Control of breathing in the exercising dog

Environmental and Physiological Factors Associated With Stamina in Dogs Exercising in High Ambient Temperatures

This IACUC approved study was performed to evaluate the environmental, physiological, and hematological components that contribute to stamina following successive bouts of exercise that included searching (5-min), agility (5-min), and ball retrieve (<10-min). Regularly exercised dogs (N = 12) were evaluated on five separate occasions. The population consisted of eight males and four females ranging in age from 8 to 23 months, which included six Labrador retrievers, three German shepherds, and one each English springer spaniel, German wirehaired pointer, and Dutch shepherd. The exercise period was up to 30 min with 5 min of intermittent rest between the exercise bouts or until a designated trainer determined that the dog appeared fatigued (e.g., curled tongue while panting, seeking shade, or voluntary reluctance to retrieve). At the end of the exercise period, pulse rate (PR), core temperature, blood lactate, and venous blood gas were collected. The median outdoor temperature was 28.9°C (84°F) (IQR; 27.2–30°C/81–86°F) and median humidity was 47% (IQR; 40–57%). Median duration of exercise was 27 min (IQR; 25–29). No dog showed signs of heat stress that required medical intervention. The components used to measure stamina in this study were total activity, post-exercise core body temperature (CBT), and increase in CBT. When controlling for breed, total activity, as measured by omnidirectional accelerometer device, could be predicted from a linear combination of the independent variables: pre-exercise activity (p = 0.008), post-exercise activity (p < 0.001), outdoor temperature (p = 0.005), reduction in base excess in extracellular fluid compartment (BEecf) (p = 0.044), and decrease in TCO₂ (p = 0.005). When controlling for breed and sex, increase in CBT could be predicted from a linear combination of the independent variables: study day (p = 0.005), increase in PR (p < 0.001), increase in lactate (p = 0.001), reduction in BEecf (p = 0.031), increase in glucose (p = 0.044), increase in hematocrit (p = 0.032), and increase in hemoglobin (p = 0.038). This study suggests that the influence of outdoor temperature, pre- and post-exercise activity, and the metabolic parameters are important components of stamina associated with exertion.

Influence of the Environment on Body Temperature of Racing Greyhounds

Heat strain is a potential risk factor for racing greyhounds in hot climates. However, there have been limited studies into the incidence of heat strain (when excess heat causes physiological or pathological effects) in racing greyhounds. The aim of this study was to determine if heat strain occurs in racing greyhounds, and, if so, whether environmental factors (e.g., ambient temperature and relative humidity) or dog-related factors (e.g., sex, bodyweight, color) are associated with the risk of heat strain. A total of 229 greyhounds were included in over 46 race meetings and seven different race venues in South Australia, Australia. Rectal temperatures of dogs were measured pre- and postrace and urine samples collected for analysis of myoglobinuria. Ambient temperature at race times ranged between 11.0 and 40.8°C and relative humidity ranged from 17 to 92%. There was a mean increase in greyhound rectal temperature of 2.1°C (range 1.1-3.1°C). A small but significant association was present between ambient temperature and increase in rectal temperature (r (2) = 0.033, P = 0.007). The mean ambient temperature at race time, of dogs with postrace rectal temperature of or exceeding 41.5°C, was significantly greater than at race time of dogs with a postrace rectal temperature ≤41.5°C (31.2 vs. 27.3°C, respectively, P = 0.004). When the ambient temperature reached 38(o)C, over one-third (39%) of dogs had a rectal temperature >41.5°C. Over half of postrace urine samples were positive by Dipstick reading for hemoglobin/myoglobin, and of 77 urine samples positive for Dipstick readings, 95% were positive for myoglobin. However, urinary myoglobin levels were not associated with ambient temperature or postrace rectal temperatures. The mean increase in rectal temperature was greater in dark (black, blue, brindle) than light (fawn and white) colored greyhounds. The results suggest heat strain occurs in racing greyhounds, evidenced by postrace rectal temperatures over 41.5°C and postrace myoglobinuria. Risk of heat strain may be increased in higher ambient temperatures and in darker colored greyhounds. Further research into the incidence of heat strain in racing greyhounds, and longer term physiological responses to heat strain, are warranted.

Thermal drive contributes to hyperventilation during exercise in sheep

The etiology of exercise hypocapnia is unknown. The contributions of exercise intensity (ExInt), lactic acid, environmental temperature, rectal temperature (Tre), and physical conditioning to the variance in arterial CO2 tension (PaCO2) in the exercising sheep were quantified. We hypothesized that thermal drive contributes to hyperventilation. Four unshorn sheep were exercised at approximately 30, 50, and 70% of maximal O2 consumption for 30 min, or until exhaustion, both before and after 5 wk of physical conditioning. In addition, two of the sheep were shorn and exercised at each intensity in a cold (<15 degrees C) environment. Tre and O2 consumption were measured continuously. Lactic acid and PaCO2 were measured at 5- to 10-min intervals. Data were analyzed by multiple regression on PaCO2. During exercise, Tre rose and PaCO2 fell, except at the lowest ExInt in the cold environment. Tre explained 77% of the variance in PaCO2, and ExInt explained 5%. All other variables were insignificant. We conclude that, in sheep, thermal drive contributes to hyperventilation during exercise.

The pattern of respiration with increasing metabolism in a small dasyurid marsupial

Previous studies have indicated that the pattern of respiration in marsupial and placental mammals may be different. Some marsupials have larger tidal volumes and slower respiratory rates under basal conditions. This study examined the respiratory responses of a small marsupial, Dasyuroides byrnei, to increasing metabolic demand. The highest metabolic rate elicited by cold exposure in a helium-oxygen atmosphere was 10.4 times the basal metabolic rate. Basal tidal volumes and respiratory rates were 138% and 46% respectively of the values predicted for placental mammals. The increasing oxygen demands of metabolism were met by increases in ventilation rather than by changes in oxygen extraction. Initially, tidal volume increased until it reached a maximum value 2.6 times that of basal tidal volume. Subsequently, ventilation was augmented by an increase in respiratory frequency. Ventilatory accommodation to an increasing oxygen demand indicated that D. byrnei has an excellent respiratory capacity to deal with the thermogenic demands of a cold environment.

Ventilatory accommodation of changing oxygen demand in sciurid rodents

Ventilation was measured across a range of O2 consumption rates in four sciurid rodents: Tamias minimus (47 g), Spermophilus lateralis (189 g), S. beecheyi (531 g), and Marmota flaviventris juveniles (1054 g) and adults (2989 g). Maximum thermogenic oxygen consumption was measured for all but adult M. flaviventris. Aerobic scopes (maximum/minimum O2 consumption rates) were 4.6, 3.8, 5.4, and 4.8 in T. minimus, S. lateralis, S. beecheyi, and juvenile M. flaviventris, respectively. Aerobic scope was at least 4.1 in adult M. flaviventris. Ventilatory accommodation of changing O2 consumption rate was qualitatively similar in the four species, with the bulk of accommodation resulting from changes in minute volume. Nevertheless, there were significant differences in the relative importance of frequency, tidal volume, and O2 extraction in accommodation. In all species, frequency and minute volume were strongly correlated to O2 consumption rate. Tidal volume was significantly correlated to O2 consumption rate in T. minimus and S. beecheyi, but not in the other species. Oxygen extraction was not significantly correlated to O2 consumption rate in any species. Analysis of factorial ventilation changes across a standardized 3.8-fold change in O2 consumption rate revealed significant differences among species in frequency and O2 extraction, but not in tidal or minute volume. When compared to a generalized allometry for mammalian resting ventilation, the four sciurid species had consistently lower respiration frequency and higher O2 extraction than predicted, perhaps because the sciurid measurements were made on unrestrained animals. There was no indication that ventilation constrained maximum O2 consumption rate.

Aerobic training effects on maximum oxygen consumption, lactate threshold and lactate disappearance during exercise recovery of dogs

1. Dogs were submitted to an aerobic training schedule and its maximum oxygen consumption, lactate threshold and lactate concentration during recovery were compared among the following conditions: not trained (UT), after 1 month of training (T1), after 2 months of training (T2) and after detraining (DT). 2. Maximum oxygen consumption increased significantly in relation to UT condition only at T2 condition. The detraining reversed this alteration. 3. Lactate threshold when expressed as Vo2 or absolute work load increased significantly after aerobic training (T2) but did not present any alteration when it was expressed as % of Vo2 max. 4. The lactate decreasing during recovery did not differ between the four experimental conditions (after 10 min). 5. The latency time for the lactate concentration to reach the top values was reduced by aerobic training (T2).

Acid-base balance during lactic acid infusion in the lizard Varanus salvator

Although reptiles rely heavily on anaerobic metabolism and lactic acid production during activity, little is known concerning their ventilatory response to the attendant metabolic acidosis. We measured arterial PCO2, H+ and lactate (L)ion concentrations, and the rates of CO2 (MCO2) and O2 (MO2) exchange in Varanus salvator (n = 9) during intravenous infusions of lactic acid (HL) or sodium lactate (NaL; 250 mM) at rest. Two protocols were used: (1) 15 min infusions of 0.42 ml/min at both 25 and 35 degrees C with measurements every 5 min; (2) 4.5 min infusions of 1.73 ml/min at 35 degrees C with measurements at 4.5 min. At 35 degrees C, control pH decreased from its value at 25 degrees C with a slope of -0.007/degrees C and PaCO2 increased. The results of HL infusion were: (1) [L]a increased, (2) MCO2 increased, (3) R (MCO2/MO2) increased, (4) pHa decreased, and (5) PaCO2 remained unchanged from control at both temperatures and at both infusion rates. The only significant changes in PaCO2 observed were following the termination of fast HL infusions, when PaCO2 decreased. In NaL infusions, only small changes were observed except in [L]a. The results indicate that: (1) delta pH/delta T in V. salvator is less than in other poikilothermic vertebrates, but consistent with other varanid lizards, (2) respiratory compensation is slight in response to acute metabolic acidosis in this species, and (3) ventilation follows changes in MCO2 rather closely, accounting for precise regulation of PaCO2 despite 4-fold increases in MCO2 elicited by bicarbonate buffering and increased metabolic rate.

Stimulation by central command of locomotion, respiration and circulation during exercise

We studied the relationships between exercise (locomotion) and respiratory and circulatory responses in 19 cats that walked or ran normally on a treadmill, and in 16 paralyzed animals during fictive locomotion, i.e., locomotory activity in motor nerves to the legs. Preparations included anesthetized cats with intact brains and unanesthetized decorticate (hypothalamic) and decerebrate (mesencephalic) animals. Spontaneous actual locomotion and fictive locomotion occurred in all preparations except the mesencephalic cats. Electrical stimulation or injection of a GABA antagonist (picrotoxin) into the hypothalamic locomotor region caused locomotion to develop. In all cases when locomotion occurred, respiration and arterial pressure increased in proportion to the level of locomotor activity despite control or ablation of respiratory feedback mechanisms. Respiration and arterial pressure increased similarly during fictive locomotion despite the absence of muscular contraction or limb movement and the lack of change of metabolic rate. We conclude that the study provides experimental support for the feed-forward, or command signal, hypothesis for the genesis of proportional changes of respiration and circulation that occur during exercise. Feedback mechanisms are not required for its operation. We suggest that command signals emanating from the hypothalamus provide the primary drive for changes of respiration and circulation during exercise.

Ventilatory transients during exercise: peripheral or central control?

The fast component of the ventilatory changes that occur at the transition phases of exercise was studied in awake dogs trained to run on a treadmill. Two questions were examined: firstly, is the fast ventilatory component modified by changes in venous return to the lungs, such as those consecutive either to increased work loads or to beta adrenergic blockade?, and secondly, is this component altered by central ventilatory depressants? The results showed that at the onset of exercise, there is no correlation between the instantaneous increment in ventilation and the intensity of exercise, but at the end of the treadmill run, the fall in ventilation is closely linked to the power of the work performed. Ventilatory transients observed either at the start or at the end of exercise remain unaffected by administration of a beta-adrenergic blocking agent. But central depressant effects on ventilation caused by narcotic analgesics or hypnotic drugs altered the breathing pattern of the fast component of exercise-induced ventilatory changes. It is concluded that the instantaneous changes in ventilation occurring at the transition phases of exercise are controlled by mechanoreceptor mechanisms, but cerebral control is superimposed on the reflex control in regulating both tidal volume and breathing rate.

Respiratory and blood gas responses in exercising birds

The effects of exercise in birds on changes in body temperature, ventilation, blood gases and air-sac gases are reviewed. Except in the case of isothermic exercise below the anaerobic threshold, birds hyperventilate during exercise. Exercise hyperventilation is greater at higher exercise intensities and at higher environmental temperatures. The domestic fowl appears to be a suitable model for the study of physiological responses to exercise in running birds. A prior period of training is necessary to accustom the birds to laboratory procedures. The possible neural and/or humoral mechanisms controlling exercise hyperpnea are listed. Intrapulmonary hypocapnia seems to exclude the possibility that lung chemoreceptors are responsible for the hyperpnea during exercise, but these receptors probably play a predominant role in the determination of ventilatory pattern.

The role of spinal cord transmission in the ventilatory response to electrically induced exercise in the anaesthetized dog

1. The ventilatory response to electrically induced ;exercise' was studied in six chloralose-anaesthetized dogs. The on-transient and steady-state responses to ;exercise' were compared in the same dogs before and after spinal cord transection at T8/9 (dermatome level T6/7) on fifteen occasions.2. Phasic hind limb ;exercise' was induced for periods of 4 min by passing current (2 Hz modulated 50 Hz sine wave) between two needles inserted through the hamstring muscles. The maximum current used was 30 mA. This was below the level previously found to produce an artifactual stimulation of breathing with the cord intact.3. Cord transection produced no significant change in either the resting values of ventilation ( V(I)) and CO(2) production ( V(CO) (2)) or the ventilatory equivalent for CO(2) during ;exercise' ( big up tri, open V(I)/ big up tri, open V(CO) (2)).4. During the steady state of exercise P(a, CO) (2) was on average significantly lower than at rest with the cord intact (mean big up tri, openP(a, CO) (2), - 2.1 mmHg; range - 5.7 to + 1), and higher, though not significantly, with the cord cut (mean P(a, CO) (2), + 1.2 mmHg; range - 1.5 to + 4.3). However, even in the absence of spinal cord transmission, the ventilatory response to exercise could not be accounted for on the basis of CO(2) sensitivity; the big up tri, open V(I)/ big up tri, openP(a,CO) (2) obtained with exercise (apparent sensitivity) was significantly greater than that obtained with CO(2) inhalation (true sensitivity) both before and after cord section.5. V(I) and V(CO) (2) increased more slowly with the cord cut than with the cord intact. This was thought to be due to a slower increase in venous return in the absence of sympathetic innervation of the lower half of the body following cord transection.6. Similar experiments were performed during muscle paralysis (following gallamine triethiodide). Ventilation was maintained with a respirator controlled by phrenic nerve activity. These experiments showed an increase in ventilation, independent of muscle contraction, which was only present when the cord was intact and which was confined to the on-transient. Only in the absence of spinal cord transmission could there be certainty that the dynamics of the ventilatory response to electrically induced ;exercise' was free of artifact.7. It was concluded that spinal cord transmission is not necessary for the steady-state ventilatory response to electrically induced exercise of the hind limbs.8. The dog with spinal cord transection provides a suitable model for the study of the chemical control of breathing during electrically induced exercise.

Effects of chronic lung denervation on breathing pattern and respiratory gas exchanges during hypoxia, hypercapnia and exercise

The influence of vagal fibres from the lung on ventilatory responses to hypercapnia, hypoxia and exercise was studied in two intact dogs (C) and two chronically lung denervated dogs (C.L.D.). In intact dogs, inspiration duration did not change as tidal volume increased in response to increased chemical drives. Chronic lung denervation did not affect the hypercapnia- or hypoxia-induced elevations in V, despite significant changes in breathing pattern. During exercise, oxygen consumption was similar for C and C.L.D. animals. V for a given oxygen uptake was the same in C and C.L.D. dogs, but VT was higher in C.L.D. animals at all levels of exercise. It is concluded that vagal fibres from the lung play a role in determining the breathing pattern, but are not required for a normal ventilatory response to hypercapnia, hypoxia and exercise. Interaction between vagal sensory input and specific structures sensitive to chemical or physical stimuli is discussed.

Control of ventilation during graded exercise in the dog

We analyzed the time courses of the VE, VT, and f responses to graded levels of exercise produced by increases in treadmill speed at preset inclines in 207 experiments on 15 tracheostomized dogs. At the onset of work, VE increased within 1-2 respiratory cycles (VE fast), and then either remained constant, or decreased. Following this time delay (TD), VE rose more slowly (VE slow) to attain a stable plateau (Dejours). The amplitude of VE fast, VT, and f during the TD were independent of the work load. However, the duration of the TD and the amplitude of the component mediating VE slow were workload dependent. The VE fast and the VE during the TD are the major components of the total VE response at low work levels (VO2 = 30-40 ml . min-1 . kg-1) and are mediated primarily by increased f, whereas, at higher VO2 (70-90 ml . min-1 . kg-1), VE slow is mediated largely by increased VT and this component is engaged earlier to make a greater contribution to the total VE response. In the conscious dog, the total VE response to exercise appears to be comprised of both neural and humoral components when thermal stress is minimal.

Carotid chemoreceptor function in ventilatory and circulatory O2 convection of exercising dogs at low and high altitude

Awake dogs were studied before (control) and after chronic bilateral carotid denervation (denervated) at rest and running for 3 min on a treadmill at 8 km . h-1 and at various grades, in an altitude chamber operated either at 140 m or at 4000 m for 3 h. Steady-state pulmonary ventilation (Vg) and breathing pattern (VT, fR), oxygen consumption (MO2), O2 concentrations (C) and pressures (P) in the arterial (a) and mixed venous blood (v), hematocrit (Ht) and acid-base status in arterial blood, and heart frequency (fH) were measured. From these data cardiac output (Vb) and stroke volume (Vs), ventilatory and circulatory requirements (Vg/Mo2, Vb/MO2), extraction of O2 from inspired gas (EairO2) and blood (EbO2), and capacitance coefficient of blood for oxygen (beta bO2) were calculated. Ventilatory responses to transient O2-inhalation were also studied and the aortic (AP) and pulmonary (PP) blood pressures measured in resting conditions. 1. After chronic carotid denervation the hypoxic chemoreflex drive of ventilation was reduced by about half, maximal MO2 remained unaffected at 140 m, but at 4000 m decreased 50% compared to 30% in controls. 2. In all experimental conditions, Vg/MO2, PaO2 and CaO2 were less in denervated animals than in controls, and EairO2, PaCO2 and H+ ion concentration were higher. 3. At 140 m, circulatory O2 convective transport was identical in the two groups of dogs. At 4000 m, beta bO2 increased similarly in both groups, but Vb and Vb/MO2 were higher in denervated dogs than in controls, in relation with reduced CaO2-CVO2 difference which contributed to restore PVO2 towards higher values. 4. At 140 m, mean resting AP and PP were similar in both groups of dogs. At 4000 m, AP increased not significantly in controls, and decreased in denervated animals; PP increased in controls, but not in denervated dogs. It is concluded that integrity of the arterial chemoreceptor drive is essential in determining the eupneic level of ventilation and normal acid-base status of the blood in both resting and exercising dogs, at low and at high altitude, and in reducing the O2 circulatory requirement at high altitude. At 4000 m, the lack of carotid chemosensitivity is accompanied by severe hypoxemia, in association with hypercapnia and acidosis, and by increased cardiac blood flow, most presumably due to decreased peripheral resistance and increased venous return; despite these compensatory changes in circulatory O2 convective transport, denervated animals reach a maximum O2 uptake at lower work load than controls.

The ventilatory responses of conscious dogs to isocapnic oxygen tests. a method of exploring the central component of respiratory drive and its dependence on O2 and CO2

Conscious unrestrained dogs trained to breathe through a respiratory mask or, after chronic tracheostomy, through a cuffed endotracheal tube were studied in an altitude chamber operated in such a way that end-tidal PO2 was maintained at 100, 75 or 60 Torr. Each hypoxic experiment was completed within 1 h of the onset of hypoxia. At all levels of oxygenation, resting pulmonary ventilation (V), obtained from the tidal volume (VT) and ventilatory period (T), and alveolar gas tensions (PAO2, PACO2) were measured cycle-by-cycle before and during isocapnic O2-tests (IOT) at various steady levels of alveolar PCO2 ranging from 30 to 48 Torr. For this, PCO2 in the inspired gas before and during IOT was adjusted so that PACO2 remained unchanged in the course of the first few breaths which followed the switch to hyperoxia. In analysing the transient changes of V in the course IOT, it was considered that an apnoea occurred when there was no measurable deflection on the integrated pneumotachogram past a duration twice the control T from the beginning of the last recorded ventilatory cycle. (1) Control V vs. PACO2 relationships showed classic positive interaction between hypercapnia and hypoxia; (2) during IOT at PAO2 of 100, 75 or 60 Torr, an apnoea occurred, V invariably falling to zero, provided that PACO2 was below 38-35 Torr according to the level of oxygenation; (3) above that threshold PACO2 value, the residual minimum ventilation (Vres) observed during IOT was linearly related to PACO2; (4) Vres vs. PACO2 relationships showed negative interaction between hypercapnia and hypoxia. It is concluded that (a) through isocapnic O2-tests, both the peripheral and central components of the ventilatory drive can be quantitatively estimated; (b) in conscious dogs, the pulmonary ventilation appears to be entirely driven by afferent activity from the arterial chemoreceptors, even in eucapnic normoxia; (c) the lower minimum ventilation seen in the course of O2-tests from a hypoxic rather than a normoxic background is still observed at PACO2 above normal, thus cannot be due only to hypocapnia related to preceding hypoxic hyperventilation must be caused by a central respiratory inhibition directly or indirectly related to depressant effect of even moderate hypoxia.

Effects of simulated altitude on O2 transport in dogs

The effects of exposure to simulated altitude on skeletal muscle and on the cardiocirculatory system rhave been studied on awake, conscious, dogs (16-39 kg) at rest and during moderate treadmill exercise at 15% incline. Measurements were obtained in Denver at a PB of 635 mm Hg and after 3 weeks of continuous exposure to a PB of 435 mm Hg using a hypobaric chamber. At both levels of PB the dogs showed marked panting, associated with pulmonary hyperventilation and low PaCO2's (28 mm Hg at 635 and 17 mm Hg at 435 mm Hg). Resting VO2's were high (12.4 and 11.4 ml/min-kg) reflecting the high respiratory outputs. [Hb] and Hct increased significantly after 3 weeks at 435 mm Hg with a 4 mm Hg decrease in P50. Q, HR, SV and systemic blood pressures were normal in both normoxic and hypoxic animals, at rest as well as during exercise. Arterial and mixed venous PO2's decreased in the hypoxic dogs (82 to 56 and 42 to 36 mm Hg, respectively) but arterial O2 content, arterial-venous O2 content difference and systemic O2 transport remained unchanged. Capillary density in the sternothyroid muscle doubled (617 to 1245 cap/mm2) after 3 weeks at a PB of 435, whereas the average muscle fiber diameter decreased about 30%. Calculations indicate that the PO2 in the 'lethal corner' of the muscle fiber is relatively higher in the hypoxic animal because of the decrease in fiber size and intercapillary distances.

A simple method for the sampling of respiratory gas in conscious dogs

A method is presented for the sampling of respiratory gas in conscious animals, using a catheter with the tip implanted in the roof of the nasal cavity and the other end exteriorized beyond the external protuberance of the occiput. The operation procedure is simple and serious postoperative complications have not been encountered. The method has been used extensively for the continuous measurement of Po2 and PCO2 in respiratory gas of conscious dogs.