A. PH secondary to left heart disease (also called pulmonary venous hypertension or postcapillary PH)

Left-sided ventricular or valvular diseases may produce an increase in LA pressure with passive backward transmission of pressure to the pulmonary circulation. As a result, PA pressure rises. In this case:^3,4

• PCWP is elevated (>15 mmHg).
  
• Diastolic PA pressure is passively increased and is equal to PCWP or is <7 mmHg higher than PCWP.
  
• Pulmonary vascular resistance (PVR) is <3 Wood units. The transpulmonary gradient, i.e., mean PA pressure minus PCWP, is <12 mmHg (some investigators use a cutoff of 20 mmHg).¹ Note that the transpulmonary gradient is the numerator in PVR calculation: PVR = transpulmonary gradient/cardiac output.

Left heart failure is the most common cause of PH. HF may be obvious in some patients, but may be occult in others, especially when isolated LV diastolic dysfunction is present.⁵ Furthermore, with chronic postcapillary PH, pulmonary arteries may undergo reactive changes and PH may become a combined pre- and post-capillary PH , in which case PCWP is elevated but diastolic PA pressure is ≥7 mmHg higher than PCWP and PVR is ≥3 Wood units.^4,6 In fact, a precapillary component accompanies PH in 20–35% of patients with advanced left HFrEF, 20–38% of HFpEF (even if not advanced), and, commonly, mitral stenosis.6–10 In addition, this situation may be seen in patients with mixed disorders, such as left HF and COPD. The active, precapillary PH component resolves after treatment of HF, but may take weeks or months to fully resolve.³

Resting PCWP may be normal despite LV failure, especially in patients appropriately treated with diuretics. The improvement of pulmonary pressure lags behind the improvement of PCWP, and these patients may have a normal PCWP with elevated PA pressure, simulating precapillary PH, except for a PVR that is only mildly increased. Exercise testing, volume loading, or pulmonary vasodilator challenge are appropriate strategies that increase PCWP in occult LV dysfunction and thus unveil the diagnosis of postcapillary PH. Patients who are suspected of having left heart disease-associated PH or mixed postcapillary PH and precapillary PH are approached as in Figure 25.1.

B. Precapillary PH

Precapillary PH is characterized by PCWP and LVEDP ≤15 mmHg, and PVR ≥3 Wood units.^1,2,4 In cases of precapillary PH associated with severe RV failure, pericardial distension and functional pericardial constriction may occur, leading to ventricular interdependence and equalization of RV and LV end-diastolic pressures, with a subsequent rise in LV end-diastolic pressure and PCWP to 15–20 mmHg.¹² As opposed to left heart disease, the increase in PCWP is, in this case, the result of PH rather than the cause of PH. Yet, in such an instance, PH may be erroneously labeled as left heart disease-associated PH. The presence of signs of LV diastolic dysfunction on echocardiography supports the diagnosis of left heart disease-associated PH, while severe RV dilatation and severe elevation of PVR >7 Wood units support the diagnosis of precapillary PH.

There are three major categories of precapillary PH:

1. Pulmonary arterial hypertension (PAH), which is related to a pulmonary vascular disease affecting the pulmonary arterioles. PAH may be idiopathic, familial, or related to connective tissue disease, toxins (amphetamines, anorexigen), cirrhosis (portopulmonary hypertension), HIV, or Eisenmenger syndrome. Idiopathic PAH may be seen at any age or sex but is more common in female patients (female/male ratio 3:1; age 36 ± 15 years). Plexiform lesion is seen on pathology: intimal and medial proliferation occluding the lumen, with multiple small holes inside this occlusion, trying to create some flow.
   

   In congenital heart disease with a large left-to-right shunt (e.g., VSD, PDA, or less often ASD), PA pressure initially increases as a result of the increase in right-sided flow, PVR remaining initially low (pressure = flow × resistance; an increase in flow leads to an increase in pressure). This “dynamic” PH resolves with shunt closure. Over time, the increased pulmonary flow induces progressive pulmonary vascular disease and severe increase in PVR to a point that PVR approaches SVR, PA pressure approaches systemic pressure, and the shunt reverses and becomes directed right-to-left or bidirectional. This is Eisenmenger syndrome and, except in ASD, is usually established in infancy or childhood. It is unusual for VSD or PDA to be diagnosed as a cause of PH in an adult. Note that small defects (VSD<1 cm, ASD<2 cm) do not usually account for PVR elevation; when found in a patient with PAH, they are incidental and should not be closed. 4

   
   

   Pulmonary veno-occlusive disease is characterized by primary venular abnormalities similar to the arteriolar abnormalities seen in idiopathic PAH and may be idiopathic or associated with scleroderma. Similar to PAH, true pulmonary arterial wedging is difficult during catheterization, but, if successful, it still creates a column of stagnant blood between the catheter and the LA; thus, the truly wedged PCWP approximates the LA pressure, albeit damped through the venular obstruction, and is normal in value. The wedged PA pressure, i.e., LA pressure, is normal, but the pulmonary capillary pressure is increased and pulmonary edema may be seen. The diagnosis is suggested by the triple combination of severe pulmonary hypertension, normal PCWP, and radiographic pulmonary edema/ground-glass opacities, as well as the lack of response to vasodilator therapy (or the deterioration with this therapy).Portopulmonary PAH is driven by portal hypertension rather than synthetic liver function. Patients may have preserved synthetic liver function but significant portal hypertension, which causes an inability to clear some substances through the liver. Portal hypertension may also cause AVMs, which result in right-to-left shunting. While AVMs are unrelated to PAH, they do nonetheless provide an escape mechanism from the high PA pressure.

   
   
2. PH secondary to thromboembolic disease. Approximately 4% of patients who develop acute PE do not fully resolve their thrombus burden and go on to develop chronic PH. This often occurs after single PE episodes. Most often, the thrombus involves the main lobar pulmonary artery or the proximal arteries (80%), with small-vessel arteriopathy and thrombosis that subsequently occur and contribute to disease progression. In a smaller category of cases, the thromboembolic process is purely distal, involving the small distal pulmonary arteries (the distal type is less likely to benefit from surgical thromboendarterectomy).
   
3. PH secondary to hypoxic lung disease. Mild PH is common in patients with COPD, but severe PH is very unusual. In fact, moderate and severe PH are only seen in 5–10% and 2% of severe COPD cases, respectively.^13,14 Also, sleep apnea does not usually lead to more than mild PH. Conversely, severe PH may be seen with advanced-stage fibrotic lung disease that obliterates the pulmonary capillaries, sarcoidosis, or obesity–hypoventilation syndrome.

A. Echocardiography

PH is often initially suggested by echocardiography. Echocardiography estimates PA pressure, and:

1. Suggests a left-sided etiology. In addition to valvular function and LV systolic function, echocardiography assesses LA pressure, LV diastolic function, and left atrial size.
   
2. Looks for signs of severity of PH: RV and RA dilatation, severe TR, and abnormal interventricular septal motion (the high, right-sided pressure or volume makes the septum bow to the left). Frequently, PA pressure cannot be directly measured by echo. Analysis of the RVOT ejection waveform may help establish PAH: 94% of patients with PAH have midsystolic notching of the pulsed RVOT envelope; the high PA pressure opposes RV ejection and causes mid-systolic slowing of ejection (mainly correlates with high PVR). This may also manifest as mid-systolic closure of the pulmonic valve on M mode (“flying W” shape).16

B. Catheterization

Catheterization is needed to confirm the diagnosis and the etiology of PH, particularly in cases of moderate-to-severe PH without a clear cause. Patients with a clear clinical and echo diagnosis of left HF do not require cardiac catheterization for PH assessment. Also, patients with an acute PE diagnosis do not require cardiac catheterization (this may, however, be required for chronic thromboembolic PH). The goals of catheterization are:

1. Confirm the diagnosis of PH by measuring PA pressure and calculating PVR (PVR = [mean PA – PCWP]/cardiac output). The spectral Doppler profile of TR is too weak or insufficient to measure the PA pressure in approximately 25–55% of patients referred for PA pressure evaluation; TR may not be present even when PH is severe.¹⁷ The echocardiographic diagnosis of PH is falsely positive in up to 50% of patients, and, overall, the PA pressure value differs from the catheterization value by >10 mmHg in 50% of patients. Echocardiography may under- or overestimate PA pressure in various PH etiologies.^17,18
   
2. Assess PCWP to determine if PH is secondary to left HF. This is the single most important measurement in PH. Yet, the assessment of PCWP may be difficult in patients with severe PH:^19–21
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