Effects of increasing left ventricular filling. Pressure in patients with acute myocardial infarction

Heart failure complicating acute myocardial infarction

Congestive heart failure (CHF) occurs in about one half of all patients with acute myocardial infarction and is a manifestation of acute alterations in left ventricular function. In the present study CHF is defined on clinical grounds, according to the presence and extent of bilateral pulmonary rales. An accompanying S 3 ventricular gallop was heard in 58% of our patients with heart failure initially, but it disappeared eventually in the majority. Dilatation of pulmonary veins and blurring of pulmonary vascular markings are useful roentgenographic signs which reflect elevations in left heart filling pressure. At times the earliest indicators of heart failure, these findings appear in general to be less sensitive than the physical examination in diagnosing CHF. Although stroke volume is decreased with CHF, cardiac index is generally maintained by increased heart rate. Left ventricular minute work and stroke work are significantly decreased, while left ventricular end-diastolic pressure is significantly increased, in patients with CHF complicating acute myocardial infarction. Arterial hypoxemia is common and the degree of arteriovenous shunting is roughly proportional to the elevation of left ventricular filling pressure. The mortality of patients with CHF is approximately three times that of patients with acute myocardial infarction and no complications. Diuretic therapy is safe and effective. Attention is called to the probability that the use of digitalis preparations in the early hours following myocardial infarction is hazardous. Furthermore, hemodynamic benefit from digitalization in the early postinfarction period remains unproven.

Hemodynamic spectrum of myocardial infarction and cardiogenic shock. A conceptual model

Despite the recent accumulation of a large hemodynamic data base describing myocardial infarction and cardiogenic shock, precise characterization of patient subsets has been elusive. This paper represents an attempt to identify the major factors contributing to this wide hemodynamic spectrum, and their interrelation using a theoretical model based upon currently emerging concepts of this disease. It is proposed that the hemodynamic alterations associated with acute infarction are a consequence both of reduction in contractile mass and alteration in left ventricular compliance. In addition, mitral insufficiency, altered contractility, and the peripheral circulation interact to produce wide divergence between clinical and hemodynamic features from case to case and during the progression of the course of the illness. This model may more rationally explain the genesis and natural history of "heart failure" and the "shock syndrome" associated with acute myocardial infarction and in addition explain the extremely variable responses to both drug therapy and to more aggressive modes of treatment of power failure.

Early increase in left ventricular compliance after myocardial infarction

The left ventricular (LV) pressure-volume (P-V) relationship is a resultant of several determinants, including initial ventricular volume, geometry, and wall stiffness. A quantitative index of one of these determinants, LV wall stiffness, was developed from a mathematical analysis of the isolated P-V relationship. Since this relationship was exponential, stiffness (dP/dV) could be expressed by the equation dP/dV = aP + b, where a and b are constants. The a constant, termed the passive elastic modulus, was independent of both pressure and volume, was modified only slightly by changes in geometry, and thus was primarily affected by changes in wall stiffness. LV wall stiffness was assessed by determination of the passive elastic modulus in eight normal canine hearts and in five hearts 1 hr after acute myocardial infarction. The value of the passive elastic modulus for the normal canine LV was found to be 0.099+/-0.006 cc(-1). In the five infarcted hearts there was a modest, but statistically insignificant, shift of the P-V curves from control, such that for the same pressure the infarcted hearts contained greater volume. However, the passive elastic modulus decreased 41% to 0.057+/-0.006 cc(-1) (P < 0.001). Thus, although LV wall stiffness may increase later in the course of myocardial infarction, it is concluded that it was significantly decreased 1 hr after infarction. Calculation of the passive elastic modulus provided a sensitive means of detecting such changes, whereas P-V curves alone were generally insensitive.

Effect of coronary artery disease and acute myocardial infarction on left ventricular compliance in man

The evaluation of left ventricular (LV) compliance by use of the pressure-volume (P-V) relationship encounters several serious difficulties. Since the P-V relationship is curvilinear, it is difficult to quantitate. Furthermore, alterations of resting heart size and geometry also produce marked changes in the P-V curve. The first derivative of the P-V relationship, however, is a precisely linear function expressed by the formula dP/dV = aP+b. The slope of this linear function, a, termed the passive elastic modulus, has been shown to be independent of initial volume and primarily and predominantly determined by changes in the stiffness of the myocardium. Myocardial wall stiffness was evaluated in three groups of subjects during LV catheterization. In 13 normal subjects a = 0.005; in 13 with coronary artery disease a = 0.011; and in 12 with acute infarction a = 0.045. The differences in stiffness among the groups were highly significant ( P < 0.005).

It was concluded that a measurable change in ventricular compliance occurs with the development of coronary artery disease and that a further increase in wall stiffness occurs with the development of acute infarction. The magnitude of increase in LV wall stiffness correlated directly with immediate prognosis: 87% of those subjects with a ΔP/ΔV greater than 0.5 mm Hg/cc died of power failure during the acute stage of their illness. These alterations in compliance may invalidate certain traditional concepts of LV function and heart failure.

Hemodynamic evaluation of left ventricular function in shock complicating myocardial infarction

Twenty-two patients with shock complicating myocardial infarction were studied hemodynamically and, despite pharmacologic therapy and regulation of intravascular volume, 16 (73%) subsequently expired. Pulmonary artery end-diastolic pressure (PAEDP) or left ventricular end-diastolic pressure (LVEDP) was > 15 mm Hg in 18 of the 22 patients, and cardiac index (CI) was ≤2.3 liters/min/m 2 in 17 of 22 patients. Fourteen of the 18 patients with PAEDP or LVEDP > 15 mm Hg expired, while two of the four patients with a PAEDP or LVEDP < 15 mm Hg survived. Thirteen of 15 individuals with a CI < 2.3 liters/min/m 2 died, and four of seven with a CI ≤ 2.3 liters/min/m 2 survived. A ventricular gallop (S 3 ) was audible in 15 patients with PAEDP or LVEDP varying from 13 to 60 mm Hg. In 11 patients with an S 3 who expired, the PAEDP or LVEDP ranged from 22 to 60 mm Hg. Mean peripheral vascular resistance was 41.3 units in survivors and 67.8 units in nonsurvivors. Six of eight patients who did not survive the period of hospitalization had a depressed response to dextran infusion manifested by a greater increase in PAEDP or LVEDP than in CI.

Hemodynamic evaluation permitted early identification of measurements associated with 100% mortality despite intensive medical treatment. These findings included: (1) PAEDP or LVEDP > 28 mm Hg and (2) PAEDP or LVEDP > 15 mm Hg in association with a CI < 2.3 liters/min/m 2 . Patients in cardiogenic shock with these hemodynamic alterations are presumably candidates for cardiocirculatory mechanical assisting devices and possibly for further surgical intervention.

Left ventricular function in acute myocardial infarction and its clinical significance

Investigations on left ventricular function in patients with acute myocardial infarction and the relatonship to clinical findings have shown: (1) limitations in the use and interpretation of central venous pressure; (2) pulmonary artery end-diastolic pressure accurately reflects left ventricular filling pressure in the absence of pulmonary vascular or mitral valve disease; (3) left ventricular filling pressure is frequently elevated in mild or clinical uncomplicated infarction; (4) left ventricular function frequently improves during the immediate as well as late convalescent period; (5) the hemodynamic and clinical evaluations may frequently be at variance; (6) a left ventricular gallop is usually associated with an abnormally elevated left ventricular filling pressure; (7) anterior infarctions present greater depression of left ventricular function than inferior infarctions; and (8) monitoring of hemodynamics can be useful in following the changes in left ventricular function and the response to therapy in patients with heart failure and cardiogenic shock.

Silent mitral insufficiency in acute myocardial infarction

Severe mitral insufficiency in the absence of an audible murmur was diagnosed by left ventricular angiography in three patients with power failure secondary to acute myocardial infarction during evaluation for mechanical circulatory assist and surgery. Mitral valve prolapse was present in two patients. Postmortem examination did not reveal an anatomic basis for the mitral insufficiency: the valve, papillary muscles, and supporting structures were all grossly normal. A single papillary muscle removed at surgery revealed a marked decrease in force development (0.22 g/mm 2 vs 0.62 ± 0.22 g/mm 2 in eight normal papillary muscles from patients with rheumatic heart disease). During isoproterenol stimulation, force development in this muscle decreased 20%, whereas in the normal muscles force development increased 73 ± 31%. Microscopically, all papillary muscles revealed evidence of extensive necrosis. Silent mitral insufficiency in acute myocardial infarction, therefore, was probably related to diminished flow velocity across the mitral valve secondary to diminished myocardial contractility. Failure to recognize and treat this entity may contribute significantly to the genesis of power failure and ultimate mortality.

Diastolic pressure-volume relationship in the canine left ventricle

Analysis of the passive pressure-volume filling curve of the left ventricle demonstrates that heart size and ventricular geometry exert major effects on the pressure-volume curve in the absence of changes in intrinsic muscle stiffness. Because the pressure-volume relationship is curvilinear, both quantitative and qualitative comparison of pressure-volume curves from different hearts is difficult. In the fresh isolated canine left ventricle, the pressure-volume relation was found to be almost perfectly exponential throughout a range of filling pressures from 5 to 30 mm Hg. Therefore, a precise linear and quantitative expression of the pressure-volume relation (dP/dV=aP+b) was developed (r=0.995). The effect of isolated changes in either initial ventricular volume (mean δa=3.1%) or ventricular geometry (mean δa=27.1%) upon the slope, or a constant of this function was small in comparison to changes induced by rigor mortis (mean δa =45%). It was concluded that the a constant was primarily affected by changes in left ventricular wall stiffness. In this manner, comparison of the pressure-volume relationship from different hearts is possible, and the contribution of changes in wall stiffness may be quantified.

Effect of volume expansion on the anginal threshold

The clinical and hemodynamic effects of rapid atrial pacing were studied in 13 patients with coronary artery disease and four normal subjects before and following acute expansion of blood volume with low molecular weight dextran. Five patients developed angina during the initial pacing period. In two of these five patients angina recurred with infusion alone, and all five of them experienced pain during the second pacing period. The anginal threshold averaged 5 min (range, 1 to 8 min) before infusion and 1.8 min (range, 0 to 5 min) following infusion. Four patients with coronary artery disease remained free of pain during the first pacing period, one of these developed pain with infusion alone, and all developed pain during pacing following infusion, the anginal threshold being 2 min (range, 0 to 3 min). The remaining four patients with coronary artery disease did not develop angina during either pacing period.

Administration of dextran was accompanied by an increase in left ventricular end-diastolic pressure in all patients during sinus rhythm and during atrial pacing. Although left ventricular volume was not determined, changes in left ventricular end-diastolic pressure were interpreted as showing directional changes in left ventricular volume. Analysis of the factors affecting myocardial oxygen requirements during the two pacing periods indicated that heart rate, systemic arterial pressure, and maximal rate of rise of left ventricular pressure were similar. Although the tension-time index rose 10.6%, the major change was the increase in left ventricular end-diastolic pressure and presumably in left ventricular volume. This increase in left ventricular volume is considered responsible for the increase in myocardial oxygen requirements following infusion of dextran as determined by the decrease in the anginal threshold in each patient of the angina group.