The effect of body warming on the ventilatory response to CO2 in the awake dog

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.

Development of in vivo ventilatory and single chemosensitive neuron responses to hypercapnia in rats

We used pressure plethysmography to study breathing patterns of neonatal and adult rats acutely exposed to elevated levels of CO2. Ventilation (VE) increased progressively with increasing inspired CO2. The rise in VE was associated with an increase in tidal volume, but not respiratory rate. In all animals studied, the CO2 sensitivity (determined from the slope of the VE vs. inspired % CO2 curve) was variable on a day to day basis. Chemosensitivity was high in neonates 1 day after birth (P1) and fell throughout the first week to a minimum at about P8. Chemosensitivity rose again to somewhat higher values in P10 through adult rats. The developmental pattern of these in vivo ventilatory responses was different than individual locus coeruleus (LC) neuron responses to increased CO2. The membrane potential (V(m)) of LC neurons was measured using perforated patch (amphotericin B) techniques in brain slices. At all ages studied, LC neurons increased their firing rate by approximately 44% in response to hypercapnic acidosis (10% CO2, pH 7.0). Thus the in vivo ventilatory response to hypercapnia was not correlated with the V(m) response of individual LC neurons to hypercapnic acidosis in neonatal rats. These data suggest that CO2 sensitivity of ventilation in rats may exist in two forms, a high-sensitivity neonatal (or fetal) form and a lower-sensitivity adult form, with a critical window of very low sensitivity during the period of transition between the two (approximately P8).

Effects of hypoxia and hypercapnia on circadian rhythms in the golden hamster (Mesocricetus auratus)

This study was designed to determine whether respiratory stimuli can influence the mammalian circadian timing system. Three-hour pulses of hypoxia (inspired O(2) concentration = 8%) or hypercapnia (inspired CO(2) concentration = 11%) were presented for 7 days at mid-subjective day (circadian time 6-9) under constant darkness. Hypoxic and hypercapnic pulses caused cumulative phase delays of 46. 4 +/- 6.9 and 25.9 +/- 12.3 min, respectively. Distance run per day was significantly reduced on hypoxic and hypercapnic pulse days, compared with nonpulsed days. Phase shifts were correlated with the reduction in daily running activity (multiple r(2) = 0.521, P = 0.036), metabolic depression (multiple r(2) = 0.772, P < 0.001), and reduction in body temperature (multiple r(2) = 0.539, P = 0.027), but not lung ventilation (multiple r(2) = 0.306, P = 0.414) during pulses. We conclude that hypoxia and hypercapnia can influence the phase and quantity of activity in free-running hamsters.

Depressed ventilatory response to hypoxia in hypothermic newborn piglets: role of glutamate

To evaluate whether changes in extracellular glutamate (Glu) levels in the central nervous system could explain the depressed hypoxic ventilatory response in hypothermic neonates, 12 anesthetized, paralyzed, and mechanically ventilated piglets <7 days old were studied. The Glu levels in the nucleus tractus solitarius obtained by microdialysis, minute phrenic output (MPO), O2 consumption, arterial blood pressure, heart rate, and arterial blood gases were measured in room air and during 15 min of isocapnic hypoxia (inspired O2 fraction = 0.10) at brain temperatures of 39.0 +/- 0.5 degrees C [normothermia (NT)] and 35.0 +/- 0.5 degrees C [hypothermia (HT)]. During NT, MPO increased significantly during hypoxia and remained above baseline. However, during HT, there was a marked decrease in MPO during hypoxia (NT vs. HT, P < 0.03). Glu levels increased significantly in hypoxia during NT; however, this increase was eliminated during HT (P < 0.02). A significant linear correlation was observed between the changes in MPO and Glu levels during hypoxia (r = 0.61, P < 0.0001). Changes in pH, arterial PO2, O2 consumption, arterial blood pressure, and heart rate during hypoxia were not different between the NT and HT groups. These results suggest that the depressed ventilatory response to hypoxia observed during HT is centrally mediated and in part related to a decrease in Glu concentration in the nucleus tractus solitarius.

Respiratory and metabolic responses of the spiny rats Proechimys yonenagae and P. iheringi to CO2

The recently described burrow-dwelling Proechimys yonenagae from the Brazilian semiarid caatinga was compared to P. iheringi from the Brazilian rain forest of Mata Atlântica in terms of interactions between body temperature (Tb), ventilation (VE) and oxygen consumption (V(O2)) during hypercapnia (5 or 10% CO2). Wistar rats were also used as a control. VE was measured by plethysmography, Tb by inserting a probe into the colon, and V(O2) by a close-flow system. During air breathing, VE did not differ between the rodents, Wistar Tb was elevated compared to P. yonenagae, and the V(O2) values of P. yonenagae and P. iheringi were significantly lower than those of Wistar rats. Hypercapnia caused hyperventilation in the three rodents, hypothermia in P. yonenage and Wistar rats, but no change in V(O2) was observed in any of the rodents. The hypercapnia-induced hyperventilation may be a major factor producing heat loss. This effect was independent of V(O2). Comparisons of the two spiny rat species suggest little adaptation of P. yonenagae to hypercapnia in burrows.

Day-night differences in the respiratory response to hypercapnia in awake adult rats

We investigated whether resting ventilation and the hypercapnic ventilatory response vary with time of day in awake adult rats. Respiratory frequency (fR), tidal volume (VT), inspired ventilation (VI), inspiratory interval (tI), carbon dioxide production (VCO2) and abdominal temperature (Tb) were measured before and during a hypercapnic stimulus (3.5% CO2 in air) at 10:00 and 22:00 h. VCO2, Tb and mean inspiratory air flow (VT/tI) were significantly higher at 22:00 h in air. VI/VCO2 was similar at 10:00 and 22:00 h. VI was significantly elevated by hypercapnia and the response at 22:00 h was 2.3 times greater than that at 10:00 h. VT/tI was unchanged at 10:00 h but significantly increased by hypercapnia at 22:00 h. VCO2 was significantly depressed at 10:00 h but not at 22:00 h. Tb was unaffected by hypercapnia. We conclude that the metabolic and ventilatory responses to hypercapnia are dependent on the time of day.

Effect of CO2 on the metabolic and ventilatory responses to ambient temperature in conscious adult and newborn rats

1. In newborn and adult rats, hypoxia decreases metabolic rate, especially at low ambient temperatures (Ta). We examined whether a similar effect can occur during hypercapnia. 2. We measured metabolism (oxygen consumption, Vo2, open flow-through method), and expiratory ventilation (VE; barometric method (adults), airflow plethysmograph (newborns)) in air and 2% or 5% CO2 in normoxia. 3. In adults, Vo2 was higher at Ta = 10 degrees C than 25 degrees C. At each Ta, CO2 breathing did not change Vo2, but increased VE, less at 10 degrees C (up to +100%) than at 25 degrees C (+161%). Blood pressure was maintained at both values of Ta and CO2, while pulse rate and body temperature were decreased in 5% CO2 at 10 degrees C. 4. In newborns, the metabolic response to lowering Ta (from 40 to 20 degrees C) much depended on behavioural responses, being larger in groups of two or four pups than in individual animals. In no case did CO2 influence the response. VE increased during 5% CO2 exposure, more so at Ta = 33 percent C (+69%) than at 25 degrees C (+49%). 5. In both adults and newborns, hypoxia (10% O2) always decreased metabolic rate. 6. We conclude that hypercapnia has no appreciable effects on metabolic rate in rats (both newborns and adults) even at low Ta, a result quite different from the hypometabolic response to hypoxia.

Influence of body temperature on responses to hypoxia and hypercapnia: implications for SIDS

1. This paper reviews current knowledge regarding interactions between body temperature and the respiratory responses to hypoxia and/or hypercapnia, with special emphasis on how these interactions might predispose towards sudden infant death syndrome (SIDS). 2. Use has been made of an adult rat model in which body core temperature is fixed by means of an intra-abdominal heat exchanger. Initial studies indicated that hyperthermia (Tb approximately 41 degrees C) enhanced the ventilatory response to hypercapnia, whereas hypothermia (Tb approximately 35 degrees C) interacted with hypoxia to depress respiration. 3. Studies involving hypothalamic lesions in urethane-anaesthetized rats have implicated the posterior hypothalamic area in the hypoxia/hypothermia interaction. Further studies are directed towards examining the role played by more caudal areas, including the raphe nuclei. 4. It has been shown that not only does the hypoxia/hypothermia interaction depress breathing but it also reduces, or sometimes eliminates, the ventilatory response to hypercapnia, which under normal circumstances provides one of the most powerful excitatory inputs to the respiratory centres. This implies that an expected reversal of the respiratory depression by build up of CO2 levels may not occur, which in turn has important implications for SIDS. 5. The literature dealing with the effects of hyperthermia on hypoxic and hypercapnic responses is also reviewed. It is concluded that environmental heat stress may only become a significant problem when it accompanies a febrile infection, under which circumstances it may seriously compromise thermoregulatory ability and alter breathing responses to chemical stimuli.

Acid-base and ventilatory adaptation in conscious dogs during chronic hypercapnia

Ventilation and cisternal cerebrospinal fluid (CSF) and arterial acid-base balance were measured in awake dogs during air control and from 1 h to 26 days of breathing 5% CO2 in air. Ventilation increased 4-fold during acute hypercapnia and then declined to a minimum at 5-10 days. Between 1-3 days and 16-26 days of hypercapnia ventilation was relatively stable at 2.5 times control. [HCO3-]CSF increased rapidly by 12 h of hypercapnia and in the steady-state [HCO3-]CSF was correlated with PCSFCO2. Between 1 h and 1.5 days of hypercapnia, increase in [HCO3-]CSF was also correlated with increase in [NH3]CSF. Despite increase in [HCO3-]CSF, there was no compensation of [H+]CSF throughout 26 days of hypercapnia. Hydrogen ion may have contributed to the control of ventilation during chronic hypercapnia since ventilation was correlated with [HCO3-]a and [HCO3-]CSF. However, a relationship between ventilation and [H+] of arterial blood and CSF during chronic hypercapnia was relatively poor or absent. Ventilatory adaptation to chronic hypercapnia could not be related to metabolism or to [NH3]CSF. The mechanism(s) by which the increase in PCO2 during chronic respiratory acidosis results in sustained elevation of ventilation remains to be resolved.