Myocardial capillaries

A. j. Milid, N. Watrous. and G, E Palade. IramunoJtold localization of exogenous albumin in murine myocardial capillaries, J, Cell Biot. 7(13 194-206 (1986). 

Figure 5 Tracer binding and transport in myocardial capillaries.

The normal coronary system consists of large epicardial or surface vessels (Rj) that offer little resistance to myocardial flow and intramyocardial arteries and arterioles (R2), which branch into a dense capillary network to supply basal blood flow (Fig. 11-1). Under normal circumstances, the resistance in R2 is much greater than that in Rj. Myocardial blood flow is inversely related to arteriolar resistance and directly related to the coronary driving pressure. 

Animal studies with the Impella pump have demonstrated a decrease in myocardial oxygen demand during ischemia and reperfusion, resulting in a smaller infarct size [23]. Initial experience in patients with cardiogenic shock treated with the Impella pump system showed improvements in cardiac output and mean blood pressure and a reduction in pulmonary capillary wedge pressure [24]. 

Several serious toxicities have been observed, with a fatahty rate of 5% in the initial studies. The major adverse effect is severe hypotension in as many as 85% of patients, which may lead to myocardial infarctions, pulmonary edema, and strokes. This hypotension is thought to be due to a capillary leak syndrome resulting from extravasation of plasma proteins and fluid into ex-travascular space and a loss of vascular tone. Patients with significant cardiac, pulmonary, renal, hepatic, or CNS conditions should not receive therapy with aldesleukin. Other adverse reactions include nausea and vomiting, diarrhea, stomatitis, anorexia, altered mental status, fevers, and fatigue. 

Mectianism of Action A cardiac inotropic agent that inhibits phosphodiesterase, which increases cyclic adenosine monophosphate and potentiates the delivery of calcium to myocardial contractile systems. Therapeutic Effect Relaxes vascular muscle, causing vasodilation. Increases cardiac output decreases pulmonary capillary wedge pressure and vascular resistance. 

Effect on CVS and blood Glucocorticoids inhibit capillary permeability and maintain myocardial contractility and also the tone of arterioles. 

ACE inhibitors have a particularly useful role in treating patients with chronic kidney disease because they diminish proteinuria and stabilize renal function (even in the absence of lowering of blood pressure). This effect is particularly valuable in diabetes, and these drugs are now recommended in diabetes even in the absence of hypertension. These benefits probably result from improved intrarenal hemodynamics, with decreased glomerular efferent arteriolar resistance and a resulting reduction of intraglomerular capillary pressure. ACE inhibitors have also proved to be extremely useful in the treatment of heart failure, and after myocardial infarction, and there is recent evidence that ACE inhibitors reduce the incidence of diabetes in patients with high cardiovascular risk (see Chapter 13). 

Measurements of arterial pressure, cardiac output, stroke work index, and pulmonary capillary wedge pressure are particularly useful in patients with acute myocardial infarction and acute heart failure. Such patients can be usefully characterized on the basis of three hemodynamic measurements arterial pressure, left ventricular filling pressure, and cardiac index. One such classification and therapies that have proved most effective are set forth in Table 13-4. When filling pressure is greater than 15 mm Hg and stroke work index is less than 20 g-m/m2, the mortality rate is high. Intermediate levels of these two variables imply a much better prognosis. 

The intramyocardial pressure, i.e., systolic squeeze, compresses the capillary bed. Myocardial blood flow is halted during systole and occurs almost entirely during diastole. Diastolic wall tension ("preload ) depends on ventricular volume and filling pressure. The organic nitrates reduce preload by decreasing venous return to the heart. 

Fever fluid retention hypotension respiratory distress rash anemia thrombocytopenia nausea and vomiting diarrhea capillary leak syndrome nephrotoxicity myocardial toxicity hepatotoxicity erythema nodosum neutrophil chemotactic defects... 

In general, the hypertrophied heart seems to be more susceptible to the deleterious effects of ischemia and reperfusion (reviewed by Friehs and Del Nido200). This may be attributed to alterations in myocardial energy metabolism and calcium handling or to anatomic and functional abnormalities of the coronary bed, such as reduced capillary density and coronary flow reserve. 

---