Mechanisms of Blood Flow in the Human Forearm during Mental Stress, Following Isometric Exercise and after Occlusion -------------------------------------------------
A common technique in measuring blood flow in the human forearm is by venous occlusion plethysmography. The fundamental principle is that if the venous return from the arm is blocked, the forearm will swell at a rate relative to the rate of the arterial inflow (Whitney, 1953). However, the rate of increase in volume of the forearm and its relationship with the arterial flow can be true only if the veins in the forearm are not completely distended because when the veins are full, a rise in the pressure and the blood will run off under the congesting cuff and so the volume of the forearm cannot rise in proportion to the flow of the artery (Benjamin, 1995).
An alternative method of measuring the blood flow is by Doppler ultrasound which is used to estimate the mean blood velocity and involve measuring the diameter of the vessel. Although this technique has an advantage to determine the conduit vessel diameter at the same time (Wilkinson,et al., 2001). However, as area is attained by squaring the diameter, a few errors made in the estimation of the diameter will result in large errors in the calculation of the flow. For this experiment, since the aim of this study was to measure the effects in the forearm blood flow caused by mental exercise, forearm exercise and venous occlusion exercise, plethysmography has the advantage over Doppler ultrasound technique.
Several mechanisms regulate local blood flow, originating either in blood vessels (myogenic and endothelial factors) or from the surrounding tissue and biochemical pathways. These mechanisms act independently of extrinsic controls such as the sympathetic nervous system and endocrine system. The balance between extrinsic control and local mechanisms determines vascular tone and blood flow. In a rest-to-exercise transition, muscle blood flow typically increases to a steady state level regulated to metabolic demand for oxygen (Tschakovsky & Sheriff, 2004). The factors responsible for the early exercise hyperemia are thought to include the muscle pump effect and rapid vasodilatory mechanisms. During exercise, sympathetic nervous activity (SNA) is increased mainly by muscle chemo-reflex whereas central command raises heart rate (HR) and cardiac output (CO) by vagal withdrawal (Rowell & O’Leary, 1990).
In this experiment, venous occlusion plethysmography was used to observe the active hyperaemia brought on by muscle contraction through squeezing of a sponge rubber for several minutes until task failure and mental exercise by doing basic mental arithmetic problems for 2 minutes. And finally, reactive hyperaemia was tested when the circulation was completely arrested for 4 minutes with the inflation of the upper cuff kept above 200mmHg.
Following conditions in this experiment, flow mediated dilation (NO) should be responsible for change in flow after mental stress, active hyperemia due to exercise as well as flow mediated dilation caused by shear stress, and ischemia caused by occlusion to be countered with reactive hyperemia.
Average baseline measurements (n=34) of the forearm blood flow averaged 25 ± 5.4(SEM) mV/sec. During the strenuous mental exercise, forearm perfusion increased by a factor of 1.2 ± 0.1(SEM) in the first minutes and gradually declined to a factor of 1.1 ± 0.1 (SEM) (figure 1). After the mental stress, forearm perfusion returned to baseline values and remained level. A t-test was conducted between baseline and during mental stress, baseline and after mental stress as well as during and after mental stress, yielded a non significant P value (P = 0.5), a non significant P value (P = 0.3) and a significant P value (P = 0.002) respectively where P<0.05 is significant.
Figure 1: Mean (n = 34) results indicating change in blood flow to the...
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