What is the pathophysiology of pulmonary hypertension (PH)?

Updated: Sep 11, 2019
  • Author: Swapnil Khoche, MBBS, DNB, FCARCSI; Chief Editor: Sheela Pai Cole, MD  more...
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Answer

Answer

The functional capacity of the vital organs is entirely dependent on the presence of sufficient perfusion and oxygenation. Perfusion and oxygenation, in turn, depend on the heart’s ability to pump oxygenated blood throughout the body. The following two interlinking circulatory systems help accomplish this [6] :

  • The low-resistance pulmonary circulation that oxygenates blood
  • The high-resistance systemic circulation that distributes blood to the rest of the body

In a steady state, the cardiac output through the two systems is equal, and Ohm’s law suggests that the pressure gradient required to pump through each system is inversely dependent on its individual resistance.

In a physiologically normal state, the heart is perfectly modeled to accommodate these different resistances. Whereas the left ventricle (LV) must generate a relatively high pressure gradient in order to overcome the high systemic vascular resistance (SVR), the RV needs to generate a lower pressure gradient to overcome the lower pulmonary vascular resistance (PVR). Furthermore, the reduced filling pressures in the RV lead to less wall stress than occurs in the LV. Accordingly, the wall of the LV is substantially thicker than that of the RV in a physiologically normal state. [4]

In a pathologic state, PH of all forms leads to an increase in resistance to flow across the pulmonary vascular bed. This creates an increased afterload for the RV, thereby impeding the ability of the RV to eject blood, and increases the end-systolic and end-diastolic volumes. [7]  In most individuals with chronic PH, the progression is gradual, allowing the right heart time for remodeling and hypertrophy in response to the increased pressure. This compensation allows increased contractility and brings stroke volume closer to baseline despite the increases in pressure and afterload.

However, patients with PH are not always in a compensated steady state, particularly when undergoing anesthesia or surgery. When a pressure-naive RV encounters elevated pulmonary pressures for the first time, or when a chronically hypertrophic RV works against a resistance far in excess of what it usually faces, it may not be able to compensate, and failure may result.

In such cases, this pressure is then transmitted in a retrograde fashion back into the venous circulation, leading to symptoms of acute right-heart failure and organ dysfunction from venous congestion. Furthermore, the decrease in forward flow through the pulmonary circulation reduces LV preload and stroke volume, thereby causing decreases in cardiac output and consequently in mean arterial pressure (MAP). [6]

Although increases in SVR can partially compensate for the decreased MAP for the purpose of maintaining tissue perfusion, there is still a decrease in overall oxygen delivery. In particular, decreased perfusion pressure and decreased cardiac output in a patient with a hypertrophic RV can greatly impede oxygen delivery to the thickened ventricular wall, leading to endocardial ischemia and worsened cardiac dysfunction.

If this situation is left untreated, the RV begins to dilate in the face of corrected afterload increase. Such dilation leads to dilation of the tricuspid annulus and subsequently to tricuspid regurgitation. This worsens forward flow and leads to increased back-pressure to the end organs (eg, the kidneys and liver), which is dependent on the difference between MAP (which is low) and central venous pressure (CVP, which is elevated). [8, 9]


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