Key Points
- Pulmonary hypertension is defined as increased pressure in the pulmonary vasculature.
–
Pressures of 25 mmHg or above are considered hypertensive.
– Be aware that some argue that the threshold should be lowered to
20 mmHg.
- Symptoms are nonspecific and may be overlooked.
– Most patients present with
shortness of breath, especially on exertion.
- Serious outcomes, include: right heart failure, arrhythmias, blood clots, and bleeding into the lungs.
– Early diagnosis and treatment is important.
Pulmonary Arterial Pressure
Review of flow of blood through the heart and lungs
Deoxygenated blood is returned to the heart via the vena cavae, passes through the right atrium, and is pumped by the right ventricle into the pulmonary trunk, which bifurcates to give rise to the right and left pulmonary arteries and their branches.
Pulmonary blood pressure is generated by the right ventricle.
Normal mean pulmonary arterial pressure is around 15 mmHg at rest.
Pulmonary circulation is characterized by low pressure, low resistance, and high compliance.
Pulmonary arterial pressure (PAP) is determined by cardiac output, pulmonary vascular resistance, and pulmonary venous pressure.
Recall that the pulmonary circulation receives the same cardiac output as the systemic circulation; the low pressure and resistance of the pulmonary vasculature allows it to receive this blood without damage.
Low pulmonary vascular resistance is the product of short, wide vessels with relatively little smooth muscle in their walls.
In our drawing, we that pulmonary arterioles have less smooth muscular tissue than systemic arterioles.
Key modulators of pulmonary vascular resistance include lung volume, perfusion pressure, oxygen, carbon dioxide, and pH levels.
For example, hypoxia causes pulmonary vasoconstriction; when chronic, this can lead to hypertension.
The high compliance of the pulmonary circulation is facilitated by the thin vessel walls with little muscular tone.
5 Groups of Pulmonary Hypertension
These Groups are defined by hemodynamic profiles and causes, and are important because proper treatment of pulmonary hypertension requires that we know its underlying pathobiology.
Group 1: Pulmonary arterial hypertension (PAH), which accounts for less than 5% of all pulmonary hypertension.
Pulmonary arterial hypertension can be idiopathic, or can be due to hereditary causes (for example, mutations of the BMPR2 gene), toxins, HIV, or connective tissue diseases.
The Hemodynamic profile of pulmonary arterial hypertension is pre-capillary, and is characterized by:
- Pulmonary arterial wedge pressure of 15 mmHg or less (Be aware that many use pulmonary arterial wedge pressure and pulmonary capillary wedge pressure interchangeably).
- Elevated pulmonary vascular resistance of 3 Wood Units or more.
(Be aware that many use pulmonary arterial wedge pressure and pulmonary capillary wedge pressure interchangeably).
Pulmonary arterial hypertension is characterized by inflammation, fibrosis, thrombosis, and vasoconstriction.
Group 2: Pulmonary hypertension due to left heart disease; approximately 70% of all cases of pulmonary hypertension are Group 2.
Causes include systolic and diastolic dysfunction,
valvular disease, and congenital disorders that lead to
left heart failure.
The hemodynamic profile of Group 2 is characterized by:
- A pulmonary arterial wedge pressure of greater than 15 mmHg; note the contrast with Group 1, which is lower.
Group 2 can be further subdivided by pulmonary vascular resistance values:
- Pulmonary vascular resistance of less than 3 Woods Units indicates post-capillary hypertension.
- Pulmonary vascular resistance of greater than 3 Woods Units indicates mixed pre- and post-capillary hypertension; this subtype of Group 2 is sometimes called "reactive."
To better understand how left heart disease leads to pulmonary hypertension, we draw the heart and a lung:
Indicate that that venous return brings oxygenated blood from the lungs to the left side of the heart.
Then, show that, when left ventricular end diastolic pressure is elevated, which occurs in left heart failure, that left atrial pressure also increases;
And, as a result, passive increases in the pulmonary vasculature occur due to venous backflow.
However, in some individuals, reactive pulmonary hypertension develops when heart disease leads to pulmonary arterial dysfunction and precapillary hypertension; thus, hypertension is "mixed" pre- and post-capillary.
In reactive pulmonary hypertension, we see increased transpulmonary gradients and pulmonary vascular resistance.
Group 3 pulmonary hypertension is due to underlying lung diseases and other disorders that lead to hypoxia; this accounts for approximately 10% of PH.
Some important causes of Group 3 pulmonary hypertension include
COPD,
interstitial lung disease, and obstructive sleep apnea, all of which cause hypoxic vasoconstriction, and, thus, elevations in pulmonary pressure.
Group 4 is chronic thromboembolic pulmonary hypertension, which accounts for less than 5% of PH.
Review
pulmonary embolism
Group 5 is pulmonary hypertension due to unknown or miscellaneous causes; approximately 15% of PH is in this group.
This group includes pulmonary hypertension associated with hematological, systemic, metabolic, and other disorders.
Thus, patients with sickle cell anemia, renal diseases, sarcoidosis, and other related disorders are at higher risk for PH.
Pre vs Post-capillary Summary
Let's summarize the 5 groups in a quick diagram to show pre- and post-capillary pulmonary hypertension.
Groups 1, 3, 4, and 5 are characterized by hypertension in the pre-capillary vessels.
Groups 2 and 5 are associated with post-capillary hypertension, but both may also progress to mixed hypertension with pre-capillary involvement.
When pulmonary hypertension is suspected, we can perform a variety of tests to determine its presence, severity, and cause.
Initial tests may yield the following findings:
- Chest radiographs may show enlarged pulmonary arteries and peripheral pruning.
- Contrast chest CT may indicate thrombi in thromboembolic PH or show signs of PH due to left heart failure, interstitial lung disease, or other chronic lung diseases;
- Echocardiography, ECG, and Cardiac MRI can show us abnormal cardiac structure and/or function that can indicate PH severity or cause; for example, abnormalities in the right ventricle can indicate that PH has progressed to right heart failure, or abnormalities in the left heart may indicate that PH is due to left heart disease.
- Other tests, such as spirometry or arterial blood gas tests, can tell us about lung function and gas diffusion.
- The definitive test for pulmonary hypertension is right heart catheterization, which measures the pressure in the right ventricle and pulmonary arteries.
Treat underlying causes, when known (for example, treating chronic lung diseases or left heart failure).
Many patients can benefit from symptomatic treatment with oxygen supplementation, diuretics, digoxin, anti-coagulants, and exercise therapy.
Vasodilators may be helpful for patients in Group 1 (Pulmonary Arterial Hypertension).
– Key examples of medications used to treat PAH include epoprostenol, nitric oxide, endothelin receptor antagonists, and calcium channel blockers.
For full references, please see our tutorial on Pulmonary Hypertension
