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Biomedical subjects

J H Mitchell

Publications and source records attributed to J H Mitchell.

At least 181 records · Page 10Linked to original sources
PubMed1981

Effects of positive pressure breathing on right and left ventricular preload and afterload.

Positive pressure ventilation with positive end-expiratory pressure (PEEP) is used to treat patients with pulmonary edema. PEEP usually raises arterial oxygen tension, but also may reduce cardiac output and stroke volume. The fall in cardiac output could be due to a fall in preload, an increase in afterload, or a decrease in myocardial contractility of either ventricle. Right ventricular afterload increases during PEEP but not enough to cause cardiac output to fall. Right atrial transmural pressure increases; right ventricular diastolic shape changes with an increase in septal to lateral diameter. Left ventricular afterload and end-diastolic volume fall during PEEP even though left atrial transmural pressure is maintained; an exaggerated decrease in left ventricular diastolic septal to lateral diameter suggests that a septal shift impairs left ventricular end-diastolic volume.

Animals↗
PubMed1981

Chronic training with static and dynamic exercise: cardiovascular adaptation, and response to exercise.

To determine the acute and chronic effects of static and dynamic exercise upon the cardiovascular system, two groups of athletes were studied and compared to untrained control individuals. Thus, 12 long distance runners (LDR) and 17 competitive weight lifters (CWL) were compared to 10 light controls (LC) and 14 heavy controls (HC). The echocardiographically measured left ventricular mass (LVM) was shown to be increased in both groups of athletes. When this mass was related to lean body mass, the LDR demonstrated a significantly increased LVM, whereas the CWL had a LVM similar to that of the HC. During static handgrip exercise, the LDR maintained a relative bradycardia and, consequently, a lower calculated double product when compared to the LC, whereas the CWL reacted similarly to the HC. Further, the LDR demonstrated higher end-diastolic and higher end-systolic volume indices than the LC during static exercise. The exercising stroke volume index and the cardiac index were, however, not significantly different in the LDR compared to the LC. In contrast to the LDR, the cardiovascular dynamics of the CWL changed in a manner very similar to that of the HC during static exercise. This information suggests, therefore, that endurance training alters both the absolute and relative left ventricular mass and the response of the cardiovascular system to static exercise. On the other hand, static exercise training increases the absolute but not the relative left ventricular mass. Also, the immediate hemodynamic response to static exercise is similar in athletes who train with this form of exercise compared to untrained control subjects.

Adult↗
PubMed1981

Cardiovascular response to static exercise in the conscious cat.

A chronic, unanesthetized cat preparation was used as an experimental model for the study of cardiovascular response to static exercise. Seven cats were operantly conditioned to hold a bar with their right forelimb against increasing resistance for 15 seconds. The exercise regimen lasted from 3 to 6 months. The cats were instrumented with a left ventricular pressure transducer and a left atrial catheter. With exercise, heart rate (HR, + 10%), left ventricular systolic pressure (LVSP, + 16%), and maximal rate of pressure development (LV dp/dt, + 15%) increased significantly. The rise in HR began prior to the onset of exercise and reached peak in less than 5 seconds and then decreased to preexercise levels, although LVSP and LV dp/dt remained elevated during exercise. The magnitude of the LVSP response (range 23 to 31 mmHg) was constant after 2 weeks of training even though the exercise intensity was increased 200 g to 700 g. After 3 months of training, the cats were exercised at the intensity first observed to induce a significant LVSP response. Peak HR, LVSP, and LV dp/dt were significantly reduced. Selective autonomic blockade with propranolol, atropine, and combined atropine-propranolol was utilized to elucidate the role of the autonomic nervous system in the cardiovascular responses that occur during voluntary static exercise. The increased HR in response to exercise was mediated primarily by the parasympathetic nervous system, whereas the increases in LV max dp/dt were mediated by the sympathetic nervous system.

Animals↗
PubMed1981

Response of arterial blood pressure to static exercise in relation to muscle mass, force development, and electromyographic activity.

In young men, the arterial blood pressure response during static exercise at 40% of maximal voluntary contraction (MVC) and the response after the contraction with a cuff inflated both were increased when a greater mass of skeletal muscle was involved. Also in men with a previously injured leg, the arterial systolic pressure and the smoothed, rectified electromyographic activity (EMG) were greater during static contraction of the large, strong leg than during static contraction of the small, weak leg at the same percent MVC. However, the arterial systolic pressure and the EMG were almost the same during contraction of either leg when the same force was developed. Finally, in young men, either the force developed was held constant at 20% MVC or the level of the initial EMG at 20% MVC was held constant for 5 minutes. During the force-constant experiments, the EMG doubled and correlated with the 40% increase in mean arterial pressure (r = 0.89). In the EMG-constant experiments, force declined to about 12% MVC with an increase of only 20% in mean arterial pressure. These studies of the response of arterial blood pressure to static muscular contractions can best be explained by both central and peripheral control mechanisms. The central control mechanism "central command" is probably related to the central activity for recruitment of motor units and the peripheral control mechanism is probably mediated by muscle afferents which are excited by metabolic changes in the contracting skeletal muscle.

Arteries↗
PubMed1980

The spinal cord ventral root: an afferent pathway of the hind-limb pressor reflex in cats.

1. In anaesthetized cats the sciatic nerve was cut and the central end was stimulated at a high frequency and voltage. This caused an increase in arterial blood pressure and a rise in heart rate. The pressure response was diminished by dorsal root section but not completely eliminated until ventral root section (L4-S3). The tachycardia response was abolished by dorsal root section alone. 2. In other cats capsaicin was injected intra-arterially into the hind limb, causing elevations in both blood pressure and heart rate. Similar to the sciatic nerve stimulation experiments, the pressor response was principally reduced by dorsal root section but was further significantly decreased by ventral root section (L1-S3). The rise in heart rate was prevented by dorsal root section alone. 3. It is concluded that, in cats, the afferent pathway of the pressor response to sciatic nerve stimulation and to hind-limb capsaicin injection are conducted principally in the dorsal roots but also to a small extent in the ventral roots of the spinal cord. Although the tachycardia response appears to be conducted only through the dorsal roots, it is possible that at lower resting heart rates and by stimulation of a large population of the unmyelinated skeletal muscle afferents, the ventral root is a functional pathway.

Afferent Pathways↗
PubMed1980

The role of muscle mass in the cardiovascular response to static contractions.

1. Eleven men performed static contractions with the fingers (digits II and III), forearm (handgrip) and the knee extensors (knee angle 90 degrees ) at 40% maximal voluntary contraction (MVC) for 2 min. In seven of the subjects handgrip and knee extension were combined, both contractions held at 40% MVC. At the end of the contraction, cuffs were inflated to 250 mmHg for 3 min around the extremity which had been contracting. Continuous measurements were performed of force, heart rate and intra-arterial blood pressure (a. brachialis; 20 cm proximally), before and during contraction as well as during the recovery with and without cuffs inflated.2. Heart rate and blood pressure increased momentarily with the onset of a contraction whereafter a gradual further increase took place. This pattern of response was similar for all muscle groups studied. The increases during the contractions were in the order: fingers, forearm, knee extensors and combined forearm-knee extensors, with the difference between each muscle group contraction being significant.3. In the recovery period from a contraction with the cuff(s) inflated, heart rate returned to control level. Blood pressure also dropped, but remained elevated above pre-contraction level until the cuff(s) was released.4. The present data during contraction are at variance with earlier observations showing that the cardiovascular response to a static contraction is proportional to the% MVC regardless of the muscle mass involved in the contraction. Our findings are in line with the traditional concept of central and peripheral nervous inputs playing a role in the cardiovascular adjustments to exercise, with both the central and the peripheral factors being related to the mass of the muscles engaged in the exercise.

Adult↗
PubMed1980

Echocardiographic left ventricular masses in distance runners and weight lifters.

Sixty individuals including 17 competitive weight lifters (CWL), 12 competitive long-distance runners (LDR), 7 amateus (noncompetitive) weight lifters (AWL), 14 heavy controls (HC), and 10 light controls (LC) were studied at supine rest with echocardiographic determination of the left venticular mass (LVM) by the Penn convention. Lean body mass (LBM) was estimated by the Wilmore-Behnke method. The absolute LVM (mean +/- SE) was increased in the two competitive athlete groups compared to controls (LDR: 195 +/- 12; CWL: 190 +/- 10 vs. LC: 122 +/- 10; HC: 151 +/- 9 g). The AWL had a mass (174 +/- 20 g) intermediate between the LDR-CWL and the HC-LC groups. A significant (P = 0.033) correlation of LVM was found with LBM although the correlation coefficient was low (r = 0.276). Normalizing LVM by LBM revealed a significantly higher mass for LDR compared to all other groups but equalized CWL and HC (LDR: 3.2 +/- 0.2; CWL: 2.5 +/- 0.1; AWL: 2.5 +/- 0.2; HC: 2.3 +/- 0.2; LC: 2.0 +/- 0.2 g). These data suggest that training for competitive long-distance running (dynamic training) elevates LVM compared to nonathletic controls and CWL. On the other hand, training for weight lifting (static training) increases absolute LVM but only to the extent that LBM is increased.

Adult↗
PubMed1980

Cardiovascular responses to static exercise in distance runners and weight lifters.

Sixty individuals including 17 competitive weight lifters (CWL), 12 competitive long-distance runners (LDR), 7 amateur (noncompetitive) weight lifters (AWL), 14 heavy controls (HC), and 10 light controls (LC) were studied at supine rest and during static exercise at 40% of maximal voluntary contraction. Blood pressures were similar in all groups at rest (R) and exercise (EX), but the heart rate (HR) and calculated double product (DP) of the LDR were lower at rest (HR: 53 +/- 2.9 beats/min, DP: 6,346 +/- 402) and at fatigue (HR: 78 +/- 5.4 beats/min. DP: 12,739 +/- 1,011) compared to the control group (R-HR: 69 +/- 2.2 beats/min, DP: 8,553 +/- 372; EX-HR: 97 +/- 3.5 beats/min, DP: 16,345 +/- 836). The LDR demonstrated higher end-diastolic volume index (EDVI) and higher end-systolic volume index (ESVI) at rest (EDVI: 84 +/- 3.7, ESVI: 31 +/- 2.7 ml/m2) and at the time of fatigue (EDVI: 90 +/- 5, ESVI: 37 +/- 2.7 ml/m2) compared to the LC group (R-EDVI: 61 +/- 4.4, ESVI: 22 +/- 2.2; EX-EDVI: 75 +/- 3.4, ESVI: 27 +/- 3.2 ml/m2). The CWL, AWL, and control groups had similar HR, DP, and cardiac volumes at rest and during exercise. These data suggest that competitive endurance (dynamic exercise) training alters the cardiovascular response to static exercise. On the other hand, weight lifting (static exercise) training does not alter the cardiovascular response to static exercise.

Adult↗
PubMed1980

Cardiovascular response to static exercise during selective autonomic blockade in the conscious cat.

Selective autonomic blockade with propranolol, atropine, and combined atropine and propranolol was used to elucidate the role of the autonomic nervous system in the cardiovascular responses that occur during voluntary static exercise in conscious cats. Seven animals were operantly conditioned to hold a bar against a fixed resistance for a constant time of 15 seconds and were then placed on an exercise regimen which consisted of small weekly increments in resistance. With a resistance of 100 g. heart rate (HR) increased by 7%. With exercise at a resistance of 200 g, HR (10%), left ventricle systolic pressure (LVSP, 16%), and LV max dp/dt (18%) increased, and significant changes in these parameters persisted throughout the remainder of the training period. beta-Adrenergic receptor blockade with propranolol abolished the increase in LV max dp/dt, whereas HR and LVSP increased. After atropine, the increase in HR was abolished at the early training stages, whereas LVSP and LV max dp/dt increased. Administration of atropine and propranolol blocked the increase in HR and LV max dp/dt responses, whereas LVSP increased. In this study, the increased HR in response to exercise was mediated primarily by the parasympathetic nervous system, whereas the increases in LV max dp/dt were mediated by the sympathetic nervous system. Furthermore, the bradycardia accompanying chronic performance of isometric exercise resulted from both an increase in vagal tone and a decrease in sympathetic tone.

Animals↗
PubMed1979

Distribution of cardiac output during induced isometric exercise in dogs.

Studies were designed to characterize the distribution of cardiac output during induced isometric exercise in anesthetized dogs. The response to isometric exercise involved significant increases in heart rate (+12 +/- 3%(SE)), mean arterial pressure (+13 +/- 2%), cardiac output (+26 +/- 8%), and respiratory minute volume (+75 +/- 26%); total peripheral resistance did not change significantly. Significant changes in blood flow were observed during isometric exercise in kidneys (-18 +/- 6%) and contracting limb muscles (+453 +/- 154%). Flow to liver (hepatic artery), spleen, brain, and myocardium remained near control values. Section of spinal dorsal roots L6-L7 abolished the responses to isometric exercise except for the increase in flow to exercising limb muscles. Alpha-adrenergic receptor blockade abolished the decrease in renal blood flow during isometric exercise; however, the increase in flow to exercising limb muscles was not affected by either alpha- or beta-adrenergic blockade.

Animals↗
PubMed1979

Cardiac changes during behavioral stress in dogs.

Alterations in heart rate (HR), left ventricular systolic pressure (LVP), and maximum rate of left ventricular pressure development (LV dP/dtmax) during a Sidman avoidance task were studied in eight chronically prepared dogs. Four of these animals comprised a nonstressed control group. In the experimental group, in addition to phasic increases in HR, LVP, and LV dP/dtmax during the avoidance period of each day, tonic increases in these measures were also observed over the 13 days of the experiment. Left ventricular systolic pressure was found to be least sensitive to the stress procedure inasmuch as the phasic changes were no longer present after the 10th day and tonic levels were within base-line values by the 13th day. When alterations in cardiac activity were observed in the nonstressed animals, there were decreases in function. It was concluded that controlled behavioral stress produces increased cardiac performance without increased bodily activity. It was also hypothesized that preavoidance increases in heart rate in experimental animals were the result of vagal influences on the heart, whereas avoidance increases in HR, LV dP/dtmax, and LVP were functions of increased beta-sympathetic activity on the heart and adaptive peripheral vascular changes.

Animals↗
PubMed1979

Reflex control of the circulation by afferents from skeletal muscle.

Functionally, the pressor reflex, rather than the depressor reflex, appears to be the normal physiologic response to exercise. Available data indicate that chemoreceptors, probably in the form of free nerve endings, are stimulated by local factors, e.g., hypoxia or hyperkalemia. Impulses from these receptors are mediated by group IV afferent nerves to the central nervous system. Then, through efferent sympathetic and parasympathetic nerves, systemic blood pressure, cardiac output, heart rate, and myocardial contractility all increase. The baroreceptors tend to modify the reflex increase in heart rate. The potential significance of local control of the systemic circulation by neural reflexes from skeletal muscle is apparent when one considers that during exercise local control allows both rapid and precise control of the circulation from the metabolically active region of the body. However, the exact role of neural reflexes from skeletal muscle during voluntary exercise still remains to be defined. Clearly, more studies are necessary to elucidate fully their function in the overall control of the circulation in conscious animals and man.

Afferent Pathways↗