Definition and classification

The pulmonary circulation is a low-pressure, lowresistance, and high-flow vascular bed. Pulmonary arterial pressure is normally 20% of systemic arterial pressure, and pulmonary vascular resistance is normally 10% of systemic vascular resistance.

The normal ranges for the systolic, diastolic, and mean pulmonary artery pressures are 20 to 30, 5 to 15, and 10 to 20 mmHg, respectively. Pulmonary vascular resistance index is normally 150 to 250 dyne.s.cm⁻⁵ (1.8 to 3.2 Wood units). The transpulmonary gradient is the difference between the mean pulmonary artery pressure and the left atrial pressure, and is normally 5 to 10 mmHg. The transpulmonary gradient is commonly used as a surrogate for pulmonary vascular resistance. If pulmonary vascular resistance is normal and left atrial pressure is greater than 5 mmHg, pulmonary artery diastolic pressure is about equal to left atrial pressure. The normal regulation of pulmonary vascular resistance is outlined in Chapter 1. Pulmonary arterial hypertension is defined as a sustained increase in mean pulmonary artery pressure above 25 mmHg at rest and 30 mmHg with exercise, with a pulmonary artery wedge pressure (PAWP) and left-ventricular end-diastolic pressure of less than 15 mmHg.¹ However, in many patients with pulmonary hypertension secondary to cardiac disease, PAWP is (or has been) greater than 15 mmHg. The classification of pulmonary hypertension is shown in shown in Table 24-1.

Table 24-1 Revised World Health Organization Classification of Pulmonary Hypertension

Pathophysiology

In clinical practice, pulmonary arterial pressure is commonly assumed to be reflective of pulmonary vascular resistance, and in most circumstances this is the case. However, based on Equation 1-6, it is clear that pulmonary arterial pressure is also dependent on cardiac output and left atrial pressure. An increase in cardiac output can lead to an increase in pulmonary arterial pressure despite a fall in pulmonary vascular resistance. Conversely, an acute rise in pulmonary vascular resistance resulting in acute right ventricular failure and thus a fall in cardiac output may cause a fall in pulmonary arterial pressure.

Under normal circumstances, as cardiac output increases in response to increased metabolic demands, pulmonary vascular resistance falls due to distension and recruitment of pulmonary capillaries; thus, pulmonary arterial pressure is relatively unchanged. With elevated pulmonary vascular resistance, this normal response of the pulmonary vascular bed is lost, and pulmonary arterial pressure rises as cardiac output increases. Over time, as pulmonary vascular disease progresses, the ability of the right heart to increase its output is reduced and cardiac output becomes relatively fixed. Eventually, right ventricular failure develops and cardiac output is reduced even under resting conditions.

The extent to which the right ventricle can cope with increased pulmonary vascular resistance depends on the time period over which the increased resistance has occurred. An untrained, thin-walled right ventricle is sensitive to acute rises in afterload. Thus, an abrupt rise in pulmonary vascular resistance (e.g., due to a massive pulmonary embolism) that requires a mean pulmonary arterial pressure of more than 40 mmHg to perfuse the pulmonary bed will rapidly lead to right ventricular failure. In contrast, if pulmonary vascular resistance increases gradually, right ventricular hypertrophy develops, which allows resting cardiac output to be maintained in the face of very high pulmonary vascular resistance (e.g., >20 Wood units). In this situation, pulmonary arterial pressure may approach or even exceed systemic arterial pressure.

With raised left atrial pressure, increased pulmonary arterial pressure can occur without raised pulmonary vascular resistance (see Equation 1-6). This is termed passive pulmonary hypertension. In passive pulmonary hypertension, the increase in pulmonary arterial pressure is usually mild and the transpulmonary gradient (see Chapter 1) is normal. The common causes of passive pulmonary hypertension are mitral valve disease and left ventricular dysfunction. The term active pulmonary hypertension denotes pulmonary hypertension due to vasoconstriction or anatomic restriction within the pulmonary bed in association with normal left atrial pressure. Pulmonary vascular resistance and transpulmonary gradient are elevated. Causes of active pulmonary hypertension include chronic lung disease and chronically elevated flow through the pulmonary circulation, such as that which occurs with uncorrected systemic-to-pulmonary shunts. The term reactive pulmonary hypertension refers to active pulmonary hypertension due to long-standing passive pulmonary hypertension. Chronically elevated left atrial pressure eventually causes pathologic changes within the pulmonary arterioles. The most common cause of reactive pulmonary hypertension in cardiac surgery patients is chronic mitral valve disease, particularly mitral stenosis. Rarely, left ventricular failure leads to reactive pulmonary hypertension. Pulmonary hypertension may be severe.

Pulmonary hypertension may be fixed or responsive. Changes in the pulmonary vascular bed that accompany active or reactive pulmonary hypertension include vasoconstriction, endothelial and smooth muscle proliferation, thrombosis, and fibrosis. Pulmonary hypertension that is associated with extensive thrombosis, fibrosis, or tissue destruction tends to be fixed, whereas pulmonary hypertension that is associated with abnormal vasoconstriction may be responsive to pulmonary vasodilating agents. Impaired production of vasodilator substances, such as nitric oxide, prostacyclin, and vasoactive intestinal peptide, and increased production of vasoconstrictor substances, such as endothelin-1 and thromboxane-A2, have been identified for a range of conditions associated with pulmonary hypertension.¹ Alveolar hypoxemia is a potent stimulus for pulmonary vasoconstriction, and it contributes to pulmonary hypertension in patients with chronic lung disease.

Pulmonary Artery Catheterization

Pulmonary artery catheterization may be used as part of a diagnostic workup (see Chapter 5) or in the ICU as a guide to management (see Chapter 8). Diagnostic right heart catheterization involves measurement of pulmonary arterial pressure, measurement of cardiac output, calculation of pulmonary vascular resistance, assessment of reversibility with pulmonary vasodilators (100% oxygen, nitroprusside, nitric oxide) and, in patients with intracardiac shunting, measurements of various oxygen saturations so as to allow estimation of the ratio of pulmonary to systemic flow (described in Chapter 5).

With chronically increased pulmonary vascular resistance, pulmonary arterial pressure may be very high but if right ventricular failure has not developed, cardiac output and central venous pressure (CVP) will be within normal limits. In this situation, there will be a large step-up in pressure between the CVP and the mean pulmonary arterial pressure. With an acute increase in pulmonary vascular resistance (e.g., due to massive pulmonary embolism), pulmonary arterial pressure will not be dramatically elevated but there may be signs of right ventricular decompensation (see subsequent material).

Echocardiography

With chronic pulmonary hypertension, the right ventricle is typically hypertrophied (wall thickness >0.5 cm and occasionally >1 cm) and dilated but, in the absence of ventricular failure, systolic function is preserved. Chronic right ventricular pressure overload leads to characteristic abnormalities in the shape and motion of the interventricular septum, which are described in Chapter 7

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