Pulmonary artery pressure, normal value

Surprisingly, pulmonary edema does not occur in patients with severe pulmonary hypertension (primary pulmonary hypertension), even though their pulmonary artery pressure may be chronically elevated as high as 45 mm Hg above the normal value of 13 mm Hg. The reason for this is that the arterial bed is severely narrowed in pulmonary hypertension thus the alveolar capillaries are not exposed to the increased pressures and there is no engorgement of blood in the pulmonary vasculature (Ingram and Braunwald, 1980). 

Pulmonary hypertension is characterized by a chronically elevated pulmonary artery pressure. As described in previous sections of this chapter, under normal conditions, the pulmonary artery pressure has a systolic value of 18 to 25 mm Hg, a diastolic value of 6 to 10 mm Hg, and a mean value ranging from 12 to 16 mm Hg. Pulmonary hypertension exists when the pulmonary artery systolic and mean pressures exceed 30 and 20 mm Hg, respectively. In the disease state, the pressure in the pulmonary artery may fluctuate widely and is often so high that it equals the blood pressure in the systemic arterial bed. As would be expected, pulmonary vascular resistance is also extremely high in patients with pulmonary hypertension. In addition, patients with this disease exhibit an enlarged right ventricle and an enlargement of the main pulmonary artery and its branches. Systemic hemodynamic parameters, however, such as cardiac output, cardiac index, systemic artery pressure, and pulmonary artery wedge pressure are usually not elevated. 

Normal resting pulmonary artery mean pressure (PAMP) is less than 25 mm Hg. This value is the sum of the left atrial pressure (LAP) and the pressure gradient needed to shift blood from the pulmonary arteries to the left atrium (fig. 1). The latter is called transpulmonary gradient (TPG). Therefore, TPG = PAMP - LAP. Pulmonary artery pressure depends on pulmonary vascular resistance (PVR) and also on the CO. The latter may change due to various reasons, in particular due to physical exercise. According to hemodynamic studies [18], physical exercise in healthy older people (especially >70 years) leads to an acute increase in PAMP more than in younger people. If CO doubles (e.g. from 5 to 101/min), PAMP and LAP rise approximately twice. This phenomenon is likely due to the worsened diastolic function of the left ventricle in the elderly. As we already mentioned, left ventricular diastolic dysfunction is frequently associated also with ESRD. 

Low-pressure receptors. The low-pressure receptors are located in the walls of the atria and the pulmonary arteries. Similar to baroreceptors, low-pressure receptors are also stretch receptors however, stimulation of these receptors is caused by changes in blood volume in these low-pressure areas. An overall increase in blood volume results in an increase in venous return an increase in the blood volume in the atria and the pulmonary arteries and stimulation of the low-pressure receptors. These receptors then elicit reflexes by way of the vasomotor center that parallel those of baroreceptors. Because an increase in blood volume will initially increase MAP, sympathetic discharge decreases and parasympathetic discharge increases so that MAP decreases toward its normal value. The simultaneous activity of baroreceptors and low-pressure receptors makes the total reflex system more effective in the control of MAP. 

Invasive hemodynamic monitoring usually is performed with a flow-directed pulmonary artery (PA) or Swan-Ganz catheter placed percutaneously through a central vein and advanced through the right side of the heart and into the PA. Inflation of a balloon proximal to the end port allows the catheter to wedge, yielding the PAOP, which estimates the pulmonary venous (left atrial) pressure and, in the absence of intracardiac shunt or mitral valve or pulmonary disease, left ventricular diastolic pressure. Additionally, cardiac output may be measured and systemic vascular resistance (SVR) calculated. Normal values for hemodynamic parameters are listed in Table 14—12. 

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