The Relative Importance of the Systolic and Diastolic Pressures
The systolic pressure represents the maximum force of the heart. It is measured by noting the first sound audible over the brachial artery using the auscultatory method. It is the summation of two factors largely; the force expended in opening the aortic valves (potential) and the force expended from that point to the end of systole, the force which is actually driving the blood to the periphery (kinetic). To start the blood in motion, the heart must overcome a dead weight equal to the sum of all the forces holding the aortic valves closed. This sum of factors, called the peripheral resistance, must be reached and passed by the force of the ventricular beat before one drop of blood is set in motion along the aorta. This factor of resistance assumes a great importance.
The systolic pressure is always fluctuating as it depends upon so many conditions, and the calls of the body except during sleep are many and various. In a study of diurnal variations in arterial blood pressure it has been found that—(1) A rise of maximum pressure averaging 8 mm. of Hg. occurs immediately on the ingestion of food. A gradual fall then takes place until the beginning of the next meal. There is also a slight general rise of the maximum pressure during the day. (2) The range of maximum pressure varies considerably in different individuals, but the highest and lowest maximum pressures are practically equidistant from the average pressure of any one individual.[4]
The pressure is lowest during sleep and gradually rises near the end of sleep, so that on awakening the pressure was the same as before sleep.
Physiologically there are many conditions which modify the systolic pressure. Sleep, position, meals, exercise, emotional states cause often wide fluctuations which may be very sudden. It should be constantly borne in mind, that the systolic pressure reading which is made, is the maximum effort of the heart at that moment only.
The diastolic pressure measures the peripheral resistance. It measures the work of the heart, the potential energy, up to the moment of the opening of the aortic valves. It is the actual pressure in the aorta. The diastolic pressure is not very variable; it is not subject to the same influences which disturb the systolic pressure. It fluctuates as a rule, within a small range. It is not affected by diet, by mental excitement, by subconscious psychic influences, to anything like the extent to which the systolic pressure is affected by the action of these factors. The diastolic pressure is determined by the tone in the arterioles and is under the control of the vasomotor sympathetic system. Any agent which causes chronic irritation of the whole vasomotor system produces increase in the peripheral resistance with consequent rise in the diastolic pressure. Any agent which acts to produce thickening of the walls of the arterioles, narrowing their lumina, produces the same effect.
Such states naturally result in increased work on the part of the heart, which as a result, hypertrophies in the left ventricle. The increase in size and strength is a compensatory process in order to keep the tissues supplied with their requisite quota of blood. Conversely, paralysis of the vasomotor system produces fall of diastolic pressure which, if long continued, results in death.
The diastolic pressure then is of importance for the following reasons:
1. It measures peripheral resistance.
2. It is the measure of the tonus of the vasomotor system.