 50, 55, 65 and 75 Torr) in random order. The following variables were linearly related to saturation: (1) the rapid increase in ventilation at the onset of hypoxia; (2) the decline in ventilation over the period of hypoxia; and (3) the undershoot in ventilation below the pre-hypoxic control values at the relief of hypoxia. The rapid decrease in ventilation at the relief of hypoxia, however, was not linearly related to saturation. The mean time to peak ventilation was 2.13 +/- 0.07 min (+/- SE) at the onset of hypoxia, which was significantly longer (P less than 0.05) than the time to minimum ventilation at the relief of hypoxia of 1.23 +/- 0.18 min. The recovery from the undershoot in ventilation was 95% +/- 3% complete after 5 min, whereas the recovery in sensitivity to hypoxia was only 35% +/- 13% complete after 5 min of euoxia.

Analyses of human respiratory flow patterns

Respiratory drives follow various afferent pathways to the respiratory centres; nevertheless, steady-state breathing patterns described in terms of tidal volumes and phase durations are largely independent of the nature of the respiratory stimulus. Flow has now been recorded during steady states from six subjects in rest and hyperpnoea induced by exercise, and hypercapnia in euoxia and in hypoxia (asphyxia). Flow patterns from different stimuli were compared isopnoeically. Quantitative methods allowed the patterns to be described in terms of several variables. The consistent small differences in isopnoeic flow patterns were: In asphyxia, the initial inspiratory acceleration was greater than in hypercapnia, and the peak flow was reached earlier. In exercise the peak flow occurred later in inspiration, and the expiratory flow was maintained high until nearer the end of the phase than with the chemical drives so that the flow pattern was less angular in shape. Stimulus-dependent effects, obvious during transient changes, are greatly attenuated rather than absent in steady states.

Ribcage contribution to CO2 response during rebreathing and steady state methods

Disagreements exist between previous studies of the contribution of the rib cage (RC) and abdomen-diaphragm (AD) components to CO2-stimulated ventilation. These studies used dissimilar techniques of CO2 stimulation and varying methods of data processing and presentation, thus precluding direct comparisons. We have therefore studied two methods of CO2 stimulation in 12 subjects, using a Read's rebreathing method and a modified steady-state technique. Respiratory inductive plethysmography was used to assess the RC and AD contributions to ventilation. Mean slopes for the ventilatory response to CO2 were the same for both methods (mean 2.56 L.min-1.mmHg-1), and the intercepts were significantly different (43.7 mmHg for rebreathing and 38.0 for modified steady state: P less than 0.001). There was a small, but significant, increase in the percentage RC contribution to ventilation during hypercapnia of 0.97%/mmHg PCO2 for rebreathing and 0.62 for steady state (P less than 0.01 and P less than 0.05, respectively), and these values were not significantly different from each other. Using our data in comparison with other studies, we have been able to show that differences in processing and presentation of data have given rise to wide variations in conclusions.

The influence of oxygen on the ventilatory response to carbon dioxide in man

1. The ventilatory response to isoxic square-wave challenges in end-tidal PCO2 was investigated at three levels of end-tidal PO2 (PET, O2) in nine healthy male subjects. 2. Twenty-seven responses against a background of mild hypoxia (PET, O2 approximately 10 kPa), sixty-seven against a background of normoxia (PET, O2 approximately 14.5 kPa) and seventy-six against a background of hyperoxia (PET, O2 approximately 70 kPa) were collected. 3. The breath-to-breath data were partitioned into a fast and a slow ventilatory component using a two-compartment model. 4. In the normoxic and hypoxic experiments the CO2 sensitivity of the fast component averaged to about 30 and 40% of the total CO2 sensitivity, respectively. In the hyperoxic experiments three subjects had no fast component in their response while in three others the CO2 sensitivity of the fast component averaged to about 24% of the total CO2 sensitivity. In the remaining three subjects the presence of a fast component was doubtful. 5. We argue that the fast component is due to the peripheral chemoreflex loop and the slow component to the central chemoreflex loop. 6. The central CO2 sensitivity and the apnoeic threshold (extrapolated end-tidal CO2 at zero ventilation in the steady state) were 15% smaller in hyperoxia than those in normoxia and hypoxia. In normoxia and mild hypoxia the central CO2 sensitivities were not significantly different. 7. We argue, that apart from peripheral oxygen-carbon dioxide interaction, there is evidence for central oxygen-carbon dioxide interaction in human subjects. 8. We conclude that in general there is a contribution to ventilation of the peripheral chemoreceptors during hyperoxia in man.

Hypoxic depression of ventilation in humans: alternative models for the chemoreflexes

The ventilatory responses of 5 volunteers to three protocols were determined. In protocol A, PETCO2 was held at 1-2 Torr above resting; and PETO2 at 100 Torr for 10 min, 50 Torr for 20 min, and 100 Torr again for 10 min. In protocol B, PETCO2 was held at 8 Torr above resting, and PETO2 varied as in protocol A. Protocol C formed a control. Each protocol was repeated at least 6 times on each subject. The data were used to evaluate four different models (models 2-5) for incorporating the depressant effect of hypoxia into a standard model (model 1) of the chemoreflex responses. In model 2, hypoxic depression was incorporated as an additive term independent of the central and peripheral chemoreflexes; in model 3 it affected the central chemoreflex gain; in model 4 it affected the peripheral chemoreflex gain; and in model 5 it affected the gain of both reflexes. From this, it was concluded only model 4 was consistent with the data; all other models were inconsistent.

On a pseudo-rebreathing technique to assess the ventilatory sensitivity to carbon dioxide in man

1. The ventilatory sensitivity to carbon dioxide obtained from a step-ramp CO2 challenge was compared to the CO2 sensitivity from the steady-state method. 2. Experiments were performed in nine healthy male subjects against a background of hyperoxia and in two subjects against a background of normoxia. 3. In each subject experiments were performed in which the stepwise increase in end-tidal PCO2 above its resting value (A) was varied (range 0-2 kPa) and the subsequent rate of rise of end-tidal PCO2 in time (R) kept constant at 0.6 or 0.8 kPa min-1. 4. The results of the hyperoxic experiments show that the slope of the non-steady-state ventilatory response to CO2 (Sn) is greatly influenced by the magnitude of A. An increase of A of 1 kPa results in a 54% increase of the ratio non-steady-state ventilatory CO2 sensitivity to steady-state ventilatory CO2 sensitivity (Ss). The magnitude of R plays a minor role in determining Sn. The normoxic experiments gave similar results. 5. In experiments performed during hyperoxia Sn approximates Ss when the magnitude of A is 0.5 kPa. 6. The results are discussed and related to a physiological model. Simulations with representative values for the model parameters are in fair agreement with experimental values.

Cardiorespiratory effects induced by acetazolamide on the ventromedullary surface of the cat

1. Inhibition of carbonic anhydrase by acetazolamide in alpha-chloralose-anaesthetized cats, in a region of the brain stem co-extensive with the glycine-sensitive area, intermediate chemosensitive area, and probably C1 catecholaminergic neurones produces hypotension, bradycardia and depression of the central respiratory drive. 2. These responses are concentration dependent, and can still be observed when the enzyme substrate (CO2) is elevated. Therefore, in both the hypercapnic and the normocapnic condition, similar responses in arterial blood pressure, heart rate and respiratory rate are observed when acetazolamide is topically applied to the glycine-sensitive area. 3. To investigate further the contribution of peripheral baro-, chemo- and cardiopulmonary receptors to these responses, acetazolamide was topically applied to the glycine-sensitive area under three different conditions: intact gallamine-paralysed (5 mg kg-1 h-1) and artificially ventilated (A), sinoaortic denervated (B), and sinoaortic denervated plus bilaterally vagotomized cats (C). Under all conditions, similar responses were observed. The fall in arterial blood pressure was 75 +/- 11 (A), 90 +/- 13 (B), and 75 +/- 9 mmHg (C). Changes in heart rate during acetazolamide application were -23 +/- 6, -20 +/- 8, and -26 +/- 6 beats min-1, respectively. The decreases in respiratory rate were 9 +/- 2 (A), 11 +/- 2 (B), and 11 +/- 2 breaths min-1 (C). 4. The data indicate that the responses to topical application of acetazolamide are mainly due to its central action at the glycine-sensitive area and are not influenced by peripheral baroreceptor and chemoreceptor inputs.

The transient ventilatory response to carbon dioxide at rest and in exercise in man

A new technique has been developed to measure the transient response to inhaled CO2 using 30 sec pulses at constant inflow. Multiple experiments are ensemble-averaged in order to define the resulting small signals. We measured the peak changes in ventilation (delta V') and in PCO2 (delta PCO2), taking the ratio (delta V'/delta PCO2) as an index of response. Six healthy volunteers performed experiments at rest, 50 W and 100 W exercise. Three runs, each containing three pulses, were performed at each workload and subsequently averaged. Analysis of variance showed no significant difference between successive pulses or among subjects. delta V' did not differ significantly with workload, but delta PCO2 was progressively smaller as workload increased, and hence the response, delta V'/delta PCO2, greater. The delay between the rise in PCO2 and the rise in ventilation was also progressively shorter as workload increased, being 16-18 sec at rest, 7-13 sec at 50 W, and 3-6 sec at 100 W. Our results suggest that there is increased sensitivity to CO2 in exercise, which may be due to progressive activation of the peripheral chemoreceptors as work load increases. The delay at rest is too long for the peripheral chemoreceptor. Therefore, with these small stimuli, the central chemoreceptor must account for the CO2 response at rest.

A dynamic analysis of the ventilatory response to hypoxia in man

1. The dynamics of the ventilatory response to isocapnic hypoxia were studied in seven healthy subjects using four different levels of hypoxia, (inspired oxygen pressures, PI,O2 equal to 110, 100, 80 and 60 mmHg) successively increasing and decreasing stepwise. 2. Five such progressions were performed for each subject, corresponding to five different durations of the steps (t) ranging between 0.33 and 5.00 min. The overall duration of one test (T) was taken as the sum of the seven successive PI,O2 hypoxic steps (t) plus one step t of air breathing. Thus, the values of T ranged between 2.6 and 40.0 min. 3. End-tidal CO2 pressure was maintained constant (+/- 1 mmHg) throughout the test by manipulation of inspired CO2 pressure. 4. We measured, as a function of T, (i) the magnitude of the loops formed by the ventilatory response curves (PA,O2-VE) as measured by their surface area (S), (ii) the magnitude of ventilatory response to each rising hypoxic step, and (iii) the difference between resting VE and VE observed at PA,O2 equal to 50 mmHg (delta V50). On average, we found one maximum in absolute value of S at T = 8 min and one minimum at T = 12 min, along with two maxima of ventilatory response at T values of 8 and 24 min. 5. The same measurements were made on tidal volume response curves (PA,O2-VT) and ventilatory frequency response curves (PA,O2-f): on average we observed two non-significant peaks in the progression with T of VT and S(VT) and two significant peaks in that of delta VT,50 for T = 8 and T = 24 min. No significant peak was observed in the progression with T of f curve parameters. 6. These results are discussed together with the current dynamic model of the ventilatory control system, which includes a central neural controller with no dynamics of its own and a linear response to chemoreceptor inputs. We discuss the physiological meaning of a negative loop area in relation to the previously described depressant effect of hypoxia upon the brain stem. 7. We conclude that the dynamics of the controlling neuronal network are responsible for the observed singularities which result from differential sensitivity properties of the controller. We propose the existence of discrete excitatory states of the controller as a possible explanation of the shape of the steady-state response curve to hypoxia and of the loop variations.

Evidence for interaction between the contributions to ventilation from the central and peripheral chemoreceptors in man

1. The question of whether there is any interaction between the peripheral and central chemoreceptor contributions to ventilation in man has been addressed. 2. Subjects were exposed to an end-tidal PCO2 of ca. 10 Torr above resting for 8 min at an end-tidal PO2 of 100 Torr. The end-tidal PCO2 was then reduced to near eucapnia. This provided a period of time when the PCO2 at the peripheral chemoreceptors would be near eucapnia, but would still be raised at the central chemoreceptors. 3. Against the background above, the effect of an hypoxic end-tidal step from a PO2 of 100 Torr to a PO2 of 50 Torr was studied, and compared with the effect of the same step when both sets of chemoreceptors were near eucapnia. 4. Three subjects were studied, each contributing twelve sets of data to each of the three protocols required for the comparisons. 5. In two of the three subjects, the ventilatory response to hypoxia was augmented when central PCO2 was high. 6. The results support the idea that there is an interaction between the central and peripheral chemoreceptors in man. The consequences of this and other possible interpretations of the results are discussed.

A dynamic analysis of the ventilatory response to carbon dioxide inhalation in man

1. The dynamics of the ventilatory response to carbon dioxide inhalation were studied in ten healthy young men using four different inspired fractions of carbon dioxide (FI, CO2) in air (0.015, 0.030, 0.045 and 0.060) successively increasing and decreasing stepwise. 2. Seven such different progressions were performed for each subject and each of seven different durations of the steps (t) ranging between 0.1 (i.e. one ventilatory cycle) and 10 min ('steady-state' conditions). The overall duration of one test (T) was taken as the sum of the seven successive FI, CO2 steps (t) plus one step, t, of air breathing. Thus, the values of T ranged between 0.8 (i.e. eight ventilatory cycles) and 80 min. Three subjects were tested twice. 3. We measured, as a function of T, the magnitude of the loops formed by the curves PA, CO2-VE and the value of the highest ventilatory response (VE max) to each progression. For all ten subjects, both functions had two maxima, one for T values of 2.6 or 8.0 min and one for T values of 24 or 40 min, and one minimum at T equal to 12 min. 4. The same measurements were made on tidal volume-response curves (PA, CO2-VT) and ventilatory frequency-response curves (PA, CO2-f) and yielded the same results except for the ventilatory frequency-response curves, for which we only found a statistically insignificant single maximum for T values of 24 or 40 min. 5. The locations of the maxima in loop magnitude and VE max were similar in duplicate tests in three subjects, whereas the quantitative values of these variables showed wide differences. 6. We compared our results with what is expected from the current linear dynamic model of ventilatory control submitted to the same forcing function: the first maximum in the loop magnitude is predicted by the model, but the second is not. The model shows no peak in the evolution of VE max. 7. We conclude that controlled system dynamics, which are the only ones included in dynamic models of ventilatory control, cannot by themselves account for our observations, and that one should take into consideration the dynamics of the controlling neuronal network.

CO2 sensitivity in humans breathing 1 or 2% CO2 in air

Ventilation increases when the concentration of CO2 in the inspired gas is increased, thereby limiting the increase in alveolar and arterial PCO2. The extent of this compensation at low levels of inspired CO2 has been debated. In five healthy humans, we have measured arterial PCO2, arterial pH and ventilation during exposure to 1 and 2% CO2 in the inspired gas. Each exposure lasted at least 7 min and arterial blood was sampled over at least 30 s during the last minute of each period. The ventilation was measured in the sixth and seventh min. The protocol included the sequences: control-test-control and test-control-test with 'test' representing CO2 loading and 'control' 0% CO2, respectively. We found that arterial PCO2 increased and pH decreased at both levels of inspired CO2. The mean increase in arterial PCO2 was 0.09 and 0.25 kPa, at CO2 1 and 2%, respectively. Three subjects were exposed to 1% CO2 in the inspired gas for 28 min flanked by similar control periods. In each period arterial blood samples were taken at 2- or 3-min intervals. Arterial PCO2 remained elevated for at least 20 min during the CO2 loading. The sensitivity to CO2 (ratio of increase in ventilation to increase in arterial PCO2) was within the range described by others at higher levels of inspired CO2. Arterial PCO2 increased by about 10% of the imposed load. We conclude that the increase in ventilation provides only incomplete compensation for exposure to CO2: arterial CO2 is increased and arterial pH decreased also at very low levels of inspired CO2.

Controlled study of respiratory responses during prolonged measurement in patients with chronic hyperventilation

The respiratory responses of 17 patients with chronic hyperventilation but without demonstrable organic disease (group H) to various manoeuvres were compared with those of 21 healthy controls (group C). The responses were tested according to a 60 min protocol in which periods of rest were replaced by exercise, voluntary hyperventilation (VHV), reading, and CO2 inhalation. 5 patients with severe resting hypocapnia were investigated overnight during sleep. Chronic hyperventilation was of two types--persistent or provoked by exercise or VHV. It was due to modest increases in tidal volume and respiratory frequency but was generally not conspicuous. End-tidal PCO2 levels were gradually corrected to near normal during sleep but not by inhalation of CO2.