Blood Flow Lab Report
Mechanisms of Blood Flow in the Human Forearm during Mental Stress, Following Isometric Exercise and after Occlusion
-------------------------------------------------
1. Introduction
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 …show more content…
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.
2. Results
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 forearm from baseline (not shown), during and after mental stress over 2 minutes. Measurements were taken at 30 second interval for 2 minutes. Trend indicates a higher deviation from the baseline during the imposition of the mental stress, which declines after 60 seconds levels off. A t-test (paired with equal variance) yielded a significant P value (P = 0.002, significant when P < 0.005). Error bars comprise of SEM.
Baseline measurements (n=33) before the commencement of the physical exercise yielded an average of 32.7 ±8.1 (SEM) mV/second. The slope increased by a factor of 5 ± 0.8 (SEM) immediately after exercise ceased, and declined gradually to 3.3 ± 0.6 (SEM) at the fourth minute (figure 2). A t-test between baseline measurements and immediately post exercise failed to produce a significant P value (P = 0.054, significant when P<0.05).
Occlusion of forearm perfusion yielded a baseline slope measurement of 53.4 ± 15.2 (SEM) mV/ seconds for n = 33, which increased immediately post occlusion by a factor of 3.3 ± 0.7, however declined sharply to 1.6 ± 0.3 within 30 seconds and continued on a gradual decline to 0.8 during the third minute before returning to baseline values during the fourth minute (figure 2).
Evaluation of the t-test between pre and post occlusion yielded a significant relationship with P < 0.05.
Figure 2: Comparison of the mean (n = 33) of the relative changes in the slope between forearm perfusion immediately after strenuous physical exercise and blood flow occlusion. Measurements were taken at 30 second intervals for 4 minutes. Trend shows a gradual return to baseline perfusion post exercise and an initial sharp decline and then a gradual levelling off post occlusion. A t-test (paired and equal variance) between post exercise post occlusion yielded a significant P value (P<0.05). The error bars comprise SEM.
Greatest increase in blood flow to the forearm was observed post exercise, while minimal deviation in the slope was observed post mental stress. A t-test between mental stress and exercise and occlusion yielded significant P values (P= 0.000003 and 0.000913 respectively), no significant relationship was observed between exercise and occlusion (P> 0.05) (figure …show more content…
3).
Figure 3: Mean relative change in slope for mental stress (n=34), exercise and occlusion (n=33) at 30 seconds after cessation of stimulus. Errors bars comprise SEM.
3. Discussion
There may have been discrepancies in the results as only flow changes in the forearm muscles were used and involuntary twitches and/or accidental motion that may have caused a change in flow during experimentation cannot be accounted for. Also, with regards to the isometric contraction of the forearm muscles during exercise, there may be differences in vascular conductance and hyperaemia depending on the individual in terms of force used and time to task failure (fatigue) that will remain unaccounted for.
There is a significant difference in flow during and after mental stress as can be seen in Figure 1 and derived from a statistical t-test with P < 0.05. This is unexpected as the muscles in the forearm would not be directly involved during mental stress. Central command and SNA may cause a slight increase in HR and a slightly elevated BP (Rowell & O’Leary, 1990), observed in Figure 1 during mental exercise. Previous studies have indicated that NO plays a role in forearm vasodilation during mental stress in humans and that most of this NO is released by cholinergic stimulation of the vascular endothelium (Dietz et al. 1994). Other studies have proposed two possible mechanisms for this. One suggests that acetylcholine (Ach) might be released from some vascular endothelial cells during (usually mechanical) stimulation of the blood vessel, and stimulate release of NO by neighbouring endothelial cells (Milner et al. 1989). The second study suggests that NO may have been released directly from the nerves (Toda & Okamura 1991; Holmqvist et al. 1991) causing vasodilation.
Immediately following exercise, the relative change in slope is quite high and declines slowly in comparison to post-occlusion.
The metabolic demand for O2 would have increased drastically during exercise and remained high for some time after as can be seen in Figure 2, where the slope increased by a factor of 5 ± 0.8 mV/s from an average baseline value of 32.7 ±8.1mV/s immediately after exercise ceased. The active hyperemia observed here may be due to a combination of tissue hypoxia and the generation of local vasodilator metabolites such as potassium ions, carbon dioxide, nitric oxide, and adenosine. More recently, ideas have emerged that NO bound to haemoglobin may be released as oxygen is removed from the red blood cells, thus evoking vasodilatation in the active muscles. This is an attractive hypothesis because blood flow to areas where oxygen demand is high, would be elevated (Stamler et al. 1997). Unless there is another source of NO for the red blood cells or some other mechanism for NO production, it seems unlikely that this mechanism is a major and/or obligatory role in exercise hyperemia (Joyner & Wilkins 2007). The explanation of exercise hyperemia, until proven otherwise, must be due to a combination of
factors.
Reactive hyperaemia is observed in forearm perfusion post-occlusion and it appears that the effects are short lived as the relative change in slope drops very sharply as observed in Figure 2. Blood flow would have been increased immediately after the release from occlusion. The hyperemia occurs following major tissue hypoxia and a build up of vasodilator metabolites including adenosine and ATP-dependant K+ channels (Bank et al. 2000) dilate arterioles and decrease vascular resistance. Then when occlusion is released, flow becomes elevated. The longer the period of occlusion, the greater the metabolic stimulus for vasodilation leading to increases in peak reactive hyperemia and duration of hyperemia (Klabunde, 2005). During the hyperemia, the tissue becomes re-oxygenated and vasodilator metabolites are washed out of the tissue. This causes the resistance vessels to regain their normal vascular tone, returning flow to baseline. An interesting point to mention is in a new study by Raff et al. published recently this year in August 2010, who found that when ischemia was applied at the upper arm, the contribution of NO to the vasodilator response following ischemia did not increase with increasing duration of ischemia. Thus, NO mediates reactive hyperemia (RH) after ischemia applied to the upper arm to a lesser extent than RH after ischemia applied to the wrist. Other factors than NO are obviously more important in mediating RH after ischemia applied to the upper arm.
It appears that the release of NO is involved in all the hyperemia observed in this experiment, but may or not be the major factor in the explanation. It could be due to a combination of factors which further research could enlighten.
-------------------------------------------------
1. Introduction
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 …show more content…
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.
2. Results
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 forearm from baseline (not shown), during and after mental stress over 2 minutes. Measurements were taken at 30 second interval for 2 minutes. Trend indicates a higher deviation from the baseline during the imposition of the mental stress, which declines after 60 seconds levels off. A t-test (paired with equal variance) yielded a significant P value (P = 0.002, significant when P < 0.005). Error bars comprise of SEM.
Baseline measurements (n=33) before the commencement of the physical exercise yielded an average of 32.7 ±8.1 (SEM) mV/second. The slope increased by a factor of 5 ± 0.8 (SEM) immediately after exercise ceased, and declined gradually to 3.3 ± 0.6 (SEM) at the fourth minute (figure 2). A t-test between baseline measurements and immediately post exercise failed to produce a significant P value (P = 0.054, significant when P<0.05).
Occlusion of forearm perfusion yielded a baseline slope measurement of 53.4 ± 15.2 (SEM) mV/ seconds for n = 33, which increased immediately post occlusion by a factor of 3.3 ± 0.7, however declined sharply to 1.6 ± 0.3 within 30 seconds and continued on a gradual decline to 0.8 during the third minute before returning to baseline values during the fourth minute (figure 2).
Evaluation of the t-test between pre and post occlusion yielded a significant relationship with P < 0.05.
Figure 2: Comparison of the mean (n = 33) of the relative changes in the slope between forearm perfusion immediately after strenuous physical exercise and blood flow occlusion. Measurements were taken at 30 second intervals for 4 minutes. Trend shows a gradual return to baseline perfusion post exercise and an initial sharp decline and then a gradual levelling off post occlusion. A t-test (paired and equal variance) between post exercise post occlusion yielded a significant P value (P<0.05). The error bars comprise SEM.
Greatest increase in blood flow to the forearm was observed post exercise, while minimal deviation in the slope was observed post mental stress. A t-test between mental stress and exercise and occlusion yielded significant P values (P= 0.000003 and 0.000913 respectively), no significant relationship was observed between exercise and occlusion (P> 0.05) (figure …show more content…
3).
Figure 3: Mean relative change in slope for mental stress (n=34), exercise and occlusion (n=33) at 30 seconds after cessation of stimulus. Errors bars comprise SEM.
3. Discussion
There may have been discrepancies in the results as only flow changes in the forearm muscles were used and involuntary twitches and/or accidental motion that may have caused a change in flow during experimentation cannot be accounted for. Also, with regards to the isometric contraction of the forearm muscles during exercise, there may be differences in vascular conductance and hyperaemia depending on the individual in terms of force used and time to task failure (fatigue) that will remain unaccounted for.
There is a significant difference in flow during and after mental stress as can be seen in Figure 1 and derived from a statistical t-test with P < 0.05. This is unexpected as the muscles in the forearm would not be directly involved during mental stress. Central command and SNA may cause a slight increase in HR and a slightly elevated BP (Rowell & O’Leary, 1990), observed in Figure 1 during mental exercise. Previous studies have indicated that NO plays a role in forearm vasodilation during mental stress in humans and that most of this NO is released by cholinergic stimulation of the vascular endothelium (Dietz et al. 1994). Other studies have proposed two possible mechanisms for this. One suggests that acetylcholine (Ach) might be released from some vascular endothelial cells during (usually mechanical) stimulation of the blood vessel, and stimulate release of NO by neighbouring endothelial cells (Milner et al. 1989). The second study suggests that NO may have been released directly from the nerves (Toda & Okamura 1991; Holmqvist et al. 1991) causing vasodilation.
Immediately following exercise, the relative change in slope is quite high and declines slowly in comparison to post-occlusion.
The metabolic demand for O2 would have increased drastically during exercise and remained high for some time after as can be seen in Figure 2, where the slope increased by a factor of 5 ± 0.8 mV/s from an average baseline value of 32.7 ±8.1mV/s immediately after exercise ceased. The active hyperemia observed here may be due to a combination of tissue hypoxia and the generation of local vasodilator metabolites such as potassium ions, carbon dioxide, nitric oxide, and adenosine. More recently, ideas have emerged that NO bound to haemoglobin may be released as oxygen is removed from the red blood cells, thus evoking vasodilatation in the active muscles. This is an attractive hypothesis because blood flow to areas where oxygen demand is high, would be elevated (Stamler et al. 1997). Unless there is another source of NO for the red blood cells or some other mechanism for NO production, it seems unlikely that this mechanism is a major and/or obligatory role in exercise hyperemia (Joyner & Wilkins 2007). The explanation of exercise hyperemia, until proven otherwise, must be due to a combination of
factors.
Reactive hyperaemia is observed in forearm perfusion post-occlusion and it appears that the effects are short lived as the relative change in slope drops very sharply as observed in Figure 2. Blood flow would have been increased immediately after the release from occlusion. The hyperemia occurs following major tissue hypoxia and a build up of vasodilator metabolites including adenosine and ATP-dependant K+ channels (Bank et al. 2000) dilate arterioles and decrease vascular resistance. Then when occlusion is released, flow becomes elevated. The longer the period of occlusion, the greater the metabolic stimulus for vasodilation leading to increases in peak reactive hyperemia and duration of hyperemia (Klabunde, 2005). During the hyperemia, the tissue becomes re-oxygenated and vasodilator metabolites are washed out of the tissue. This causes the resistance vessels to regain their normal vascular tone, returning flow to baseline. An interesting point to mention is in a new study by Raff et al. published recently this year in August 2010, who found that when ischemia was applied at the upper arm, the contribution of NO to the vasodilator response following ischemia did not increase with increasing duration of ischemia. Thus, NO mediates reactive hyperemia (RH) after ischemia applied to the upper arm to a lesser extent than RH after ischemia applied to the wrist. Other factors than NO are obviously more important in mediating RH after ischemia applied to the upper arm.
It appears that the release of NO is involved in all the hyperemia observed in this experiment, but may or not be the major factor in the explanation. It could be due to a combination of factors which further research could enlighten.