I. Zila

Heart rate recovery after exercise: relations to heart rate variability and complexity

Physical exercise is associated with parasympathetic withdrawal and increased sympathetic activity resulting in heart rate increase. The rate of post-exercise cardiodeceleration is used as an index of cardiac vagal reactivation. Analysis of heart rate variability (HRV) and complexity can provide useful information about autonomic control of the cardiovascular system. The aim of the present study was to ascertain the association between heart rate decrease after exercise and HRV parameters. Heart rate was monitored in 17 healthy male subjects (mean age: 20 years) during the pre-exercise phase (25 min supine, 5 min standing), during exercise (8 min of the step test with an ascending frequency corresponding to 70% of individual maximal power output) and during the recovery phase (30 min supine). HRV analysis in the time and frequency domains and evaluation of a newly developed complexity measure - sample entropy - were performed on selected segments of heart rate time series. During recovery, heart rate decreased gradually but did not attain pre-exercise values within 30 min after exercise. On the other hand, HRV gradually increased, but did not regain rest values during the study period. Heart rate complexity was slightly reduced after exercise and attained rest values after 30-min recovery. The rate of cardiodeceleration did not correlate with pre-exercise HRV parameters, but positively correlated with HRV measures and sample entropy obtained from the early phases of recovery. In conclusion, the cardiodeceleration rate is independent of HRV measures during the rest period but it is related to early post-exercise recovery HRV measures, confirming a parasympathetic contribution to this phase.

On- and off-responses of heart rate to exercise - relations to heart rate variability

During physical exercise, heart rate (HR) increases by parasympathetic withdrawal and increase of sympathetic activity to the heart. HR variability (HRV) in time and frequency domains provides information about autonomic control of the cardiovascular system. Non‐linear analysis using the Poincaré plot method is able to reveal supplementary information about cardiac autonomic control. The aim of this study was to determine the association between HRV parameters, the initial increase of HR at the onset of exercise (on‐response) and HR decrease in the recovery phase after acute exercise (off‐response). HR was continuously monitored in 17 healthy male subjects (mean age: 20·3 ± 0·2 (SEM) years) at rest (25 min supine; 5 min standing), during exercise (8 min of step test at 70% of maximal power output) and in the recovery phase (30 min supine). HRV analysis in time and frequency domains and evaluation of the Poincaré plot measures (length, widths) were performed on selected segments of HR time series. HR on‐ and off‐responses were quantified using an exponential curve fitting technique. The time constants Ton and Toff, representing the rate of on‐ and off‐responses to exercise, were computed. Postexercise HRV indices and time constant of on‐response – Ton – to exercise were negatively correlated. From preexercise HRV indices, only Poincaré plot parameters were correlated with Ton. No correlation between HRV indices and parameters of off‐response was found. In conclusion, preexercise HRV parameters are not closely correlated with the rate of cardioacceleration at the onset of exercise and cannot predict the rate of HR recovery. On the other hand, postexercise HRV parameters are related to the rate of initial adjustment of HR to exercise referring to the importance of rapid HR on‐response for a faster recovery after exercise.

Effects of diuretic-induced hypovolemia/isosmotic dehydration on cardiorespiratory responses to hyperthermia and its physical treatment in rabbits

Under conditions of heat stress and hyperosmotic dehydration, both animals and humans reduce thermoregulatory evaporation and regulate deep body temperature at elevated levels. Regarding the mechanisms, the main role in producing these thermoregulatory changes during dehydration is attributed to the increased osmolality of body fluids, although the role of the decreased plasma volume without changes in plasma osmolality (hypovolemia/isosmotic dehydration) has not been so far investigated. There are also controversial experimental results regarding the effects of dehydration on heat stress-induced cutaneous vasodilation. Therefore, this paper studied the effects of hypovolemia/isosmotic dehydration on cardiorespiratory responses to hyperthermia and its physical treatment in 17 anaesthetized adult rabbits. The animals were divided into two groups: normovolemic group (NV; n = 10) and hypovolemic group (HV; n = 7). In the HV group, hypovolemia/isosmotic dehydration (decrease in plasma volume by 16.1 ± 1.2%) was induced by furosemide (5 mg kg−1 i.v.) without change in measured plasma Na+ concentration. Hyperthermia (the rise in body temperature (BT) to 42°C by a gradual body surface heating) caused significant increase in minute ventilation (VE) in both groups. However, VE values were significantly higher in the HV rabbits compared to the NV animals despite the lower breathing frequency (p < 0.05). The panting was absent in the HV rabbits at the BT of 42°C, unlike the NV animals. From cardiovascular variables, the vasoconstrictor response in visceral (mesenteric) region during hyperthermia in hypovolemic/isosmotic animals was attenuated (p < 0.05), whereas the heat stress-induced cutaneous vasodilation was not influenced by hypovolemia. Recovery of the BT by body surface cooling was accompanied by further increase in VE in the NV group, whereas VE decreased (p < 0.05) in the HV animals. Cooling led to recovery of the cardiovascular parameters. There were found no significant cardiorespiratory differences between the groups (NV:HV) during cooling. The lower frequency of breathing and attenuation of the mesenteric vasoconstriction during exogenous hyperthermia are present not only during hyperosmotic dehydration induced by water deprivation, but they also occur under conditions of furosemide-induced isosmotic dehydration/hypovolemia in rabbits. The heat stress-induced cutaneous vasodilation regarding its biological importance was not influenced by hypovolemia/isosmotic dehydration. Therefore, it is suggested that hypovolemia alone is sufficient to produce described respiratory, thermoregulatory and cardiovascular changes in dehydrated rabbits during exogenous hyperthermia, whereas hyperosmolality is not a requisite.

Effects of Elevated Body Temperature on Control of Breathing

Effects of Elevated Body Temperature on Control of Breathing Changes in body temperature can be evoked mainly by alterations in the peripheral temperature, or modified by shifts in the central body temperature. Two conditions can lead to abnormal elevation of body temperature: hyperthermia or fever. As regards respiratory system, exposure to heat stress is accompanied by marked alterations in breathing, especially by an increase in ventilation. Ventilation rises due to an increase in central output from hypothalamus or brainstem, an increase in peripheral output via skin temperature receptors, an increase in central or/and peripheral chemoreceptor output or sensitivity and can be also mediated through changes in thermoregulatory mechanisms. This review summarizes results of previous studies as well as of experiments done in our laboratory in order to elucidate the mechanisms included in respiratory changes under heat stress.

Diuretic-induced dehydration/hypovolemia inhibits thermal panting in rabbits

Respiratory and thermoregulatory responses to hyperthermia during isosmotic dehydration/hypovolemia were studied in 17 anaesthetized adult rabbits divided into two groups: normovolemic group (NV; n=10) and hypovolemic group (HV; n=7). Hypovolemia/isosmotic dehydration (a decrease in plasma volume by 16.1+/-1.2%) was induced by furosemide (5 mg kg(-1) i.v.). During hyperthermia (the rise in body temperature to 42 degrees C by a gradual body surface heating), the HV rabbits had lower (P<0.05) respiratory frequency and higher (P<0.05) tidal volume than the NV animals. The panting was absent in the HV rabbits at the BT of 42 degrees C, unlike the NV animals. The lower respiratory frequency and the absence of panting during exogenous hyperthermia in dehydrated animals are present not only during hyperosmotic dehydration induced by water deprivation [Doris, P.A., Baker, M.A., 1981. Hypothalamic control of thermoregulation during dehydration. Brain Res. 206 (1), 219-222], but they also occur in the furosemide-induced isosmotic dehydration/hypovolemia.

Effects of acute normovolemic haemodilution on cardiorespiratory changes in hyperthermia and its physical treatment

As only one experimental study examining the effects of haemodilution on circulatory responses to hyperthermia has so far been published and there is no information on respiratory responses to hyperthermia during haemodilution or anaemia, this paper studied the effects of acute normovolemic haemodilution on cardiorespiratory changes during 42°C hyperthermia and its recovery by body surface cooling in 16 anaesthetized adult rabbits. The animals were divided into two groups: haemodiluted group (Hct = 18.6 ± 0.4%) and control group (Hct = 41.1 ± 0.9%). In the haemodiluted group, acute normovolemic haemodilution was induced by 60% replacement of total blood volume with dextran. Haemodilution produced significant increases in minute volume (VE, p<0.02), heart rate (HR, p<0.02) and central venous pressure (CVP, p<0.02), but there was no significant change in mean arterial pressure (MAP). Hyperthermia caused significant increases in VE and HR in both the groups; however, VE and HR values were significantly higher in the haemodiluted group compared to the controls. CVP decreased (p<0.05) during 42°C hyperthermia only in the controls in the panting phase. MAP did not significantly change during over-heating in controls, but it significantly (p<0.02) rose in the haemodiluted group. In the recovery phase, cooling led to significant decreases in HR and MAP in both the groups and to further increases in VE (p<0.05) in controls, whereas VE decreased in the haemodiluted animals. There were found no significant cardiorespiratory differences between the two groups during cooling. Hyperthermia was accompanied by the higher values of minute volume and heart rate in the haemodiluted animals, indicating a greater activation of the respiratory and cardiovascular systems, which could result in diminished functional cardiorespiratory reserve and a higher risk of respiratory and circulatory failures in haemodiluted/anaemic animals during hyperthermia. In the phase of recovery of the body temperature there were no significant cardiorespiratory differences found between the two groups.