Short-term effect of tidal pleural pressure swings on pulmonary blood flow during rest and exercise

Because pleural pressure (Ppl) has important effects on venous return and left ventricular function, it is possible that the magnitude of respiratory fluctuations in Ppl importantly influences cardiac output (pulmonary blood flow, QL) during exercise. To examine this question, we increased (+15 cmH2O) and decreased (-11 cmH2O) the amplitude of fluctuations in Ppl by elastic loading and unloading, respectively, during steady-state exercise (50 W) and estimated the corresponding changes in QL from measurement of breath-by-breath alveolar O2 consumption [(Vo2)A] by a modification of the technique of Beaver et al. (J. Appl. Physiol. 51: 1662-1675, 1981). Load changes were applied for three breaths. Using oscilloscopic volume feedback, subjects maintained constant breathing pattern and end-expiratory volume during control and experimental breaths. This procedure minimized errors in computing (Vo2)A. Furthermore, because over the brief period of load change (especially the first 1 or 2 breaths) mixed venous and arterial O2 contents were not likely to have changed, changes in (Vo2)A reflected changes in QL according to the Fick principle. In six normal subjects, neither loading nor unloading had any effect on (Vo2)A in the first, second, or third breath (P greater than 0.5). Additional studies at rest produced equally negative results. We conclude that the magnitude of respiratory fluctuations in Ppl has no short-term effect on pulmonary blood flow at rest or during mild exercise.     

Increases in intramuscular pressure raise arterial blood pressure during dynamic exercise.

This investigation was designed to determine the role of intramuscular pressure-sensitive mechanoreceptors and chemically sensitive metaboreceptors in affecting the blood pressure response to dynamic exercise in humans. Sixteen subjects performed incremental (20 W/min) cycle exercise to fatigue under four conditions: control, exercise with thigh cuff occlusion of 90 Torr (Cuff occlusion), exercise with lower body positive pressure (LBPP) of 45 Torr, and a combination of thigh cuff occlusion and LBPP (combination). Indexes of central command (heart rate, oxygen uptake, ratings of perceived exertion, and electromyographic activity), cardiac output, stroke volume, and total peripheral resistance were not significantly different between the four conditions. Mechanical stimulation during LBPP and combination conditions resulted in significant elevations in intramuscular pressure and mean arterial pressure from control at rest and throughout the incremental exercise protocol (P < 0.05). Conversely, there existed no significant changes in mean arterial pressure when the metaboreflex was stimulated by cuff occlusion. These findings suggest that under normal conditions the mechanoreflex is tonically active and is the primary mediator of exercise pressor reflex-induced alterations in arterial blood pressure during submaximal dynamic exercise in humans.     

24-hour homeodynamic states of arterial blood pressure and pulse interval in conscious rats.

In models that describe the homeostasis of the circulation, arterial blood pressure is usually expressed as a single value, which is regarded as the set point in such systems. The aim of the study was to identify in rats from 24-h beat-to-beat recordings the value of blood pressure that describes best such a set point of the cardiovascular system. Normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR), kept on a 12:12-h lights on-off cycle, were instrumented for computerized 24-h beat-to-beat recording of mean arterial pressure (MAP) and pulse interval (PI). Three-dimensional frequency distributions were constructed by plotting for each beat its MAP vs. its PI. During the dark period, the concurrent distribution of MAP and PI showed two distinct modes while during the light period a single mode was found. Comparable patterns were found in SHRs and WKYs. These three different modes were significantly different from the mathematically calculated mean values of MAP and PI over these periods. Thus in rats the 24-h behavior of the cardiovascular system is better described by dynamic shifts between different modes (homeodynamic states) than by a single set point.     

Repeated bout effect is not correlated with intraindividual variability during muscle-damaging exercise

The aim of this study was to test the hypothesis that the repeated bout effect depends on intraindividual variability during a second bout of eccentric exercise. Eleven healthy men performed 2 resistance training bouts consisting of maximal eccentric exercise (EE1 and EE2) using the knee extensor muscles. The interval between the exercise bouts was 2 weeks and consisted of 10 sets of 12 repetitions at 160° · s(-1). Maximal isokinetic concentric torque at 30° · s(-1) was measured before the bouts and 2 minutes and 24 hours thereafter. Muscle soreness score and creatine kinase activity were determined before and after exercise. Intraindividual variability in torque during each eccentric repetition was measured during exercise. Repeated bout effect manifested after EE2: Muscle soreness was less, the shift in optimal knee joint angle to a longer muscle length was less, and the decrease in isokinetic concentric torque 2 minutes after exercise was less for EE2 compared with that for EE1. During concentric (isokinetic) contraction, length-dependent changes in isokinetic torque (IT) occurred after both EE1 and EE2: The shorter the muscle length, the greater the change in IT. There was a significant relationship between the decrease in maximal isokinetic knee extension torque 24 hours after EE1 and intraindividual variability of EE1 (R2 = 0.71, p < 0.05), but this relationship was not significant for EE2 (R2 = 0.18). It seems that intraindividual variability during eccentric exercise protects against muscle fatigue and damage during the first exercise bout but not during a repeat bout. These findings may be useful to coaches who wish to improve muscle function in resistance training with less depression in muscle function and discomfort of their athletes, specifically, when muscle is most sensitive to muscle-damaging exercise.     

Comparison of three methods for beat-to-beat-interval extraction from continuous blood pressure and electrocardiogram with respect to heart rate variability analysis.

In recent years the analysis of heart rate variability (HRV) has become a suitable method for characterizing autonomous cardiovascular regulation. The aim of this study was to investigate the differences in HRV estimated from continuous blood pressure (BP) measurement by different methods in comparison to electrocardiogram (ECG) signals. The beat-to-beat intervals (BBI) were simultaneously extracted from the ECG and blood pressure of 9 cardiac patients (10 min, Colin system, 1000-Hz sampling frequency). For both data types, slope, peak, and correlation detection algorithms were applied. The short-term variability was calculated using concurrent 10-min BP and ECG segments. The root mean square errors in comparison to ECG slope detection were: 1.74 ms for ECG correlation detection; 5.42 ms for ECG peak detection; 5.45 ms for BP slope detection; 5.75 ms for BP correlation detection; and 11.96 ms for BP peak detection. Our results show that the variability obtained with ECG is the most reliable. Moreover, slope detection is superior to peak detection and slightly superior to correlation detection. In particular, for ECG signals with higher frequency characteristics, peak detection often exhibits more artificial variability. Besides measurement noise, respiratory modulation and pulse transit time play an important role in determining BBI. The slope detection method applied to ECG should be preferred, because it is more robust as regards morphological changes in the signals, as well as physiological properties. As the ECG is not recorded in most animal studies, distal pulse wave measurement in combination with correlation or slope detection may be considered an acceptable alternative.     

Middle cerebral artery flow velocity and pulse pressure during dynamic exercise in humans

Exercise challenges cerebral autoregulation (CA) by a large increase in pulse pressure (PP) that may make systolic pressure exceed what is normally considered the upper range of CA. This study examined the relationship between systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean arterial pressure (MAP) and systolic (V(s)), diastolic (V(d)). and mean (V(m)) middle cerebral artery (MCA) blood flow velocity during mild, moderate, and heavy cycling exercise. Dynamic CA and steady-state changes in MCA V in relation to changes in arterial pressure were evaluated using transfer function analysis. PP increased by 37% and 57% during moderate and heavy exercise, respectively (P < 0.05), and the pulsatility of MCA V increased markedly. Thus exercise increased MCA V(m) and V(s) (P < 0.05) but tended to decrease MCA V(d) (P = 0.06). However, the normalized low-frequency transfer function gain between MAP and MCA V(m) and between SBP and MCA V(s) remained unchanged from rest to exercise, whereas that between DBP and MCA V(d) increased from rest to heavy exercise (P < 0.05). These findings suggest that during exercise, CA is challenged by a rapid decrease rather than by a rapid increase in blood pressure. However, dynamic CA remains able to modulate blood flow around the exercise-induced increase in MCA V(m), even during high-intensity exercise.     

Optimal frequency ranges for extracting information on cardiovascular autonomic control from the blood pressure and pulse interval spectrograms in mice

The analysis of blood pressure (BP) and heart rate (HR) variability by spectral methods has proven a useful tool in many animal species for the assessment of the vagal and sympathetic contributions to oscillations of BP and HR. Continuous BP measurements obtained in mice by telemetry were used to characterize the spectral bandwidths of autonomic relevance by using an approach with no a priori. The paradigm was based on the autonomic blockades obtained with conventional drugs (atropine, prazosin, atenolol). The spectral changes were estimated in all of the combinations of spectral bandwidths. The effect of hydralazine was also tested using the same systematic analysis, to detect the zones of sympathetic activation resulting reflexly from the vasodilatory action of the drug. Two zones of interest in the study of the autonomic control of BP and HR were observed. The first zone covered the 0.15-0.60 Hz range of the systolic BP spectrum and corresponds to the low-frequency zone (or Mayer waves). This zone reflects sympathetic control since the power spectral density of this zone was significantly reduced with alpha1-adrenoceptor blockade (prazosin), while it was significantly amplified as a result of a reflex sympathetic activation (hydralazine). The second zone covered the 2.5-5.0 Hz range of the pulse interval spectrum and corresponded to the high-frequency zone (respiratory sinus arrhythmia) under vagal control (blocked by atropine). These zones are recommended for testing the autonomic control of circulation in mice.     

Effect of heavy exercise on spectral baroreflex sensitivity, heart rate, and blood pressure variability in well-trained humans

The aim of the study was to assess the instantaneous spectral components of heart rate variability (HRV) and systolic blood pressure variability (SBPV) and determine the low-frequency (LF) and high-frequency baroreflex sensitivity (HF-BRS) during a graded maximal exercise test. The first hypothesis was that the hyperpnea elicited by heavy exercise could entail a significant increase in HF-SBPV by mechanical effect once the first and second ventilatory thresholds (VTs) were exceeded. It was secondly hypothesized that vagal tone progressively withdrawing with increasing load, HF-BRS could decrease during the exercise test. Fifteen well-trained subjects participated in this study. Electrocardiogram (ECG), blood pressure, and gas exchanges were recorded during a cycloergometer test. Ventilatory equivalents were computed from gas exchange parameters to assess VTs. Spectral analysis was applied on cardiovascular series to compute RR and systolic blood pressure power spectral densities, cross-spectral coherence, gain, and alpha index of BRS. Three exercise intensity stages were compared: below (A1), between (A2), and above (A3) VTs. From A1 to A3, both HF-SBPV (A1: 45 +/- 6, A2: 65 +/- 10, and A3: 120 +/- 23 mm2Hg, P < 0.001) and HF-HRV increased (A1: 20 +/- 5, A2: 23 +/- 8, and A3:40 +/- 11 ms2, P < 0.02), maintaining HF-BRS (gain, A1: 0.68 +/- 0.12, A2: 0.63 +/- 0.08, and A3: 0.57 +/- 0.09; alpha index, A1: 0.58 +/- 0.08, A2: 0.48 +/- 0.06, and A3: 0.50 +/- 0.09 ms/mmHg, not significant). However, LF-BRS decreased (gain, A1: 0.39 +/- 0.06, A2: 0.17 +/- 0.02, and A3: 0.11 +/- 0.01, P < 0.001; alpha index, A1: 0.46 +/- 0.07, A2: 0.20 +/- 0.02, and A3: 0.14 +/- 0.01 ms/mmHg, P < 0.001). As expected, once VTs were exceeded, hyperpnea induced a marked increase in both HF-HRV and HF-SBPV. However, this concomitant increase allowed the maintenance of HF-BRS, presumably by a mechanoelectric feedback mechanism.     

Geographic distributions of within-population variability in blood pressure

Data on mean blood pressure with standard deviation were extracted from the medical and anthropologic literature for as many populations as possible. The populations were classified as traditional, transitional, or modern. Both mean and within-population variability were found to be higher in more modern populations, confirming the prediction that variability of a character of low heritability is higher in an adverse environment, where it is more difficult to maintain homeostasis. In addition, variability increases with age, indicating a breakdown in homeostasis with aging. On average, males had higher diastolic blood pressure than females, and on average, females had greater variability in systolic blood pressure than males. There was a highly significant negative relationship between latitude and within-population variability in blood pressure.     

Beta-adrenoceptors and the regulation of blood pressure and plasma renin during exercise

The relative role of beta 1- and beta 2-adrenoceptors in the regulation of blood pressure and plasma renin at rest and during exercise was studied in 17 normal male volunteers. They performed, in a randomized order and according to a double-blind crossover study design, three graded and uninterrupted exercise tests until exhaustion after being pretreated during 3 consecutive days with a placebo, with a predominantly beta 1-blocker (atenolol, 50 mg once/day), or with a predominantly beta 2-blocker (ICI 118551, 20 mg 3 times/day). Both drugs caused a decrease of heart rate, but the reduction by ICI 118551 was less pronounced at rest and no additional decline occurred at exercise. ICI 118551 did not affect blood pressure at rest, but during exercise diastolic blood pressure was higher than after a placebo. Atenolol lowered systolic blood pressure at rest and suppressed the exercise-induced increase in systolic blood pressure. At rest and during exercise plasma renin activity was lowered by predominantly beta 1-blockade and unchanged during beta 2-antagonism. The exercise-induced increase in plasma renin was, however, not affected by the beta 1-blocker. After atenolol the urinary excretion of aldosterone was decreased but the plasma aldosterone concentration was not changed. ICI 118551 did not alter plasma or urinary aldosterone. Our results therefore provide further evidence that the adrenoceptors mediating the release of renin at rest and during exercise in humans are partially of the beta 1-subtype, whereas beta 2-adrenergic receptors probably play only a minor role in the control of renin secretion, especially at exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cultural variations in the placebo effect: ulcers, anxiety, and blood pressure

An analysis of the control groups in double-blind trials of medicines demonstrates broad variation--from 0 to 100 percent--in placebo effectiveness rates for the same treatment for the same condition. In two cases considered here, drug healing rates covary with placebo healing rates; placebo healing is the ultimate and inescapable "complementary medicine." Several factors can account for the dramatic variation in placebo healing rates, including cultural ones. But because variation differs by illness, large placebo effects for one condition do not necessarily anticipate large placebo effects for other conditions as well. Deeper understanding of the intimate relationship between cultural and biological processes will require close ethnographic scrutiny of the meaningfulness of medical treatment in different societies.