Appendix: Clinical Determination of Mean-Cardiovascular Pressure
A useful approximation of the mean-cardiovascular pressure can be made without the need of stopping the heart and letting the pressure equalize in the entire cardiovascular system. The assumption is made that the same pressure would be found by isolating a representative sample of the arteries, capillaries, and veins at ventricular diastole, and letting the pressure equilibrate in that sample. By instantaneously interrupting arterial inflow to the arm and venous outflow from it, the pressure will fall in the arteries and rise in the veins until they are equal. This equalized pressure, which will occur within 30 seconds, approximates that found when circulation is stopped in the entire body.
Equipment:
A narrow pneumatic blood pressure cuff (approximately one inch in width) is used so that during inflation it will not displace any blood volume distally into the arm.
A one liter air pressure reservoir, pressurized to 300 mm. Hg, is connected to the pressure cuff with a valve interposed to allow instantaneous inflation of the cuff. The reservoir is pressurized by a regular blood pressure inflating bulb and valve, interposed with a "Y" connecter between the reservoir and the valve.
Two pressure transducers and recorders are needed, to allow pressure recording without any loss of fluid from the vascular bed, which would occur with a simple manometer system.
Technique:
- The patient should be lying perfectly horizontal.
- Both an artery and a vein are cannulated at the antecubital area of the arm and connected to the pressure transducers.
- The blood pressure cuff is applied above the biceps bulge of the arm. It must be applied loose enough that it does not cause any venous obstruction, as evidenced by the observation that it produces no elevation of venous pressure above that seen before its application.
- The cuff must be applied tight enough that its inflation, to 300 mm. Hg pressure, completely interrupts the arterial flow to the arm. Complete interruption can be assumed if arterial and venous pressure approach equalization after 30 seconds, and do not continue to rise thereafter.
- The arm should be abducted sufficiently from the side of the body, so as to obtain the lowest venous pressure reading, thereby being certain that any abduction is not mechanically interfering with venous flow in the axilla.
- The arm should be horizontal, with the anterior surface of the antecubital skin at mid-chest position.
- The thickness of the chest should be accurately measured. The pressures are small, so careful standardization is necessary in order to get meaningful data.
- The patient is instructed to leave the arm perfectly relaxed, and not to contract any muscle.
- The valve or switch in the pressure line is turned, thereby inflating the cuff, which simultaneously interrupts venous and arterial flow to and from the arm.
Pressure readings are made 30 seconds after the occlusion. As the equalization process progresses, the pressure gradient becomes so small between the arteries and veins that the pressures may never completely equalize. However, the average of the arterial and venous pressures recorded at 30 seconds can be interpreted as the mean-cardiovascular pressure.
Establishing such a standardization makes these approximate readings clinically useful. In any low arterial blood pressure situation, the mean-cardiovascular pressure indicates whether the problem is from low blood volume or myocardial failure. Mean-cardiovascular pressure is a diagnostic tool that can separate high output hypertension from the more common arteriolar resistance hypertension, thus indicating the proper therapy regimen.