Exercise Physiology Section 8 PDF

Section 8 Autonomic response during exercise Prof.dr.heba shawky ii- Heart rate variation during postural change ii- Heart rate variation during postural change (procedure): 1-Continuous monitoring of HR for 30 seconds prior to and 60 seconds after standing. 2-In normal individuals, reflex...

Section 8 Autonomic response during exercise Prof.dr.heba shawky ii- Heart rate variation during postural change ii- Heart rate variation during postural change (procedure): 1-Continuous monitoring of HR for 30 seconds prior to and 60 seconds after standing. 2-In normal individuals, reflex acceleration of heart rate is maximal approximately 15 seconds after standing and then gradually slows back to near-supine rate. Failure of heart rate to increase with the development of symptomatic orthostatic hypotension is indicative of autonomic dysfunction. Causes of autonomic dysfunction: I- Diabetes Mellitus II- Nutritional causes such as vit. B12 deficiency III- Toxic /metabolic causes such as porphyria IV- Central causes such as cerebral vascular accidents, central hemorrhages, and syringomyelia. V- Primary (idiopathic) degeneration of autonomic postganglionic fibers iii- Heart rate response to valsalva maneuver: - The Valsalva is forced expiration against resistance - The manoeuvre can result in complex transient cardiovascular effect associated with the increased intrathoracic and intra-abdominal pressure with corresponding changes in blood pressure (BP). - It is relatively simple, inexpensive, non-invasive and reproducible method. - The test requires the maintenance of forced expiration against resistance for 15 seconds, with intrathoraic pressure of about 40 mmHg (measured by aneroid manometer). Phases of Valsalva: Phase I (onset of strain): Blood pressure increases slightly due to increased intrathoracic pressure (transmitting the intra thoracic pressure to the rest of the circulation) Phase II (continued strain): Arterial pressure decreases, so heart rate begins to increase. (Continuous elevation of intrathoracic and intra-abdominal pressure impedes venous return to the heart with decrease in cardiac output and arterial pressure.The reduced arterial pressure will be sensed by arterial baroreceptors, causing increased sympathetic activity to the heart lead to corresponding tachycardia) Phase III (release): Further drop in blood pressure due to a sudden drop in intrathoracic pressure, and the heart rate increase is sustained Phase IV (Recovery): - Is associated with increased cardiac output, "overshoot" hypertension, and finally a reflex bradycardia. - The restoration of venous return causes a continuous rise in diastolic heart filling, thus improving the cardiac function leading to; * An increased stroke volume and cardiac output Pressure overshoot The arterial BP rises in this phase typically 20–40 mmHg above the baseline pressure, although in some individuals, it can increase even to 80 mmHg above control values. (This is due to the fact that the previously released noradrenaline is still circulating in the system, thus preventing the vessels to dilate, and to the inertia of the constricted vessels and the high blood volume ejected from the left ventricle to the constricted arterial tree leads therefore to a significant increase in the arterial pressure (so-called pressure overshoot)  Reflex bradycardia The activity of arterial baroreceptors which responding to the increased level of pressure initiates reflex slowing down of the heart rate. N.B.: - However valsalva maneuver is commonly accepted as a safe method of testing cardiac function or the integrity of the autonomic nervous system, the manoeuvre is not free of potential side effects - The manoeuver results in an increased intra-ocular pressure which can lead to retinal or macular haemorrhage