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Biomedical subjects

D S Miles

Publications and source records attributed to D S Miles.

51 records · Page 3Linked to original sources
PubMed1982

Transthoracic impedance: differences between men and women with implications for impedance cardiography.

Impedance cardiography (IC) is a reliable noninvasive technique for monitoring stroke volume (SV) and cardiac output. Transthoracic impedance (Zo) is one variable in the equation used for the calculation of SV. Thoracic impedance reflects the resistivity offered by tissues and air and the length and cross-sectional area of the thoracic volume. The purpose of this study was to evaluate possible differences in Zo between men and women. Measurements (Mean +/- S.D.) of Zo in 29 men (age 25.6 +/- 4.6 yr) and 35 women (24.2 +/- 6.0 yr) in the seated posture revealed Zo values were significantly (p less than 0.05) greater for the women (31.5 +/- 3.3 ohm) than the men (23.5 +/- 1.84 ohm). The observed differences in Zo cannot be attributed to thoracic length; distances between the monitoring electrodes were similar for the men (25.1 cm) and the women (25.7 cm). We hypothesize that the greater Zo observed for women is due to a smaller thoracic cross-sectional area and greater resistivity resulting primarily from relatively more fat tissue, smaller heart size, and lesser central blood volume as compared with men.

Adolescent↗
PubMed1981

Hypoxia effects on plasma volume shifts at rest, work, and recovery in supine posture.

The purpose of this study was to determine the effects acute exposure to hypoxia (less than 1.5 h; gas mixture 12.6% O2-87.4% N2) has on plasma volume (PV) shifts during rest and exercise. Nine unacclimatized males performed identical protocols in the supine posture in both normoxic (N) and hypoxic (H) conditions. The protocol was rest 60 min, submaximal exercise (PWC 140, work rate eliciting a heart rate of 140 beats/min) for 30 min, immediately followed by maximal exercise, and 10 min of passive recovery. There were slight but significant losses of 2.7 and 1.4% PV at rest in H and N conditions, respectively. At the same relative intensity of submaximal exercise (work loads reduced by 22% in H conditions), PV losses were nearly identical (N = 11.2%, H = 11.8%). There was a further PV efflux subsequent to maximal exercise (N = 7.9%, H = 5.2%). The maximum PV efflux, from the beginning of rest to the end of maximal exercise, was 20% for both conditions. Total plasma protein (PP) content was unchanged during rest or exercise for either N or H conditions. After 10 min of recovery, restitution of PV was 10% below preexercise values for both N and H conditions. We conclude that acute hypoxic exposure does not influence the loss of VP or PP during submaximal or maximal exercise.

Adult↗
PubMed1981

Influence of respiration on stroke volume determined by impedance cardiography.

The conventional method of calculating stroke volume (SV) with impedance has been to measure changes in thoracic impedance for only those beats of the impedance cardiogram (IC) which cross on a specific baseline. This severely limits the number of usable beats since respiration produces oscillation of the IC around this baseline. This study investigated the influence of respiration on SV calculated independent of the baseline in seven women, 20-44 years old, in each of three postures: seated, supine, and standing. SV was determined in three continuous respiratory cycles from the following beats: 1. all; 2. all inspiratory (I); 3. I on baseline; 4. all expiratory (E); 5. E on baseline; 6. all end-E; 7. end-E on baseline (conventional). The absolute values for SV, heart rate (HR), and cardiac output (Q) were posture dependent. SVs did not differ among the respiratory phases in the standing and seated postures; in the supine posture, the smallest SV occurred at end-E on baseline. HR varied with respiration in both the standing and seated postures but not in the supine posture. Despite significant changes in HR in the seated and standing postures, Qs did not differ among the respiratory phases. Supine Qs were significantly smaller during end-E due primarily to a small SV. Consequently, when reporting Q values for supine subjects, it is important to designate the respiratory phase and baseline criteria. Our results indicate that SVs calculated independent of the baseline in general agree with the more conventional method of calculation. Therefore, we recommend the calculation of SV independent of the baseline. The reliability of the data will be increased by increasing the number of observations and SVs can be determined under circumstances where it is difficult to obtain end-E beats on the baseline.

Adult↗
PubMed1981

Evaluation of myocardial performance during wheelchair ergometer exercise.

Since a relatively high incidence of cardiovascular disease is associated with wheelchair confinement, exercise stress testing techniques should be utilized to assess myocardial performance of wheelchair-dependent individuals. Therefore, the purpose of this study was to apply the techniques of impedance cardiography to graded wheelchair-type exercise. For this, 9 wheelchair-dependent volunteers completed a progressive intensity, discontinuous test on a wheelchair ergometer (WERG) at power outputs (PO) of 10, 20, and 30 watts. An impedance cardiogram (ZCG), electrocardiogram (ECG), and phonocardiogram (PCG) were recorded for 15 sec immediately following steady state exercise. Stroke volume (delta V) calculated from the ZCG was multiplied by steady state heart rate (HR) to estimate cardiac output (Q). The simultaneous recording of the ZCG, ECG, and PCG permitted calculation of systolic time intervals and impedance cardiography contractility indices. Values for delta V, HR, Q, arteriovenous oxygen difference, systolic and diastolic blood pressure, mean systolic ejection rate, the first derivative of the impedance change, and the Heather Index increased with increments in PO. The Q-S2 interval, left ventricular ejection time (LVET), the pre-ejection period (PEP), the ratio of PEP to LVET, and the R-Z interval decreased with increases in exercise intensity. A linear relationship was found between Q and oxygen uptake which was similar to that reported by other investigators for arm exercise. These data indicate that impedance cardiography may be used in conjunction with electrocardiography and phonocardiography for the non-invasive assessment of myocardial performance during wheelchair exercise testing.

Adult↗
PubMed1980

Alactic capacity and power: reliability and interpretation.

The purpose of this investigation was to determine test-retest reliability for measuring maximal values of alactic capacity (ALCmax) and alactic power (ALPmax). For this, 31 men and 13 women each completed two ALCmax and ALPmax tests. Test-retest correlation coefficients for ALCmax and ALPmax values were 0.78 and 0.90, respectively. In addition, for both variables no significant differences were found between test no. 1 and test no. 2. Mean ALCmax values of 429 and 317 J . kg-1, as well as mean ALPmax values of 3.79 and 3.08 W . kg-1 were found for the men and women, respectively. Both ALCmax and ALPmax values were lower (p less than 0.05) for women than men. However, for the capacity power ratio (ALCmax . ALPmax-1) no difference was found between men and women. A correlation coefficient of r = 0.61 was found between mean values for ALCmax and ALPmax . ALCmax values were discussed as being related to skeletal muscle phosphagen concentrations, and ALPmax values were discussed as being related to phosphagen splitting rate. The observed sex differences were suggested to reflect a greater percent body fat in women than men.

Adenosine Triphosphate↗
PubMed1980

Physiological adjustments of women to prolonged work during acute hypoxia.

Five women (23-32 yr) performed bicycle work in a hypobaric chamber for 2 h at 41% of their respective altitude maximal oxygen uptakes (VO2) at 758, 586, 523, and 446 Torr barometric pressures (PB). Steady-state VO2 was achieved within 5 min work at all altitudes. Pulmonary ventilation (29.2 +/- 1.9 (mean +/- SE) 1/min, BTPS), respiratory rate (22 +/- 2 breaths/min), cardiac output (8.5 +/- 1.4 1/min), heart rate (115 +/- 6 beats/min), and stroke volume (75 +/- 13 ml) were similar at all altitudes, but time-related changes differed with altitude. Blood lactates did not change with work duration and were similar at 758 and 586 Torr PB but progressively elevated at 523 and 446 Torr. Blood norepinephrine, measured only at 758 and 446 Torr PB, increased with work but not altitude; epinephrine increased only at altitude. Norepinephrine levels and respiratory, cardiovascular, and thermoregulatory functions were essentially dependent on relative work load; blood lactates and epinephrine levels were not. Many physiological functions in these women performing sustained light work during acute altitude exposure were remarkably stable in contrast to previously reported studies on men.

Acclimatization↗
PubMed1980

Absolute and relative work capacity in women at 758, 586, and 523 torr barometric pressure.

Six young women performed an incremental bicycle work test at sea level barometric pressure (PB = 758 torr) and during acute exposure (1 h) to simulated altitudes of PB 586 and 523 torr. Submaximal oxygen uptake (VO2) for a given workload was independent of altitude but maximal oxygen uptake (VO2 max) decreased 10 and 13%, respectively, at the higher altitudes. Although heart rate (fC) was consistently higher at altitude for a given VO2, the slope of fC vs, VO2 was independent of altitude and VO2 max. Exercise fC appeared to be a function of the relative workload i.e. VO2 as a percentage of VO2 max measured at each PB. Carbon dioxide (CO2) elimination increased with altitude for a given VO2 but also was a function of the relative workload. Pulmonary ventilation (BTPS), however, was consistently 10-15% higher at altitude when expressed as a percent of VO2 max, primarily due to an increase in respiratory rate. Compared to published studies on males, this increased ventilation may impart a slight advantage to women in maintaining arterial oxygenation, but ventilatory reserve may be decreased and limited at higher altitudes. At altitudes down to PB 523 torr, the control of fC responses and decrements in maximal oxygen uptake in women were similar to males, but ventilatory control mechanisms differed.

Adult↗
PubMed1980

Effects of toeclip use on metabolic and cardiopulmonary responses to bicycle ergometer exercise.

Toeclips secure the feet to bicycle pedals and are thought to enable greater anticipation of leg musculature in the pedaling motion. To determine the effects of toeclip use on metabolic and cardiopulmonary responses to BERG exercise, 13 female subjects, who had no prior experience with the devices, performed maximal and submaximal work with and without the use of toeclips. Similar oxygen uptake (V02), physical work capacity, pulmonary ventilation (VE), respiratory exchange ratio (R), heart rate (HR), and blood lactate concentration (LA) were found for maximal effort exercise under both conditions. For submaximal exercise (at 30, 60, and 80% maximal V02) performed with and without toeclips, no differences were found for V02, VE, R, HR, LA, cardiac output, stroke volume, and arteriovenous oxygen difference. These data suggest that toeclip use during BERG exercise may not alter metabolic and cardiopulmonary responses of individuals who have no prior experience with the devices.

Adult↗
PubMed1980

Cardiovascular, metabolic, and ventilatory responses of women to equivalent cycle ergometer and treadmill exercise.

The relationships between age and cardiorespiratory and metabolic adjustments during cycle ergometer exercise and treadmill walking were investigated at 30%, 60%, 80%, and maximal aerobic power (VO2max). The subjects were 18 females between the ages of 17-40. Tr-admill walking resulted in an 8 percent greater maximal aerobic power than that observed during cycle ergometer exercise. Higher lactate concentrations, respiratory exchange ratios and lower heart rates during the cycle ergometer test supported the possibility of localized fatigue from anaerobic metabolism as a limitation to aerobic power potential. Contrary to previous observations in males, female subjects die not demonstrate a lower stroke volume or higher heart rate during submaximal cycling compared to treadmill walking. At exercise intensities above 30% Vi2max, plasma bicarbonate and pH were consistently lower following exercise on the cycle ergometer. Respiratory compensation as calculated from "excess ventilation," delta VE/delta HCO3-, was approximately 4 1 min-1/mEq 1-1. Compensation was found to be independent of the type of exercise but linearly related (r=0.99) to the resultant metabolic acidosis. It was concluded that cardiovascular responses to equivalent cycle ergometer and treadmill exercise are similar; however, occurrence of a greater metabolic acidosis during cycle ergometer exercise suggests that a smaller muscle mass is used.

Adolescent↗
PubMed1979

Post-competition blood lactate concentrations in collegiate swimmers.

The purpose of this investigation was to quantitate post-competition lactate (LA) concentrations of swimmers during a competitive collegiate meet. Blood LA was measured by an enzymatic method on 23 subjects 5 min after each race event. The largest mean LA concentration of 25.7 mM/L was observed in swimmers after competing in the 200-yd individual medley. Swimmers in the 200-yd butterfly, back, breast and freestyle races had similar mean blood LA concentrations (ranging from 16.4 to 20.6 mM/L). Swimmers in the two longest events, the 500-yd and 1,000-yd free style races, had mean LA concentrations of 15.6 and 10.0 mM/L, respectively. To account for the effects of motivation, LA concentrations were measured following maximal effort noncompetitive 100 and 200-yd swims. LA concentrations were slightly greater in conjunction with faster performances for the competitive as compared to the noncompetitive 100 and 200-yd swims.

Adult↗
PubMed1979

Maximal work capacity of women during acute hypoxia.

Six healthy women (22--34 yr of age) performed maximal bicycle work in a hypobaric chamber at sea level and at simulated altitudes of 2,130 and 3,050 m (barometric pressures, 758, 586, and 523 Torr). Maximal oxygen uptake (VO2max) decreased 10 and 15% from sea-level values at 2,130 and 3,050 m, respectively. At these altitudes minute ventilation (VEBTPS) increased 17 and 22% respectively, a consequence of increased respiratory rate (fR). Respiratory exchange ratios increased 10 and 14%, and oxygen pulse decreased 9 and 12%, respectively, at 2,130 and 3,050 m. Maximal blood lactates, heart rates, cardiac outputs, and plasma volume shifts were unaffected by these altitudes. Although during maximal work the percentage increases in VEBTPS, fR, and R that resulted from altitude exposure were greater in women than those previously reported for men, the decrements in VO2max were comparable to those in men. The results show that relative to their performance at sea level, men and women have equal ability to perform maximal work at altitudes up to 3,050 m.

Acute Disease↗
PubMed1979

Effect of prolonged running on physiological responses to subsequent exercise.

The purpose of this study was to compare metabolic and cardiopulmonary responses for submaximal and maximal exercise performed several days preceding (pre-test) and 45 min after (post-test) 21 miles of high intensity (70% VO2 max) treadmill running. Seven aerobically trained subjects' oxygen uptake, oxygen pulse, respiratory exchange ratio, heart rate, pulmonary ventilation, ventilatory equivalent of oxygen, and blood lactate concentration were determined for exercise during the pre- and post-test sessions. No differences were found for submaximal oxygen uptake, oxygen pulse, pulmonary ventilation and ventilatory equivalent of oxygen between the pre- and post-test values. Generally, submaximal heart rate responses were higher, and respiratory exchange ratio values were lower during the post-test. Reductions of maximal work time (12%), maximal oxygen uptake (6%) and maximal blood lactate concentration (47%) were found during the post-test. Thermal stress and glycogen depletion are possible mechanisms which may be responsible for these observed differences.

Adult↗
PubMed1978

Metabolic responses of untrained individuals to warm-up.

Seven untrained male subjects were studied for the effects of mild warm-up on oxygen uptake and lactic acid production. Each subject completed two standardized workloads on a bicycle ergometer requiring 75% of their physical work capacity. Protocols of the two tests consisted of either no warm-up or a 4-min warm-up preceding a 5-min exercise at approximately 80% of their maximal oxygen uptake. The contrasting protocols did not reveal any significant differences between heart rate, lactic acid, and oxygen uptake. The dominant influence on the metabolic processes was the absolute workload of the tasks and not the presence or absence of preliminary related activity. It was concluded that an untrained individual lacks the cardiovascular and cellular adaptations necessary to demonstrate metabolic benefits from warm-up.

Adult↗