USMLE® Step 1 style questions USMLE
A 39-year-old woman comes to the office for her routine cardiology examination. She was diagnosed with limited cutaneous scleroderma two years ago, for which she is currently undergoing treatment with methotrexate and nifedipine. A portion of her ECG is shown below. Which of the following abnormalities is seen on this patient's ECG?
The pulmonary circulation starts with the right ventricle.
The pulmonary arteries divide into smaller arteries known as pulmonary arterioles and then eventually into pulmonary capillaries which surround the alveoli - which are the millions of tiny air sacs where gas exchange happens.
At that point, oxygen enters the blood and carbon dioxide enters the alveoli.
The pulmonary capillaries drain into small veins that join to form the two pulmonary veins exiting each lung, and these pulmonary veins complete the circuit by delivering oxygen-rich blood into the left atrium.
The normal pulmonary artery pressure is about 25/10 mmHg with a mean arterial pressure of 15 mmHg.
Pulmonary hypertension most commonly develops as a result of left heart disease.
Here the pulmonary blood vessels are normal and undamaged, but the left side of the heart is unable to pump efficiently – for example because of heart failure or valvular dysfunction.
Another cause of pulmonary hypertension is chronic lung disease, which typically causes hypoxic vasoconstriction.
That’s when some area in the lung is diseased and is unable to deliver oxygen to the blood.
To help adapt to this, the pulmonary arterioles in that area, start to constrict - and this effectively shuttles blood away from those damaged areas of the lung, and towards healthy lung tissue. But if the problem is widespread, like in individuals with emphysema, the mechanism can backfire.
That’s because there’s widespread vasoconstriction of pulmonary arterioles, and that increases pulmonary vascular resistance in general.
Increased resistance makes it hard for the right ventricle to pump out blood – a bit like pushing water through a narrow pipe as opposed to a wider one.
So to make the same amount of blood flow through the pulmonary arterioles, the right side of the heart has to generate increased pressure and that results in pulmonary hypertension.
The clots can block pulmonary vessels which increases the resistance to blood flow, and they can also endothelial cells in the vessels to release histamine and serotonin, which constricts the pulmonary arterioles.
Together the blockage and the narrowing of the blood vessels causes a rise in the pulmonary blood pressure.
One type of pulmonary hypertension is pulmonary arterial hypertension which is when there’s elevated pressure in the pulmonary arterioles, but the pressure in their capillaries and pulmonary veins is still normal.
Some congenital heart defects can cause pulmonary arterial hypertension.
A long-standing left-to-right cardiac shunt caused by a ventricular septal defect, atrial septal defect, or less commonly, a patent ductus arteriosus can result in pulmonary hypertension and eventual reversal to a right-to-left shunt, which is called Eisenmenger’s syndrome.