These paragraphs are a brief summary of the effects of disordered pulmonary vascular resistance. They assume you are familiar with the section on general haemodynamic principles.
One of the most important factors in the prognosis and management of congenital heart disease is pulmonary vascular resistance. Normally pulmonary vascular resistance is much lower than systemic (15-20% of it), and so the pressure in the pulmonary arteries needs only to be a fraction of systemic arterial pressure to maintain the same flow. As a consequence the right ventricular myocardium is thinner than the left and the pulmonary arteries (and arterioles) have thinner walls than the systemic arteries. This is not the case in the foetus where the widely patent ductus arteriosus allows blood to divert from the pulmonary circuit to the systemic; it follows that not only are the pulmonary arteries at systemic pressure but the pulmonary resistance is much higher than systemic in order to reduce pulmonary perfusion to a trickle. This is achieved by arteriolar spasm. As a result the foetal right ventricle is as thick walled as the left, and the pulmonary arteries are as thick walled as the systemic.
At birth the arteriolar spasm relaxes, the ductus closes, and pulmonary blood flow rises to the normal cardiac output. However, the pulmonary resistance falls only to systemic levels, and the pulmonary artery pressure remains at or around systemic pressure. Over the first two to three weeks of life, the walls of the pulmonary arteries become thin and the pulmonary resistance and pressure fall to normal levels. Also, the right ventricle begins to involute, but this process is not completed until nine to twelve months of age.
This normal sequence of events can be disrupted by any malformation that allows the systemic blood flow free access to both pulmonary and systemic circuits. The classic example is a large ventricular septal defect, i.e. one where the defect is equal to or larger than the aortic valve. The fall of pulmonary vascular resistance to normal levels will mean that the lungs will accept a greater flow than the systemic circulation, and a shunt is created. The critical thing to remember I that if a VSD is large, any gradient across it is trivial and ventricular pressures must be the same (and great vessel systolic pressures must therefore be equal). If pulmonary resistance falls then the shunt must increase to the point where the systemic output is normallised. If, for example, the pulmonary resistance falls to a quarter of systemic resistance, then the pulmonary flow must be four times systemic flow.
Often in the presence of a large VSD, the normal post natal fall in pulmonary resistance is delayed due to persistence of both the arteriolar spasm and the pulmonary arterial wall thickness. Pulmonary resistance may remain at systemic level for several weeks, so a large ventricular septal defect need not present early in neonatal life since the shunt will not become symptomatic until the resistance falls. In some cases the pulmonary resistance never falls and the shunt does not develop. If the resistance does slowly drop, the left to right shunt increases: as the body will demand a normal systemic cardiac output (see the module on basic haemodynamics) the pulmonary circulation will become hyperdynamic. Even with large shunts the pulmonary resistance usually remains higher than normal, mainly due to continuing arteriolar spasm. The walls of the arteries usually retain a systemic thickness, and so may not readily dilate with increasing flow; this can reduce the radiographic appearance of plethora.
In the longer term, pulmonary vascular resistance can be more severely altered by pulmonary vascular occlusive disease (PVOD). PVOD is a progressive cellular obliteration of the pulmonary arterioles, initially by smooth muscle hypertrophy, later by intimal proliferation which can progress to occlusion. There is a histological classification, the four Heath grades. Level four is when many vessels show occlusive lesions with attempts at recanalisation. The greatest stimulus to its development is a mixture of pulmonary hypertension, plethora and cyanosis, particularly in transposition of the great vessels when it can develop in the first six months of life. Next, in order of severity, are the pulmonary hypertensive ventricular septal and great vessel shunts as described above; here the occlusive disease can become established at about two years of age. Next come the causes of plethora at low pulmonary pressure; atrial septal defects; small or medium sized ventricular septal defect and patent ductus arteriosus (where the size of the defect limits the size of the shunt rather than pulmonary vascular resistance); and occasionally in chronically raised pulmonary venous pressure.
The disease is inexorable and will steadily raise pulmonary resistance up to and beyond systemic level. With ventricular septal defect and patent ductus arteriosus, once pulmonary resistance exceeds systemic, the shunt reverses to become right to left; this is known as the Eisenmenger response. With atrial septal defects pulmonary artery pressure may rise above systemic before the secondary increase in right ventricular diastolic compliance reverses the shunt. PVOD, once established, continues to progress when the responsible defect is closed, even if pulmonary resistance is not yet at systemic level. In these cases surgical closure of the defect can actually shorten life expectancy, since Eisenmenger reversal of a shunt delays the onset of right ventricular failure. In many cases of large shunts it is necessary, at catheterisation, to test the pulmonary circuit with oxygen or isoprenaline in order to distinguish between a raised pulmonary resistance due to arteriolar spasm (which is abolished by them) and the unvarying resistance of occlusive disease.
Arteriolar spasm can be re-induced later in life, through to adulthood, by acute rises in pulmonary venous pressure or pulmonary blood flow, by alveolar hypoventilation, or by combinations of the three.