Pulmonary circulation

What it normally does

• The right ventricle moves the same cardiac output as the left - about 5 L/min at rest - but against a mean pulmonary artery pressure of only about 14 mmHg (normal is roughly 14 plus or minus 3, and 20 mmHg or less is taken as the upper limit of normal). Its free wall is only a few millimetres thick and crescentic in cross-section: it is built to eject volume at low pressure, not to generate pressure. An unprepared right ventricle can acutely raise mean pulmonary artery pressure to only about 40 mmHg (systolic of the order of 50 to 60) before it dilates and fails; a ventricle hypertrophied over months can generate considerably more.

  a sudden rise in afterload - a large pulmonary embolus - overwhelms it within hours, while the same pressure arrived at slowly is tolerated for years. It also explains the paradox that a systolic pressure above about 60 mmHg on echocardiography argues against a purely acute cause: the right ventricle could not have got there overnight.

• The bed is enormously compliant and much of it is unrecruited at rest, particularly at the lung apices where alveolar pressure can exceed capillary pressure. On exercise, closed capillaries open and open ones widen (recruitment and distension), so pulmonary vascular resistance - normally of the order of 1 Wood unit, against about 15 for the systemic circulation - falls further, and flow can rise threefold or more for only a small rise in pressure.

  a large fraction of the bed (conventionally more than half) can be obstructed or destroyed before resting pressure climbs and symptoms appear, which is why both pulmonary embolism and pulmonary hypertension present late. Conversely, when that reserve is lost the pressure-flow relationship becomes steep and exercise produces a large pressure jump instead of a fall in resistance.

• Pulmonary arterioles do the opposite of systemic ones: a fall in ALVEOLAR oxygen tension (not arterial) makes them constrict. Hypoxic pulmonary vasoconstriction begins as alveolar PO2 falls below roughly 70 mmHg and becomes marked below about 60; the sensor is in the smooth muscle itself, where hypoxia inhibits voltage-gated potassium channels, depolarising the cell and opening L-type calcium channels. Blood is pushed away from poorly ventilated alveoli toward well-ventilated ones.

  when the stimulus is patchy it matches perfusion to ventilation and protects arterial oxygenation - but when the whole lung is hypoxic the entire bed constricts at once and the pressure rises. It is also why raising alveolar oxygen lowers pulmonary artery pressure, and why any drug that dilates pulmonary vessels indiscriminately can worsen shunt and drop the arterial oxygen.

• The bed sits in series with essentially the whole venous return, so it acts as the body's filter: material travelling in systemic venous blood - thrombus from a deep leg or pelvic vein, air, fat, amniotic fluid, injected particulate - is delivered to a pulmonary artery. Two qualifications matter. The bronchial and thebesian veins are the exception to the series arrangement, draining oxygenated-then-deoxygenated blood into the pulmonary veins and left heart (the normal anatomical shunt, a couple of per cent of output, and the reason a normal PaO2 is below alveolar PO2). And a right-to-left shunt such as a patent foramen ovale lets venous material bypass the filter entirely.

  leg vein thrombosis presents as a lung problem, and the same anatomy explains paradoxical embolism - a venous clot causing a stroke - in a patient with a patent foramen ovale.

• The pulmonary capillaries are a through-passage draining by the pulmonary veins into the left atrium, with no valve in between, so capillary hydrostatic pressure is set downstream by left atrial pressure (normal mean about 4 to 12 mmHg, approximated clinically by the pulmonary capillary wedge pressure, with 15 mmHg or less taken as normal at catheter). Low capillary pressure across a blood-gas barrier well under a micron thick, plus plasma oncotic pressure of about 25 mmHg and continuous lymphatic drainage, is what keeps the air spaces dry.

  anything that raises left atrial pressure is felt by the lungs first: interstitial oedema begins once mean left atrial pressure rises above roughly 18 to 20 mmHg acutely and alveolar flooding follows as it approaches plasma oncotic pressure. When the rise is chronic the lymphatics enlarge and higher pressures are carried with surprisingly few crackles.

What goes wrong

• Pulmonary embolism (PE)← from “The bed sits in series with essentially the wh…”

  A thrombus from a deep leg or pelvic vein travels through the right heart and wedges in a pulmonary artery - the filter function working against the patient. That lung segment is still ventilated but no longer perfused, so it becomes dead space, ventilation is wasted and the person breathes fast; hypoxaemia comes less from the dead space itself than from redistributed flow, shunt through atelectatic lung and a low mixed venous oxygen when output falls. Because the bed is compliant and recruitable, small emboli barely move the pressure. A large or saddle embolus, with reflex and humoral vasoconstriction on top, raises afterload past what an unprepared right ventricle can generate; it dilates, the septum bows into the left ventricle, filling of the left side falls, and cardiac output collapses.

  The lung's filter role is the whole story. Clear chest, clear film, hypoxic and tachypnoeic - think vascular. Massive PE is defined by haemodynamics (sustained hypotension), not by clot size on the scan. In pregnancy or renal impairment a V/Q or perfusion scan is often preferred to CTPA.

  You would find: Sudden breathlessness and pleuritic chest pain with a clear chest on auscultation and a chest x-ray that is normal or near-normal. Sinus tachycardia is the commonest ECG finding (S1Q3T3 and right heart strain occur but are uncommon); oxygen saturation is often low, though a normal saturation does not exclude PE; CO2 is low from over-breathing. Hypoxia with a clean-sounding chest is the tell. Stratify with a validated rule such as Wells or the revised Geneva score, then D-dimer if the patient is not high-risk (an age-adjusted cut-off, age times 10 micrograms/L in those over 50, reduces false positives) or straight to CTPA if they are. A swollen calf points to the source; hypotension means the right ventricle is failing.

• Cor pulmonale from chronic hypoxic lung disease← from “Pulmonary arterioles do the opposite of system…”

  Hypoxic vasoconstriction is designed to be local. When the whole lung is hypoxic - COPD, obstructive sleep apnoea, obesity hypoventilation, bronchiectasis and chronic suppurative lung disease, which carry a heavy burden in Aboriginal and Torres Strait Islander communities - the entire bed constricts together. Sustained hypoxia also drives remodelling: the arteriolar media thickens, muscle extends into vessels that were previously non-muscular, compliance is lost, and pulmonary artery pressure stays high even when the hypoxia is corrected. The right ventricle hypertrophies, then dilates and fails.

  Right heart failure caused by lung disease, not by the left heart - so exclude left-sided disease before accepting the label. The vasoconstrictor component reverses with oxygen; the remodelled component does not, which is why late disease responds less. Correcting hypoxia, not vasodilating, is the treatment.

  You would find: Someone with long-standing lung disease who now has a raised JVP (more than about 3 to 4 cm vertically above the sternal angle), ankle oedema and a tender, pulsatile enlarged liver. Loud pulmonary component of the second heart sound and a parasternal heave at the left sternal edge. Resting oxygen saturation is low, and both the saturation and the pulmonary pressure improve with supplemental oxygen - which is the diagnostic and therapeutic point.

• Pulmonary arterial hypertension (group 1)← from “The bed is enormously compliant and much of it…”

  The small pulmonary arteries proliferate, thicken and narrow in their own right, so the bed loses the compliance and recruitment it depends on. Haemodynamically this is pre-capillary disease: mean pulmonary artery pressure above 20 mmHg at rest with a pulmonary artery wedge pressure of 15 mmHg or less and pulmonary vascular resistance above 2 Wood units. Because resistance can no longer fall with flow, cardiac output is effectively fixed - breathlessness on exertion, and syncope at peak effort when systemic vasodilatation is not matched by any rise in output. The pressure-loaded right ventricle hypertrophies, then fails.

  Diagnosis is haemodynamic, not echocardiographic - echo screens, catheter confirms. Wedge pressure is what separates group 1 (pre-capillary, wedge 15 or less) from group 2 left heart disease (post-capillary, wedge above 15), and that distinction decides whether pulmonary vasodilators help or harm. Group 1 is the rare group; do not treat the common ones as if they were it.

  You would find: Progressive exertional breathlessness with a clear chest and near-normal spirometry - the mismatch between how breathless the patient is and how normal the lungs look. Loud P2, a prominent a wave in the JVP while the right ventricle is hypertrophied and sinus rhythm is preserved, and a large v wave with a pansystolic tricuspid regurgitation murmur once the ventricle dilates. Exertional syncope is an ominous sign. Echocardiography estimates right ventricular systolic pressure and screens; right heart catheterisation is required to confirm the diagnosis and to separate pre-capillary from post-capillary disease.

• Pulmonary congestion, pulmonary oedema and post-capillary pulmonary hypertension from left heart disease← from “The pulmonary capillaries are a through-passag…”

  The pulmonary capillaries drain into the left atrium with nothing in between, so left-sided pressure is transmitted straight back into them. A stiff or failing left ventricle, or a narrowed mitral valve, raises left atrial pressure; capillary hydrostatic pressure rises past the point where the alveoli can stay dry and fluid moves first into the interstitium and then into the air spaces. Sustained for months, the pulmonary arterioles remodel and add a fixed pre-capillary resistance on top of the passive back-pressure. Left heart disease is the commonest cause of pulmonary hypertension in Australia and worldwide.

  Post-capillary: mean pulmonary artery pressure above 20 mmHg with a wedge pressure above 15 mmHg. The treatment is the left-sided lesion and the filling pressure, not the pulmonary vasculature - group 1 vasodilators used here worsen pulmonary oedema.

  You would find: Breathless lying flat, waking at night gasping, fine crackles at both bases, frothy or pink sputum, with Kerley B lines, upper-lobe blood diversion and perihilar shadowing on chest x-ray. In rheumatic mitral stenosis - still seen in young Aboriginal and Torres Strait Islander patients - a loud first heart sound, an opening snap, a rumbling mid-diastolic murmur at the apex, exertional breathlessness and haemoptysis.

What we give, and how it works

Open a drug to see what it binds, what that does to the cell, and what you then see in the patient.