Effects of hypoxia, hypercapnia and changes in body temperature on the pattern of breathing in cats

The human ventilatory response to stress: rate or depth?

Many stressors cause an increase in ventilation in humans. This is predominantly reported as an increase in minute ventilation (V̇E). But, the same V̇E can be achieved by a wide variety of changes in the depth (tidal volume, V_T ) and number of breaths (respiratory frequency, ƒ_R ). This review investigates the impact of stressors including: cold, heat, hypoxia, pain and panic on the contributions of ƒ_R and V_T to V̇E to see if they differ with different stressors. Where possible we also consider the potential mechanisms that underpin the responses identified, and propose mechanisms by which differences in ƒ_R and V_T are mediated. Our aim being to consider if there is an overall differential control of ƒ_R and V_T that applies in a wide range of conditions. We consider moderating factors, including exercise, sex, intensity and duration of stimuli. For the stressors reviewed, as the stress becomes extreme V̇E generally becomes increased more by ƒ_R than V_T . We also present some tentative evidence that the pattern of ƒ_R and V_T could provide some useful diagnostic information for a variety of clinical conditions. In The Physiological Society's year of 'Making Sense of Stress', this review has wide-ranging implications that are not limited to one discipline, but are integrative and relevant for physiology, psychophysiology, neuroscience and pathophysiology.

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Ventilatory responses to changes in temperature in mammals and other vertebrates

This article reviews the relationship between pulmonary ventilation (VE) and metabolic rate (oxygen consumption) during changes in ambient temperature. The main focus is on mammals, although for comparative purposes the VE responses of ectothermic vertebrates are also discussed. First, the effects of temperature on pulmonary mechanics, chemoreceptors, and airway receptors are summarized. Then we review the main VE responses to cold and warm stimuli and their interaction with exercise, hypoxia, or hypercapnia. In these cases, mammals attempt to maintain both oxygenation and body temperature, although conflicts can arise because of the respiratory heat loss associated with the increase in ventilation. Finally, we consider the VE responses of mammals when body temperature changes, as during torpor, fever, sleep, and hypothermia. In ectotherms, during changes in temperature, VE control becomes part of a general strategy to maintain constant relative alkalinity and ensure a constancy of pH-dependent protein functions (alphastat regulation). In mammals on the other hand, VE control is aimed to balance metabolic needs with homeothermy. Therefore, alphastat regulation in mammals seems to have a low priority, and it may be adopted only in exceptional cases.

Volume-timing relationships during cough and resistive loading in the cat

The relationship between pulmonary volume-related feedback and inspiratory (CTI) and expiratory (CTE) phase durations during cough was determined. Cough was produced in anesthetized cats by mechanical stimulation of the intrathoracic tracheal lumen. During eupnea, the animals were exposed to single-breath inspiratory and expiratory resistive loads. Cough was associated with large increases in inspiratory volume (VI) and expiratory volume (VE) but no change in phase durations compared with eupnea. There was no relationship between VI and CTI during coughing. A linear relationship with a negative slope existed between VI and eupneic inspiratory time during control and inspiratory resistive loading trials. There was no relationship between VE and CTE during all coughs. However, when the first cough in a series or a single cough was analyzed, the VE/CTE relationship had a positive slope. A linear relationship with a negative slope existed between VE and eupneic expiratory time during control and expiratory resistive loading trials. These results support separate ventilatory pattern regulation during cough that does not include modulation of phase durations by pulmonary volume-related feedback.

Central histamine contributed to temperature-induced polypnea in mice

Breathing pattern is influenced by body temperature. However, the central mechanism for changing breathing patterns is unknown. Central histamine is involved in heat loss mechanisms in behavioral studies, but little is known about its effect on breathing patterns. We examined first the effect of body temperature on breathing patterns with increasing hypercapnia in conscious mice and then that of the depletion of central histamine by S(+)-alpha-fluoromethylhistidine hydrochloride (alpha-FMH) (100 mg/kg ip), a specific inhibitor of histidine decarboxylase, at normal and raised body temperatures. A raised body temperature increased respiratory frequency with reductions in both inspiratory and expiratory time and decreased tidal volume. On the other hand, alpha-FMH lowered respiratory frequency with a prolongation of expiratory time at the raised temperature; however, this was not observed at a normal temperature. These results indicate that central histamine contributes to an increase in respiratory frequency as a result of a reduction in expiratory time when body temperature is raised.

Hering-Breuer reflex in conscious newborn rats: effects of changes in ambient temperature during hypoxia

In a previous study in conscious normoxic newborn rats, we found that the strength of the Hering-Breuer reflex (HB reflex) was greater (188%) at high (36 degrees C) than at low (24 degrees C) ambient temperature (T(a); D. Merazzi and J. P. Mortola. Pediatr. Res. 45: 370-376, 1999). We now asked what the effect would be of changes in T(a) during hypoxia. Rat pups at 3-4 days of age were studied in a double-chamber airflow plethysmograph. The HB reflex was induced by negative body surface pressures of 5 or 10 cmH(2)O and quantified from the inhibition of breathing during maintained lung inflation. Rats were first studied at T(a) = 32 degrees C in normoxia, followed by hypoxia (10% O(2) breathing). During hypoxia, oxygen consumption (VO(2)) averaged 47%, and HB reflex 115%, of the corresponding normoxic values, confirming that in the newborn, differently from the adult, hypoxia does not decrease the strength of the HB reflex. As hypoxia was maintained, lowering T(a) to 24 degrees C or increasing it to 36 degrees C, on average, had no significant effects on VO(2) and the HB reflex. However, with 5-cmH(2)O inflations, the HB reflex during the combined hypoxia and hyperthermia was significantly stronger than in normoxia. We conclude that in conscious newborn rats during normoxia the T(a) sensitivity of the HB reflex is largely mediated by the effects of T(a) on thermogenesis and VO(2); in hypoxia, because thermogenesis is depressed and VO(2) varies little with T(a), the HB reflex is T(a) independent. The observation that the reflex response to lung inflations during hypoxic hyperthermia can be greater than in normoxia may be of importance in the pathophysiology of apneas during the neonatal period.

Effects of changes in ambient temperature on the Hering-Breüer reflex of the conscious newborn rat

We questioned to what extent changes in temperature could affect the newborn's ventilatory inhibition provoked by lung inflation, or Hering-Breüer (HB) inflation reflex. Conscious newborn rats (3-4 d old) were studied in a double chamber airflow plethysmograph at ambient temperatures of 32 degrees C (slightly below their thermoneutrality), 24 degrees C (cold), and 36 degrees C (warm). At these ambient temperatures, the corresponding body temperatures averaged 35.4, 31.0, and 37 degrees C. The HB reflex was triggered by negative body surface pressures of 5 or 10 cm H2O, and quantified as the duration of the expiratory time during the maintained inflation, either in absolute values or in relation to the control expiratory time. In cold the HB reflex decreased to 80%, and in warm it increased to 150%, of the values measured at 32 degrees C. Oxygen consumption, measured by an open flow system, averaged 59, 47, and 29 mL x kg(-1) x min(-1) at, respectively, 24, 32, and 36 degrees C. In cold, the ventilatory response to hypoxia (10% O2) was almost absent, that to hypercapnia (5% CO2) was greater, and that to hypoxia and hypercapnia combined was less than in warm. We conclude that in newborn rats the strength of the vagal inhibition on breathing, evaluated in the form of the HB reflex, is sensitive to temperature, becoming stronger as temperature increases. One contributing factor is the temperature-induced change in metabolic rate, whereas the role of temperature-induced changes in ventilatory chemosensitivity is variable. The strong temperature-dependency of the neonatal HB reflex implies that in newborns exposed to a warm environment breathing is more susceptible to inhibitory inputs.

The effect of a rise in body temperature on the central-chemoreflex ventilatory response to carbon dioxide

We assessed the central-chemoreflex ventilatory responses to carbon dioxide in six male volunteers using a hyperoxic rebreathing technique. Hyperventilation prior to rebreathing allowed both the threshold and the sensitivity of the response to be measured. We used immersion in water to control the body temperature (tympanic). The water temperature was adjusted to be either thermo-neutral or hot so that body temperature either remained normal [+0.2 (0.04) degrees C, mean (SEM)] or was elevated by 1.5 (0.08) degrees C. The sensitivities of the central-chemoreflex ventilatory responses to carbon dioxide were increased at elevated body temperatures, changing from a mean of 1.8 (0.2) 1.min-1. Torr-1 to 2.7 (0.1) 1.min-1. Torr-1. However, the thresholds did not change with temperature, and the mean threshold was 48(1) Torr at both normal and elevated temperatures. For all of the volunteers, ventilation was increased at elevated body temperatures for all levels of carbon dioxide, mainly by changes in respiratory frequency due to reductions in expiratory times. At subthreshold levels of carbon dioxide, mean ventilation changed significantly from 6.3(1.1) 1.min-1 at normal temperatures to 10.8 (1.9) 1.min-1 at elevated temperatures. Heart rates also increased significantly with temperature, changing from a mean of 66 (4) beats.min-1 to 102 (3) beats.min-1 at threshold levels of carbon dioxide. The mean rates of rise of carbon dioxide partial pressure during rebreathing were significantly increased with temperature as well, changing from 0.075 (0.008) Torr.min-1 to 0.089 (0.004) Torr.min-1. We concluded that elevating the body temperatures of our subjects not only increased their ventilation, heart rates and metabolic rates at all levels of carbon dioxide, but it also increased the sensitivity of their central chemoreflex ventilatory responses to carbon dioxide. Despite these increases, the thresholds of the central-chemoreflex ventilatory responses to carbon dioxide remained unchanged.

[Respiratory effects of moderate hypothermia (36 degrees C-28 degrees C) in dogs under halothane anesthesia]

Changes in systemic haemodynamic variables (mean arterial pressure, MAP; heart rate, HR; cardiac output, Qc), in oxygen consumption, VO2, and in ventilation (minute ventilation, V; respiratory frequency, f; tidal volume, VT; and arterial blood gases) with particular attention to respiratory times (duration of inspiration, TI; duration of expiration, TE; duration of the breathing cycle, TTOT), to respiratory timing (TI/TTOT) and respiratory drive (VT/TI) were studied during moderate progressive hypothermia (36 degrees C to 28 degrees C) during stable halothane anaesthesia (MAC = 1.5) in six dogs. MAP, HR and Qc decreased; V and f decreased, the decrease in f being correlated with that in temperature (r = 0.66; P < 0.01). Tidal volume did not change. The PaO2 and pHa decreased while PaCO2 increased slightly. The decrease in ventilation was related to changes in respiratory times (TI and TE) which increased (TE more than TI) and in respiratory drive (VT/TI which decreased due to the increase in TI). The relation between VT/TI and TI/TTOT changes was not constant during cooling. Changes in respiratory times and drive could be due to the effect of cold on medullar respiratory control.

Thermal effects on ventilation in cats: participation of carotid body chemoreceptors

In pentobarbitone anesthetized cats, raising body temperature from 37 to 40 degrees C by external heat increased respiratory frequency, tidal volume, frequency of spontaneous gasps and mean inspiratory flow. It reduced end-tidal CO2 pressure, together with inspiratory and expiratory durations. After bilateral section of the carotid nerves, raising body temperature still induced hyperventilation, but the increase in gasp frequency was less pronounced and no significant change in tidal volume was observed. In comparison to steady ventilatory values in the intact condition, significant reductions in tidal volume at 38 degrees C and in gasp frequency at 37, 39 and 40 degrees C were observed after bilateral carotid neurotomy. Brief hyperoxic tests induced transient decreases in tidal volume and increases in end-tidal CO2 pressure which were significantly larger at 40 degrees C than at 37 degrees C. These changes disappeared after bilateral carotid neurotomy. Anesthetic block of both carotid nerves produced transient reductions in tidal volume at any given temperature. We conclude that carotid body afferents contribute to the hyperventilation evoked by hyperthermia. After their interruption, such contribution is replaceable from other thermal afferents.

Diaphragm length and breathing pattern changes during hypoxia and hypercapnia

In this study diaphragmatic length changes were measured during quiet breathing and during augmentation of breathing with hypoxia and hypercapnia in supine anesthetized dogs. The breathing pattern and the VT-TI relationship during hypoxia were different than those during hypercapnia. The crural diaphragm shortened more than the costal diaphragm with both stimuli, and the amount of shortening in relation to the tidal volume implied that there was considerable distortion of the chest wall during hyperventilation. The velocity of shortening of both parts of the diaphragm at similar levels of ventilation was greater during hypoxia than hypercapnia. The velocities found with hyperventilation suggested that force-velocity considerations did not reduce force generation. Hypoxic stimulation resulted in a reduction in the resting length of both parts of the diaphragm, and was associated with a positive shift in baseline pleural pressure which implied gas trapping. The large tidal diaphragmatic shortening found with augmented breathing and the shorter resting length with hypoxia indicated that length-force properties are important in force generation.

Ventilatory recovery from hypothermia in anesthetized cats

Ventilation and breathing pattern were recorded in a group of seven anesthetized cats during rewarming from 24 to 38 degrees C of esophageal temperature. It was found that at 24 degrees C, ventilation was very much depressed accounting for an alveolar hypoventilation resulting in hypoxia and hypercapnia. During rewarming, ventilation increased steadily; this was caused by sequential changes in central inspiratory activity (VT/Ti) and Ti/Tt ratio reflecting breath timing. Changes in VT/Ti have been initially attributed to an improvement in chemoresponsiveness and subsequently, to an involvement of supra-pontine thermoregulatory control areas during rewarming. Marked changes in breath timing, especially observed between 28 and 34 degrees C, have been attributed to a direct effect of rewarming upon the brain stem respiratory network. It has the result, that during hypothermia, several components of the respiratory control system are differently affected causing marked changes in breathing pattern and ventilation. They are accompanied by modifications in arterial blood pressure and heart rate.

Verification of a model for the mechanisms controlling expiratory duration in rabbits under various conditions

A mathematical model for the mechanisms controlling expiratory (E) duration was verified through comparison of its prediction with the relationships between TE and waveform parameters of various input patterns obtained by bilateral, simultaneous electrical stimulation of the largest vagal afferent fibers in vagotomized, paralyzed rabbits at normal or elevated chemical drive under light or deep barbiturate anesthesia. The model accurately reproduced experimental results in each animal and condition; it suggests that in the absence of vagal input (v.i.) an exponentially decaying central activity (phi) prevents inspiratory (I) onset until a time independent threshold (phi thr) is reached; the vagal component of phi results from fixed gain, long time constant (tau = 1.07 +/- 0.03 sec; mean +/- SE for N = 37 for N = 37 animals) temporal summation of expiratory v.i., being unaffected by inspiratory v.i.; central and vagal components share a common mechanism, the same value of tau applying to v.i. integration and to phi decay in the absence of v.i. under all conditions; the amplitude of the central component is proportional to tidal changes in central I activity, independently of the condition studied; and phi thr increases at elevated chemical drive, is unaffected by v.i., and decreases under deep anesthesia.

Effects of body temperature, passive limb motion and level of anesthesia on the activity of the inspiratory muscles

The relationships between relative tidal activity (moving average EMG) of the diaphragm (AdiT) and of the external intercostal or parasternal muscles (AicT) and between the rate of rise of these activities (Adi and Aic) were assessed during rebreathing in rabbits with various body temperatures (BT: 34-41 degrees C) before and after vagotomy (VGT), at rest and during passive limb motion (PLM), and in vagotomized rabbits with or without thoracic dorsal rhizotomy (TDR) under light (LBA) or deep barbiturate anesthesia (DBA). Both relationships had the form AicT = a AdiTb and Aic = a' Adib'. In intact normothermic animals under LBA mean values for b and b' were 1.47 and 1.37, a and a' being unity by definition. No changes in b or b' occurred even with TDR: this suggests that the relation between the central command to phrenic and to inspiratory intercostal alpha-motoneurones was the same under all conditions. Neither BT nor PLM modified a', but a changed owing to BT and PLM dependence of the relation between central inspiratory drive and off-switch threshold. Both VGT, independently of BT, and DBA decreased a and a' before but not after TDR, when a and a' reached the lowest values (0.12 and 0.22). Hence VGT and DBA, but not BT and PLM, change the relation between the central command to inspiratory intercostal alpha- and gamma-motoneurones, the multiplicative effect of alpha-gamma linkage on AicT and Aic being prevented by TDR.

Effect of temperature on the ventilatory response curve to carbon dioxide in anaesthetized cats

Effects of body temperature on the ventilatory control system were studied in 17 anaesthetized cats. At different body temperatures (stabilized within 0.1 degrees C) CO2 response curves were measured in each cat. In with chloralose-urethan anaesthetized cats it was found that in the body temperature range of 34-40 degrees C, in which neither shivering nor panting occurred, no statistically significant trend with temperature was found in the slope (S) and the extrapolated intercept on the PaCO2-axis (B) of the linear CO2 response curve during hyperoxia as well as hypoxia. In two with pentobarbital anaesthetized cats similar results were obtained. The resting ventilation (at FICO2 = 0) did not change significantly, while the resting PaCO2 during hyperoxia showed a trend to increase with temperature just reaching the level of significance (P less than 0.05). Breathing frequency increased significantly with temperature (P less than 0.0005). When body temperature was elevated above 41 degrees C both the slope (S) and the intercept of the CO2 response curve (B) decreased. In three cats ventriculo-cisternal perfusion was performed and no apparent influence of body temperature was found on the relation between the PCSFCO2 and PETCO2 and on the VE-PCSFCO2 response curves. These findings show that body temperature has no important modifying effect on the ventilatory response to CO2 in anaesthetized cats in the temperature range of 34-40 degrees C.

Breathing movements in fetal lambs and the effect of hypercapnia

1. A breath-by-breath analysis was made of tracheal pressure records from fetal lambs in utero at 117--130 days gestation. 2. Episodes of rapid irregular breathing were accompanied by diaphragmatic e.m.g. activity and were characterized by a gradual increase in breathing rate initially and a terminal decline. Within each episode breathing was very irregular. 3. Inspiratory time increased with increasing expiratory time rapidly as the rate fell to about 80/min, and thereafter more slowly. 4. The principal effects of hypercapnia (a rise in fetal Pa, CO2 by approximately 11 mmHg) were to increase the depth of fetal breathing, by nearly twofold, and to make breathing much more regular. A change in rate was doubtful because of the skew distribution of normal breath intervals.

Central respiratory effects of carbon dioxide, and carotid sinus nerve and muscle afferents

1. In anaesthetized and paralysed cats, artificially ventilated and with the vagi and carotid sinus nerves cut, the effects of (a) changing end-tidal CO2, (b) stimulation of carotid sinus nerves, and (c) squeezing the calf muscles, on neural respiratory output have been studied. 2. The peak value of integrated phrenic activity, which is the neural equivalent of tidal volume (VTn), the rate of rise of integrated phrenic activity (Slope), inspiratory duration (TI), expiratory duration (TE), and total cycle length (Ttot) were measured. It was found that Slope-VTn, VTn-Ttot and VTn-TE relationships were similar whether respiration was stimulated by carotid sinus nerve stimulation or by increasing end-tidal CO2. However, when respiration was stimulated by squeezing the calf muscles, a given Slope was associated with a smaller VTn and a given VTn was associated with smaller Ttot and TE values than obtained with the other two inputs. 3. It is concluded that these results are compatible with the hypotheses that (1) inputs from central and peripheral chemoreceptors have similar actions at various sites in the respiratory centre complex, and (2) afferents activated by squeezing the limb muscles have additional facilitatory actions on mechanisms terminating inspiration and expiration.

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

1. The pattern of breathing during the approach to the steady state following step changes of end-tidal P(CO2) and P(O2) has been determined in normal conscious human subjects. Three types of step were studied: (a) steps of P(A, CO2) against a constant background of hyperoxia (P(A, O2) approximately 200), an almost pure intracranial chemoreceptor stimulus, (b) steps of P(A, O2) between approximately 50 and 80 torr against a background of constant mild hypercapnia, an arterial chemoreceptor stimulus, and (c) steps of P(A, CO2) against a background of constant hypoxia (P(A, O2) approximately 50), a mixed stimulus. Steps were small and the responses barely detectable by the subjects.2. Steps of CO(2) in hyperoxia produced the slowest approach to the steady state. A single exponential fitted the ventilation response up to about 4 min (mean half time 83 sec for the ;up' and 69 sec for the ;down' transients). During the transient the pattern of change of tidal volume (V(T)) and expiratory time (T(E)) was the same as in the steady state. Inspiratory time (T(I)), however, in the early part of the transient, changed in the opposite direction to T(E), returning to its steady value only after 1(1/2)-3 min. This effect occurred in both ;up' and ;down' transients and resulted in a smaller change of respiratory frequency than would have been predicted from the steady-state response.3. Hypoxic steps produced the fastest approach to the steady state with mean half-times for ventilation of 10.9 sec for the ;up' transients and 6.6 sec for the ;down'. T(I) followed the same pattern during the transient as in the steady state, whereas T(E), following the step out of hypoxia, lengthened to far beyond its final steady value within five breaths of the step, only returning to its steady-state value 3-4 min after the step. This resulted in an exaggerated change of frequency during the early part of the transient.4. Steps of CO(2) in hypoxia, a mixed peripheral and central chemoreceptor stimulus, showed a ventilation response which was best fitted by two exponentials, the half-times of which were consistent with those obtained for the separate responses. The patterning was also consistent with a mixed response, more so for T(I) than for T(E).5. The steady-state pattern derived from the pre-switch means was consistent with the pattern previously described.6. Possible mechanisms are discussed. It is suggested that these results could explain the different patterns seen in the past by those using re-breathing and steady-state techniques.7. The validity of using one or two breath oxygen or nitrogen tests (or other similar tests) as a quantitative measure of the hypoxic response in man is questioned.

Experimentally induced Cheyne-Stokes breathing

We have studied the propensity for periodic breathing to occur in cats anaesthetized with pentobarbitone breathing either spontaneously or with the aid of a 'servo-respirator' governed continuously by the efferent phrenic nerve activity. Sustained periodic breathing could be induced increasing 'controller gain', either by increasing the gain of the respirator, or by lung deflation, which reflexly increased controller responses to both hypoxia and hypercapnia. Periodic breathing was potentiated both by hypoxia and by diminishing the central (CO2, H+)-drive by focal cooling at the ventral surface of the medulla, two procedures which increase the relative influence of hypoxic drive. Less hypoxia was needed to produce periodic breathing at high rather than low controller gains. Reducing controller gain to zero by constant artificial respiration always abolished periodic breathing. Periodic breathing was also eradicated when the relative importance of CO2 drive was enhanced by breathing the cats with CO2-enriched gas mixtures or with 100% O2. The results are consistent with theoretical predictions for the occurrence of oscillations in the mechanisms for the chemical control of breathing and indicate that increasing controller gas can produce periodic breathing. The results further emphasize the importance of the (CO2, H+)-drive in preserving ventilatory stability.

Behavior of expiratory neurons in response to mechanical and chemical loading

The response of medullary expiratory neurons to added mechanical and chemical loads was studied in anesthetized cats. Alterations in burst characteristics and central timing were compared in the intact and bilaterally vagotomized cat. The following results were obtained: (1) Graded expiratory airflow resistances caused progressive increases in burst duration, spikes per burst and firing rate; similar effects were noted for end-inspiratory tracheal occlusions and continuous positive breathing; all facilitation was eliminated by vagotomy. (2) Graded inspiratory airflow resistances delayed the onset of an expiratory burst but did not change the overall burst characteristics. (3) Acute hypercapnia increased ventilation without noticeable changes in expiratory burst characteristics; acute hypoxia produced a reduction in burst duration concomitant with changes in ventilation. It is concluded that (1) expiratory neurons are responsive to vagally mediated volume information and (2) transient hypoxia and hypercapnia sufficient to increase ventilation does not increase the firing rate of expiratory neurons but exerts differential effects with respect to timing. It is suggested that expiratory duration is related to the time integral of expired volume and that the increase in FRC imposed by expiratory loads does not alter the central timing of the next inspiration.

Central and direct vagal dependent control of expiratory duration in anaesthetized rabbits

In anaesthetized rabbits, total or partial (only inflation reflex nearly abolished) DC current vagal block was performed during inspiration (ITB and IPB), or expiration (ETB and EPB), or throughout the breathing cycle (CTB and CPB). During CTB inspiratory (Ti) and expiratory duration (Te) increased as after vagotomy. With ITB Ti equally Tivag; Te increased, but remained shorter than Tevag. During ETB, Ti was unchanged, Te increased, but remained shorter than Tevag. The sum of deltaTe during ITB and ETB equalled deltaTevag. During CPB and IPB, Ti and Te behaved as during ITB. With EPB, Ti was unchanged and Te shortened. Preferential stimulation of large myelinated fibers in the central vagal stumps during expiration lengthened Te. Inspiratory stimulation shortened both Ti and Te, restored breath timing of ETB, but not that of pre-vagotomy control. Hence, Te of eupneic breaths should depend on a central mechanism relating Te to preceding Ti and on expiratory vagal discharge, having both a small lengthening (from stretch receptors) and a variable shortening effect (from irritant receptors). Both central and peripheral mechanisms are affected by CO2 breathing.

Lung reflexes in rabbits during pulmonary stretch receptor block by sulphur dioxide

Anaesthetized rabbits were given 200 ppm sulphur dioxide to breathe for 10 min. This abolished activity in 23 of 26 pulmonary stretch receptors, while leaving that of lung irritant receptors unimpaired. The Breuer-Hering reflex was abolished and breathing became deeper and slower. Inspiratory time (tI) was increased and expiratory time (tE) decreased. Subsequent vagotomy increased tidal volume (VT), tI and tE. In animals with stretch receptors blocked, injections of phenyl diguanide and histamine still increased breathing frequency and decreased VT, indicating that reflexes from lung irritant and J-receptors were intact. Inhalation of 8% CO2 caused a bigger increase in frequency and tidal volume in rabbits with stretch receptor block compared with controls or those after vagotomy. Induction of pneumothorax with stretch receptor block transiently prolonged tI and shortened tE; removal of the pneumothorax also transiently shortened tE and usually also decreased tI. The results suggest that lung irritant receptors reflexly shorten tE in all our experimental conditions, but have various effects on tI which may depend on the timing of the irritant receptor discharge and refractoriness of the inspiratory response.

The relation between tidal volume and inspiratory and expiratory times during steady-state carbon dioxide inhalation in man

1. Ambiguities and discrepancies in the published descriptions of the patterns of breathing in man have been re-investigated.Steady-state hyperpnoea during rest was induced in normal subjects of both sexes by means of CO(2) inhalation, usually in high O(2), but sometimes in low (P(A, O2) approximately 200 and 55 torr respectively).2. The relations between mean tidal volume V(T) and mean times for inspiration T(I) and expiration T(E) were satisfactorily divided into lower and upper parts (ranges 1 and 2) in nineteen out of thirty-three experiments using an objective least-squares method, and polynomials were fitted separately to each of the following pairs of variables V(T,lower), T(I); V(T,lower), T(E); V(T,upper), T(I); V(T,upper), T(E).3. The breakpoints occurred when V(T) was about one third of vital capacity, but there was much variation between subjects.4. In range 2, V(T) was inversely related to both T(I) and T(E) and there appeared to be linkage between T(I) and T(E), all as described by Clark & Euler (1972). The relation suggested by Hey, Lloyd, Cunningham, Jukes & Bolton (1966) over range 2 was not confirmed.5. The main part of the relation described by Hey et al. (1966) is concerned with range 1. Here the relation between V(T) and T(I) was variable; slight, but significant negative correlation predominated. V(T) was, on the other hand, strongly and inversely related to T(E), and plots of (T(E) - T(I)) against V(T) over both ranges showed even less scatter. Hence (a) T(E) is probably related to V(T) through two mechanisms, one involving a link with T(I) and another prominent one being quite independent of T(I), (b) most changes of respiratory frequency in normal man are brought about by change of T(E) rather than of T(I), and (c) there is no region of constant frequency as implied by Clark & Euler (1972).

Reduction in inspiratory activity in response to sternal vibration

It has been shown previously that there is a reduction in tidal volume in response to longitudinal sternal vibration at 100 Hz. In the present study it was shown that the effect of such vibration is to reduce tidal volume (VT) and prolong inspiratory time (ti) in such a way that points from vibrated and non-vibrated breaths fall on the same VT:ti curve. This indicates that the normal mechanisms which terminate inspiration are unaffected by vibration. The effect of vibration is simply to reduce the rate at which inspiration proceeds. This was illustrated here when vibration reduced the rate of fall of intrapleural pressure during inspiration, and also reduced the instantaneous ventilation at any level of chemical drive. Electrophysiological recordings made here from phrenic motoneurones support these findings. It is concluded that sensory nerves in the chest wall, which can be excited by vibration, can inhibit inspiration.

Separation of the inspiratory and expiratory reflex effects of alternate-breath oscillation of PACO2 during hypoxia

Four healthy young men and women, selected for the responsiveness and steadiness of their breathing, were studied in rest and mild exercise (58 runs) while receiving alternate inspirates of low and high PCO2 (0 and 8.6 kPa). PACO2 oscillated between ca. 6 and 7.5 kPa (45-55 torr); PACO2 was held steady at more than one level between 6 and 9.6 kPa (45-72 torr). Using cross-correlation analysis, the phase relations were determined between the alternating PACO2 and the following reflex outputs: mean inspiratory and expiratory flows (VI and VE) and the reciprocal of the duration of expiration (1/TE), the two expiratory variables being lumped together for purposes of expression, but not of calculation. T1, being relatively unaffected alternating PACO2, was not re-studied (see companion paper). The common patterns of significant reflex alternation were: VI alone, usually in phase (with PACO2), 24%; VE alone, usually in phase, 17%; both inspiratory and expiratory variables, in phase with CO2 and each other, 15%; both inspiratory and expiratory variables, the expiratory being out of phase with both CO2 and with the inspiratory, 23%. Some runs showed a mixture of phase relations. In 71%, end-expiratory lung volume (VL,E' formerly called FRC) alternated significantly. It is concluded that expiratory events can be influenced by peripheral chemoreceptors independently of inspiration.

Interaction between vagal and chemoreceptors afferents in ventilatory response to transient hypercapnia (anaesthetized rabbit)

In rabbits anaesthetized with ethyl-carbamate, stimulation of chemoreceptors afferents was allowed by transient hypercapnia, before and after vagal blockade by DC current. In these relatively fast breathing animals, the transient hypercapnia produced light changes of inspiratory tidal volume (VI), inspiratory (TI) and expiratory durations (TE). Despite the identity of transient hypercapnia, it ensued that: (1) the higher the spontaneous VI and the lower the respiratory frequency (fR), the greater their respective changes (deltaVI and deltafR) during the ventilatory response; (2) after vagal blockade, greater changes in VI, TI, TE and mean inspiratory flow rate (VI/TI) occurred than in control state, while the relation between deltafR and fR was more significant than in control state. Respective roles played by vagal and chemoreceptors afferents in the ventilatory response to transient hypercapnia are discussed.

Respiratory and thermoregulatory responses of rabbits breathing carbon dioxide during heat exposure

1. Rabbits were clipped and exposed in turn to three environmental conditions: control (C), cold exposure (CE) and water deprivation (WD). Following each type of treatment, the rabbits were exposed to an ambient temperature (Ta) of 35 degrees C for 1 hr. Throughout this period they breathed either normal atmospheric air or 6% CO2 in air. 2. During heat exposure, measurements were made of the respiratory responses and of the O2 consumption (Vo2) of the rabbits. Rectal temperature (Tre) was measured immediately before and again immediately after heat exposure. 3. When subjected to cold exposure or water deprivation the rabbits showed an initial decrease in respiratory frequency (RF) and an initial increase in VT when compared with controls. There was no difference in VE. Rabbits breathing 6% CO2 showed an increase in VT and VE and a decrease in RF when compared with rabbits breathing atmospheric air. In all cases a change in VT or RF was associated with a reciprocal change in the other parameter. 4. The respiratory responses to breathing 6% CO2 were essentially similar in treated and control rabbits, from which it is concluded that neither cold exposure nor water deprivation alter the sensitivity of the medullary respiratory centre to the respiratory drive from the central chemosensors. 5. The increase in Tre during heat exposure was significantly less in rabbits breathing 6% CO2 than in rabbits breathing atmospheric air. However, there was no significant over-all difference in VO2 between rabbits breathing CO2 and those breathing air. From this it is concluded that increased ventilation induced by CO2 causes a greater dissipation of heat than does thermally-induced panting. 6. It is concluded that VT is controlled by the level of blood PCO2 whereas RF is controlled by thermoregulatory requirements. It is further concluded that the reciprocal relationship between VT and RF is regulated in such a way as to maintain VE at the appropriate level for effecting gaseous exchange and evaporative heat loss.

Respiratory frequency control during hypercapnia in vagotomized, anesthetized cats

There is a small, but significant, increase in frequency during hypercapnia in vagotomized, anesthetized animals, indicating involvement of an extravagal mechanism in the response. The intent of this study was to determine the source of this second mechanism regulating frequency during hypercapnia. Experiments were performed on 22 vagotomized, anesthetized (Dial) cats. Frequency (f), inspiratory time (ti) and expiratory time (te) responses to CO2 were monitored before and after sectioning of afferent nerves from the carotid bodies (carotid sinus nerve section), chest wall (dorsal rhizotomies, T1-T12) and diaphragm (dorsal rhizotomies. C4-C7). Most vagotomized animals responded to 6% CO2 with an increased frequency, decreased ti and no consistent change in te. The responses to CO2 were essentially unaltered following chest wall and diaphragm deafferentation. Sodium cyanide stimulation of the carotid bodies produced similar respiratory pattern changes as CO2; furthermore, the f and ti changes with CO2 were still present following carotid body deafferentiation. The results of this study suggest that: (1) afferents from chest wall and diaphragm mechanoreceptors are not responsible for the vagal-like effects on ti and f during hypercapnia, (2) afferents from lung mechanoreceptors, via the vagus nerves, are the only inputs from respiratory mechanoreceptors causing an increased f during hypercapnia, (3) the extravagal mechanism responsible for the decreased ti and increased f during hypercapnia is inherent to the medullary-pontine rhythm generator, and (4) input from the chemoreceptors can elicit the response.

The effect of lung reflexes on the pattern of breathing in cats

Tidal volume (VT), and inspiratory (tI) AND EXPIRATORY (TE) durations have been measured in anaesthetized cats on stimulation of alveolar type J (nociceptive) and lung irritant receptors by intravenous injections of phenyl diguanide and of histamine acid phosphate respectively. The reflexes were studied during eupnoea, hypercapnic and hypoxic hyperpnoea, during rebreathing from hyperventilation apnoea and at different body temperatures. In all conditions the drugs caused rapid shallow breathing with reduction in VT, tI and tE. The VT/tI relationship for injection of the drugs was different from that caused by hypercapnic stimulation of breathing, but the tI/tE relationship was proportionally similar for all conditions. Recording single unit activity from phrenic motor fibres showed that the lung reflexes had little action on the initial frequency of discharge of the fibres, but cut short the discharge earlier than for the controls. The results are interpreted in terms of the ways in which lung reflexes can modify the pattern of breathing.

Pattern of breathing during hypoxia or hypercapnia of the awake or anesthetized cat

The breathing patterns during hypoxia and hypercapnia are similar in anesthetized cats but are qualitatively different in awake cats. The differences seen in the awake animals can be explained by either the central depressive effect of hypoxia or by a specific effect of hypercapnia on supra-pontine structures. The Breuer-Hering reflex sensitivity, i.e. the VT-TI relationship, appears, in the awake cat, quite similar to that recently described in man. The inspiratory activity is shown to be controlled by mechanisms dependent on the nature of the respiratory stimulation. The recent model proposed for the control of inspiration during anesthesia must be modified to account for the results observed in awake animal.

The effect of limb movements on the regulation of depth and rate of breathing

In anesthetized dogs and rabbits passive or active limb movements (1) shifted to the left the relationship between tidal volume (Vt) and inspiratory time (Ti), (2) lowered the relationship between expiratory time (Te) and Ti and decreased its slope, and (3) increased the output to the inspiratory muscles (Vt/Ti). These effects increased with increasing the frequency of movements. Similar effects were obtained after vagotomy. When the stimulus was started during expiration. Te was shorted in spite of the previous unaffected Ti, Arterial PCO2 during exercise was similar (active movements) or below (passive movements) control value. Since other chemical and physical humoral factors do not seem involved, the whole increase of ventilation should be produced by neurogenic stimuli. The time course of Te, Ti and VT/Ti at the onset and at the offset of limb movements indicates an abrupt and a slow component in the neurogenic drive. A single contraction of the limb during expiration of inspiration affected the timing and the VT/Ti of 3-8 breaths.

Effects of lesions in the parabrachial nucleus on the mechanisms for central and reflex termination of inspiration in the cat

The effects of PCO2 and body temperature on the time course and peak amplitude of the central inspiratory activity (CIA) and the inspiratory "off-switch" threshold was studied in apneustic and non-apneustic cats. Apneusis resulted from lesions of the inspiratory inhibiting structures of the medial parabrachial nucleus (NPBM) and by interrupting vagal volume feedback. The cats were paralyzed and ventilated either proportionally to their phrenic output or at predetermined rate and volume. The dependence of the rate of rise and maximal amplitude of phrenic activity on PCO2 and body temperature were comparable in apneustic and non-lesioned animals. The Hering-Breuer volume threshold for inspiratory termination was increased following the rostral pontine lesions. Both hyperthermia and hypercapnia caused augmentation of the absolute rate of rise of inspiratory activity but hypercapnia, in contrast to hyperthermia, caused virtually no change in the fractional increment per unit time. With hypercapnia the inspiratory "off-switch" threshold was raised in the apneustic animals in intact ones, whereas hyperthermia did not seem to influence this threshold. In apneustic conditions expiratory duration remained constant, independent of the large variations in the inspiratory durations. Our results suggest that the NPBM merely provides an excitatory, threshold-lowering input to the inspiratory "off-switch" mechanism.

Temperature effects on the inflation reflex during expiratory time in the cat

In order to obtain further information on the mechanisms controlling expiratory duration (TE) we have studied the effects of changes in body temperature on reflex characteristics of the TE-prolonging inflation reflex. In cats under light pentobarbitone anesthesia controlled volume changes were applied at various times in expiration and the changes in TE were measured from records of phrenic activity. Inflations falling in the last 20% of the control expiratory time were without effect. This reflex insensitive phase remained a constant fraction of expiratory time when respiratory rate was changed with temperature. The relative reflex responsiveness increased, and the volume threshold was reduced with increasing respiratory rate. The inflation reflex characteristics showed integration of the incoming pulmonary stretch receptor activity. The integrating mechanism exhibited a "leaky" character, the decay rate of which changed with body temperature. Functionally the duration of the inspiration-inhibiting activity controlling expiratory duration can be regarded as dependent on 3 main factors 1) the initial peak level of the inhibitory activity, 2) the amount and timing of the vagal afferent activity which adds to this inhibition, and 3) its rate of decay. The results suggest that all these factors are influenced by changes in temperature.

Role of peripheral and central chemosensitive afferents in the control of depth and frequency of breathing

In anaesthetized cats the response to hypercapnia was studied during normoxia, hypoxia (increased peripheral chemosensitive afferents, IPA) and after carotid sinus denervation (decreased peripheral chemosensitive afferents, DPA) in terms of: (a) central output to the inspiratory muscles, (b) bulbopontine respiratory activity and (c) threshold-inhibition curve for termination of inspiration (Vr/Ti relationship). IPA increased, whereas DPA decreased the central output, as estimated from the rate of change of the subatmospheric pressure developed in the trachea during occlusion of the airways. IPA and DPA did not modify the bulbo-pontine rhythm, as estimated from the timing of occluded breaths (phasic vagal afferents nil), nor the Vt/Ti relationship, both of which have been shown to vary as a function of PACO2. It is concluded that peripheral chemosensitive afferents influence only the output of the respiratory centres, whereas central chemosensitive afferents influence also the bulbo-pontine respiratory rhythm and the threshold inhibition curve for termination of inspiration.

Immediate response to expiratory threshold load

In anesthetized rabbits, cats and dogs the end-inspiratory volume referred to normal FRC (deltaVe.i.) of the first breath during expiratory threshold load (ETL) was greater than control deltaVe.i. The increase of deltaVe.i. was roughly proportional to the increase of end-expiratory volume relative to normal FRC (deltaVe.e.) caused by ETL. During squeezing of the rib cage or of the abdomen or after cordotomy at T1 the phenomenon occurred nearly to the same extent. This suggests that afferent impulses from the chest wall are not essential in determining the phenomenon. The first inspiration during ETL occurred after a delay that was roughly proportional to the increase of deltaVe.e: the arterial Pco2, however, was not significantly increased at the time of the first inspiration during ETL. After bilateral vagotomy the first inspiration during ETL was no longer increased. An explantation of the increased deltaVe.i. of the first inspiration during ETL is provided on the basis of the vagal dependent tidal volume-inspiratory duration characteristic of the respiratory center.

Contribution of hypercapnic stimuli and of vagal afferents to the timing of breathing in anesthetized cats

In tracheotomized, anesthetized cats, the authors studied the role of phasic and tonic vagal discharge and of hypercapnic stimuli on the timing of breathing. The effect of tonic vagal discharge was separated from that of the other two parameters by comparing the duration of inspiration and of expiration of control breaths with those obtained during occlusions of the airways at FRC (bulbo-pontine activity). The effect of tonic vagal discharge was then separated from that of hypercapnic stimuli by comparing the bulbo-pontine activity before and after vagotomy. Hypercapnia caused shortening of inspiratory and expiratory duration set by the bulbo-pontine pacemaker and increased sensitivity of the respiratory centers for a given phasic vagal input (displacement to the left of the tidal volume vs inspiratory duration relationship). Vagotomy did not modify the bulbo-pontine duration of inspiration nor its possibility to shorten in hypercapnia; by contrast it caused a lengthening of the expiratory time which did not shorten any more in hypercapnia, suggesting that tonic vagal discharge mainly influences the expiratory duration.

Tonic vagal influences on inspiratory duration

The tidal volume (VT), inspiratory (Ti) and expiratory (Te) durations were determined in anesthetized rabbits, cats and dogs under normal conditions, during continuous positive or negative pressure breathing. Elastic loads up to complete occlusion of the airways were applied in order to determine the VT-Ti and Te-Ti relationships. The larger the end-expiratory lung volume the steeper the VT-Ti relationship, the smaller the increase of Ti and the greater the increase of Te produced by the load. Hence, the larger the end-expiratory lung volume the steeper the Te-Ti relationship. In vagotomized animals airways occlusion did not affect Ti and Te. Ti of occluded breaths in vagotomized animals was longer than that of occluded breaths in intact animals, whereas Te was similar at the resting volume of the respiratory system (FRC), shorter above FRC and longer below FRC. These results show that both phasic and tonic vagal discharges determine Ti and Te.