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THE PULSE PRESSURE
While the systolic pressure has been receiving great attention from the general practitioner the other phenomena of blood-pressure, the diastolic pressure, and the pulse pressure, which is the difference between the systolic and diastolic pressure have, until quite recently, received but scant attention. Possibly this is due to the fact that the methods of measuring the diastolic pressure have not been uniform and thus no comparison was possible between the results of different observers, while even those of a single observer were so variable that no conclusions could be drawn.
It is now accepted that the auscultatory method of determining blood-pressure is the most accurate and uniform of all the methods available for bedside work, so the early difficulty no longer exists. There appear to be two schools of teaching (see page 90) regarding the method of determining the pulse pressure by auscultation, one of which teaches that the disappearance of all sound is the diastolic point, the other that the point of transition from the third to the fourth phase (fourth point of Swan) is the diastolic point. The difference between these two methods is sufficient, except in the exceptional case when the change from fourth to fifth point is almost simultaneous, to alter to an appreciable degree the pulse pressure. This difference, according to Swan,' averages 14 mm., being, of course, lower when the fifth point is taken as the criterion. Accepting 45 mm. as a fair average pulse pressure it is evident that the difference in reading resulting from differing interpretation, can amount to 3343 per cent. more of the whole. Throughout this work, unless otherwise specified, the data employed are based upon diastolic readings made at the fifth point, that is at the moment of disappearance of all sound.
Concerning the importance of a knowledge of the pulse pressure, it seems logical to believe that the pulse pressure, which is the actual head of pressure driving the blood into the peripheral vessels, and which represents the systolic output of the heart, should be of first importance in cardiac prognosis (see functional tests, Chapter XIX, page 331).
The Determination of the Pulse Pressure.—The pulse pressure cannot be determined until both the systolic and the diastolic pressure have been estimated, as it is computed by subtracting the diastolic pressure from the systolic pressure, and the accuracy with which these are computed will determine the accuracy of the pulse pressure. We believe the pulse pressure to be highly important because it is not so much the knowledge of the actual pressure (systolic) under which an organ receives its blood, as it is the knowledge of the amount of blood received by the organ per unit of time, that enables us to judge of functional capacity. The volume of blood delivered by the heart to the great vessels per minute, even if not actually measured will be indicated by the pulse pressure, times the heart rate per minute. At the same time this calculation indicates the velocity of the blood, for we believe that, within certain fairly restricted limits, there is a relation between pulse rate, pulse pressure and blood velocity, the accuracy of this opinion has been demonstrated, experimentally as well as theoretically.”
1 Loc. cit.
It has also been shown that a reduction in pulse pressure means a lessened velocity of blood flow, although the reverse is not necessarily true.
The Relation of Pulse Pressure to Other Pressure Values.—Clinical evidence points clearly to the fact that in the normal adult, the pulse pressure approximates one-third of the systolic pressure and one-half of the diastolic pressure, thereby giving a relation of 3:2:1 for the systolic, the diastolic and the pulse pressure (see chart, page 117), although slight variations from this are compatible with a normal cardiovascular system. Upon the other hand, practically all those conditions which tend to an elevated systolic pressure of 180 or more disturb this ratio, as do also conditions of low pressure, accompanied by vasomotor weakness, so that in many chronic cardiovascular and renal conditions and in shock, the pulse pressure tends to approximate the diastolic pressure and may at times equal or even exceed it (see chart, page 118). This statement should not be held invariable, as Warfield” has cited cases of myocardial disease with normal blood-pressure values five minutes before death.
The normal average pulse pressure may be considered to lie between 35 and 50 mm.
Low Pulse Pressure.—The most consistent low pulse pressures are met in cases of pulmonary tuberculosis, in typhoid fever between the third and fifth weeks, and in many wasting diseases and exhausting fevers. The reduction of pulse pressure in these cases is accomplished chiefly at the expense of the systolic pressure, which is reduced relatively more than is the diastolic pressure.
1 Louis M. Warfield, Jour. A. M. A., Oct. 4, 1913, lxi, 14, p. 1254. Louis M. Warfield, Amer. Jour. Med. Sci., December, 1914, cxlviii, 6.
Large Pulse Pressure. According to W. J. Stone the pulse pressure is usually increased in anemia. As examples he cites three cases in which the systolic bloodpressure varied from 120 to 135 and the diastolic pressure from 60 to 85, giving the pulse pressure 45 to 65.
The patient with the lowest red count (1,630,000) had the lowest diastolic pressure and in consequence a pulse pressure of 65. The pulse pressure in aortic insufficiency is always large, probably larger than in any other condition (see chart, page 332).
In chronic nephritis and in arteriosclerosis with good cardiac compensation the pulse pressure is increased, while in myocardial insufficiency it varies depending apparently upon the size of the heart, being high where the heart is hypertrophied and small where the heart is small. As a general rule it may be stated that in general blood-pressure depression, irrespective of whether the preceding pressures were above normal or not, the presence of a small pulse pressure usually means a gravely affected heart and an unfavorable prognosis.
Significance of Pulse Pressure Changes.-In general, when the systolic pressure is normal, an excessive pulse pressure signifies peripheral dilatation. When the systolic pressure is high, the increased pulse pressure is to be regarded as normal and of no special significance, especially if the 3:2:1 ratio is approximately maintained. Decreased pulse pressure indicates marked peripheral construction and may be an unfavorable prognostic sign; as in disease conditions a decreasing pulse pressure usually means cardiac failure. This condition often occurs in the latter stages of the high-systolic-increased-pulse pressure cases, in which the occurrence of a low-systolic pressure and an increased pulse pressure usually means circulatory failure through loss of vasomotor tone and heart-muscle weakness. This is seen in febrile conditions, cachectic states, malignant disease and in convalescene from long illness.
1 Jour. A. M. A., Oct. 4, 1913, lxi, 14, p. 1256.
The Blood-pressure Quotient. It is believed that the pulse pressure, divided by the systolic pressure, gives an indication of the parts played by vascular resistance and heart work respectively (see Functional Tests, page 331). The determination of the blood-pressure quotient is based upon the belief that increased heart work is shown by an increase in systolic pressure accompanied by an enlarging pulse pressure, while on the other hand an increase in peripheral resistance alone will increase systolic pressure and decrease pulse pressure. In addition reduced peripheral resistance lowers systolic pressure and increases pulse pressure; therefore, if the systolic pressure and the pulse pressure under the same condition vary proportionately, that is, both increase, then the cause lies in the heart, while if in reverse direction or if not proportionately, then the cause lies at least in part in changes in vascular tone. The total cardiac output in a given time also bears a relation to the number of systoles per minute.