Pulmonary Artery Pressure (PAP) is the force exerted by blood against the walls of the pulmonary arteries, the vessels that carry deoxygenated blood from the right side of the heart to the lungs. The right ventricle must generate this pressure to push blood through the capillaries where it collects oxygen and releases carbon dioxide. Maintaining a low pressure in this circuit is necessary for the right side of the heart to function efficiently. Unlike systemic blood pressure, pulmonary pressure operates at a much lower level. This lower pressure allows the delicate pulmonary capillaries to facilitate gas exchange without damage, making PAP measurement a direct indicator of heart and lung function.
The Standard Numerical Range
The normal range for pulmonary artery pressure is significantly lower than systemic blood pressure, reflecting the low-resistance nature of the lung’s blood vessels. PAP is described using three values, measured in millimeters of mercury (\(\text{mmHg}\)): systolic, diastolic, and mean pressure.
The pulmonary artery systolic pressure (PASP) is the peak pressure when the right ventricle contracts, generally falling between \(15 \text{ and } 30 \text{ mmHg}\) at rest. The pulmonary artery diastolic pressure (PADP) is the minimum pressure when the right ventricle is relaxed, typically ranging between \(4 \text{ and } 12 \text{ mmHg}\).
The mean pulmonary artery pressure (mPAP), the average pressure over a full heartbeat cycle, is the most important measurement for clinical diagnosis. A normal mPAP is approximately \(14 \pm 3 \text{ mmHg}\) at rest, meaning the upper limit of healthy pressure is around \(20 \text{ mmHg}\). An mPAP greater than \(20 \text{ mmHg}\) at rest is the accepted threshold for diagnosing elevated pulmonary pressure, known as pulmonary hypertension. This cutoff was recently lowered from \(25 \text{ mmHg}\) to allow for earlier identification. Elevated pressure suggests the pulmonary blood vessels have become narrowed or stiff, increasing the resistance the heart must overcome.
Methods for Measuring Pulmonary Artery Pressure
PAP requires specialized medical procedures for accurate measurement.
Right Heart Catheterization (RHC)
The definitive and most precise method is Right Heart Catheterization (RHC), considered the gold standard for diagnosis because it directly measures pressures within the heart chambers and pulmonary arteries. During RHC, a thin, flexible catheter is inserted into a vein (usually in the neck or groin) and threaded through the right side of the heart into the pulmonary artery. The catheter contains pressure sensors that provide real-time, accurate measurements of systolic, diastolic, and mean PAP. This procedure also allows clinicians to measure the pulmonary artery wedge pressure (PAWP) and pulmonary vascular resistance (PVR), which help determine the specific type of pressure elevation.
Echocardiography
The most common, non-invasive method used for initial screening is transthoracic echocardiography, an ultrasound of the heart. This method uses sound waves to create images of the heart’s structure and function. Echocardiography estimates the pulmonary artery systolic pressure by measuring the velocity of blood regurgitating backward through the tricuspid valve, known as the tricuspid regurgitant jet velocity. A tricuspid regurgitant velocity above \(2.8 \text{ meters per second}\) suggests the pressure is likely elevated. While echocardiography is a quick, safe tool for initial assessment, it provides an estimate rather than a direct measurement. If the estimated pressure is high, RHC is typically recommended to confirm the diagnosis.
Understanding Pulmonary Hypertension
Pulmonary hypertension (PH) is a serious condition where pressure in the lung arteries is chronically too high. This constant increase forces the right ventricle to work significantly harder against the increased resistance in the pulmonary circulation. Over time, this excessive workload causes the right ventricle’s muscle wall to thicken and enlarge (hypertrophy).
This chronic strain often leads to right-sided heart failure, where the right ventricle can no longer effectively pump blood to the lungs, causing blood to back up into the systemic circulation. Patients commonly report non-specific symptoms such as increasing shortness of breath, particularly during physical activity, and persistent fatigue. Other signs include dizziness, chest pain, or fainting (syncope).
PH is a complication arising from many different underlying conditions, organized into five clinical groups by the World Health Organization (WHO):
- Group 1: Pulmonary Arterial Hypertension (PAH), where the disease originates in the small arteries of the lungs, often due to unknown or genetic factors.
- Group 2: The most common form, resulting from high pressure backing up from left-sided heart disease, such as heart failure.
- Group 3: Associated with lung diseases and low oxygen levels, such as chronic obstructive pulmonary disease (COPD) or interstitial lung disease.
- Group 4: Occurs due to chronic blood clots or obstructions in the pulmonary arteries, known as chronic thromboembolic pulmonary hypertension (CTEPH).
- Group 5: Encompasses PH caused by various other conditions, including certain blood disorders or metabolic diseases.