MANUAL PHYSIOLOGY PRACTICAL - Repository:The Medical ...

FM/UOJ
Experiment C 5
PLETHYSMOGRAPHY
Plethysmography is a technique for measuring volume changes. This technique
is used to measure blood flow to an organ. Occlusion of efflux of blood (veins) without
affecting the influx increases the volume of the organ. This technique has a
disadvantage in that the measurement should be made within a short period because the
vascular bed of the organ will be filled with blood soon and the influx will be to
measure the blood flow to fore arm under different conditions.
Instruments:
Plethysmograph, Volume recorder, Kymograph, Sphygmomanometer
Demonstration
The subject sits comfortably with the right arm inserted into the
plethysmograph, which is placed on the table. A rubber sleeve seals the opening of the
plethymograph around the arm. The rubber applied over the forearm should not
compress the veins and cause venous obstruction before starting the experiment. One
outlet from the plethysmograph is connected to a volume recorder, which writes on a
moving drum of a kymograph.
A 50 ml syringe is connected to the other opening for calibration of the change
in volume in the plethysmograph. Air is injected into the plethysmograph in varying
volumes and the deflection of the pen for each volume is recorded.
The sphygmomanometer cuff is applied to the arm of the same limb and the
blood pressure measured. This cuff occludes the veins of the arm when the pressure is
increased to about 40 mm Hg (less than the diastolic pressure). Each recording begins
as soon as the vein is occluded. The duration can be determined from the length of the
tracing and the drum velocity. The rate at which the pointer rises is proportional to the
blood flow. The rate of rising by the pointer slows down and the tracing flattens out as
the veins are full and the blood flow has slowed down. Therefore the initial portion of
the tracing only is used to calculate the flow.
The blood flow to the right fore arm is determined after the following
procedures.
a. Resting blood flow
b. After keeping the left hand in warm water for 2 minutes (at about 45°C)
c. After keeping the left hand in cold water for 2 minutes (at about 10°C)
d. After subjecting the right forearm to ischaemia for four minutes.
e. After performing muscular work by the right forearm for four minutes.
f. After performing muscular work with venous occlusion for four minutes.
g. After moderate work by the leg muscles on a cycle ergo meter for four minutes.
Circulation
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