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Refer to the basic anatomy section for details regarding the blood pumping function of the heart. In brief, the blood returning from the body enters the right upper chamber. Simultaneously the blood from the lungs, which has now been ‘oxygenated’, enters the left upper chamber. It is held there until the lower chambers are ready to receive them. Thereafter, the lower chambers relax and accept the blood from the upper chambers and then pump the blood out of the heart. The right chamber pumps the blood to the lungs to get oxygenated and the left chamber pumps to the body to provide oxygen and nutrition (figure 1). The cycle repeats as the next batch of blood returns to the heart. This continues on, like a roller coaster ride, one car leaves with passengers to circulate the ride while the next car pulls into the station to receive more.
Thus, there are two parallel circuits. One going from the right chambers to the left and another from the left to the right (figure 2).
The two circuits have different characteristics, have two different blood pressures and show interdependence as well as independence of each other. The circuit connecting the left with the right, which goes through the body, is called systemic circulation. The other circuit which goes through the lungs is called pulmonary circulation (figure 2).
What causes high pressure in lung circuit?
Normally, the pressures within the systemic circuit are significantly higher than the pulmonary circuit. Under certain circumstances, the pressures within the pulmonary circuit may rise and this is called pulmonary hypertension. There could be multiple reasons for the same.
When left chambers are to blame:
One of the most common reasons is the failure of the left heart pump. If the blood does not move forward from the left chambers, then the pressure built up within the chamber backs up into the pulmonary circuit. Just like at a traffic signal, if the cars at the front do not move when it turns green, the traffic gets backed up. So left ventricle failure (and excessive pressure built up) is one of the most common cause of pulmonary hypertension (figure 3).
Here the treatment of the left chamber failure is the primary focus of treatment. In later stages, the other parts of the circuit get involved, as explained below.
When the lungs are to blame:
Some conditions in the lungs can raise the pressure within the lungs. Coming back to traffic analogy, if 3 lanes in a 4-lane freeway are blocked, the traffic backs up. Similarly, if the pulmonary circuit gets blocked – from blood clots, from scar around the arteries called fibrosis or from unknown reasons called idiopathic, pulmonary hypertension results. Conditions causing long-standing low oxygen states, such as ‘COPD’, can result in changes in the pulmonary circuit called remodelling. This is like a 4-lane highway becoming 2-lane permanently. Undiagnosed and untreated sleep apnea (stopping to breathe while sleeping or snoring) can cause low oxygen states during sleep, again causing pulmonary hypertension.
When the pressure in the pulmonary circuit rises, the right chambers must work hard to push any blood through, into an already pressurized circuit. Unfortunately, the right ventricle is not adept at that. This is like a low horsepower car with a trailer attached at the back, attempting to climb a steep hill. If the pressure in the pulmonary circuit rises suddenly and excessively, as happens with sudden massive blood clots in lungs, the right ventricle fails. The failure of the right ventricle pump makes it unable for blood to reach the left side and suddenly the left chamber runs dry (figure 4).
Less blood in the left ventricle means less blood pumped into the body. This results in a drop in the blood pressure, poor blood supply to organs and multi-organ failure.
On the other hand, if the pressure in the pulmonary circuit rises slowly and gradually, the right ventricle gets time to adapt to the rising pressure. As the pressure rises, the workload on the right chambers increases. High pressure in the right chambers results in high pressure in the veins bringing blood back from the body. If the freeway gets blocked, the cars back up into the ramp feeding the freeway. If this persists, all roads leaving the city get ‘congested’ and the city halts. This ‘congestion’ is also manifest in the body with multiorgan failure, as the blood vessels leaving each organ get congested (figure 5). The organs most commonly affected are the liver and the kidneys.
Irrespective of the mode of rising pressure in the lungs, the end result is progressive right chamber failure. This results in congestion as described above which starts affecting the other organs (figure 5).
When kidneys start failing, they stop making much urine, resulting in retention of water and salt. While liver is a factory making raw materials for many things including proteins called albumin. Albumin is necessary to keep the fluids in the blood from leaking out. So, both organ failure causes swelling of legs, arms, belly etc., from fluid retention and leakage.
Long standing high pressure in the lungs cause changes in the circuit called remodelling. As mentioned above, this is like a 4-lane freeway becoming 2-lane permanently and slowly even 1-lane. The functioning of the lungs gets seriously affected and oxygenation of blood suffers. This could cause shortness of breath, dizziness or tiredness.
If the cause is detected early and treated, the above changes can be reversible. However, if the cause is irreversible or untreatable, then the above-mentioned changes must be treated individually. The narrow blood vessels in the lungs are attempted to be opened wider with medications. Long-standing low oxygen levels are treated with home oxygen therapy, sleep apnea with CPAP machine. Multiple blood clots in the lungs over a long time can cause progressive pulmonary hypertension and this is treated with stripping of the blood vessel wall called endarterectomy. All these treatment options unfortunately are easier said than done. Different patients may have different results, going from great responders to non-responders.