**6. The pathophysiology of L->R shunt lesions**

ASD, VSD and PDA are the main lesions which shunt from left to right.

In Atrial septal defect, the size of the shunt is determined by the size of the defect and the degree of pulmonary vascular resistance. Small shunts cause no enlargement of cardiac chambers, while large shunt cause significant RA and RV dilatation, mid- diastolic flow murmur though the tricuspid valve and ejection systolic murmur at the left upper sternal border due to increased flow across the pulmonary valve. The left chambers do not increase in size as the increased return to LA is decompressed into the RA through the defect.

The pathophysiology of the shunt in VSD and PDA are similar. The magnitude of shunt is determined by the size of the defect and the degree of pulmonary vascular resistance (PVR); PVR is more important in the large defects where it determines the degree of shunt. Since PVR is elevated at birth and falls by 6-8 wks, children with large VSD typically become symptomatic with signs of congestive heart failure (CHF) at this time.

A small defect causes no enlargement of cardiac chambers, the ECG and X-ray are normal, there is ejection systolic murmur and P2 is normal. For moderate sized defects, there is LA and LV enlargement unlike ASD and as the RV is contracting at the time of left to right shunt, it undergoes no volume load. However the increased blood pumped by the RV causes middiastolic murmur across the mitral valve.

Large defects and PDA cause CHF in infancy, especially if the PVR is low. This reflects in biventricular hypertrophy with increased saturations, pan systolic murmur and loud P2. As the PVR increases, the heart size becomes normal on Chest X-ray though the pulmonary segment remains prominent, the murmur is reduced, and there is pure RVH in ECG

In PDA in addition there is enlargement of aorta and transverse aortic arch, which is usually not very evident in on X-ray as the aortic arch does not form a part of cardiac silhouette. As the PVR increases, there can be differential cyanosis with lower limb saturations being lower than the upper limb reflecting right to left shunt at the level of PDA.

Any newborn with deep cyanosis and cardiomegaly (egg on side appearance) and increased pulmonary vascular marking and no murmur can be diagnosed to have TGA. Patients with VSD present a little late with features of early CHF and these patients are prone to develop very early pulmonary hypertension, because they get relatively desaturated blood under high pressure. The unique feature of pulmonary circulation of 'hypoxic vasoconstriction ' acceler‐ ates the onset of pulmonary vascular disease.TGA, VSD and PS can occur and the presentation

The Basis of Management of Congenital Heart Disease

http://dx.doi.org/10.5772/53654

235

Truncus arteriosus has complete mixing of systemic and pulmonary blood and the level of

This is commonest cause of cyanosis in children in developing countries. Though classically supposed to have the features of a) VSD b) Aortic over-ride c) RV hypertrophy and d) Right

**a.** A large VSD at least as big as the aortic annulus to equalise pressures in both ventricles. **b.** Right ventricular outflow tract obstruction – which can be at any level infundibular,

If a child presents clinically with signs of small VSD with RVH, it strongly suggests diagnosis

The intensity and the duration of heart murmur are inversely proportional to the severity of pulmonary stenosis. In pulmonary atresia or in cyanotic spells when there is critical reduction in pulmonary flow there may be no murmur at all due to absence of flow across the RVOT There are no signs of CHF in TOF because no chamber is under volume overload and only RV

The degree of cyanosis depends on the balance between the systemic and pulmonary resis‐ tances, and decrease in SVR due to activities like crying and defecation can increase the degree

Hyperpnoea plays an important role in the perpetuation of cyanotic spell as it increases the venous return and more desaturated blood enters the systemic circulation due to override.

**b.** Reducing hyperpnoea by sedation – morphine, sodium bicarbonate which reduces

**d.** Stabilisation of vascular reactivity – propranolol prevents sudden decrease in SVR and its

of which depends on the degree of reduction of pulmonary blood flow.

ventricular outflow tract (RVOT) obstruction, the two important features are

of TOF, the probability of which increases in the presence of right aortic arch.

is under pressure overload which is not suprasystemic and is well tolerated.

The role of RVOT spasm in the development of cyanotic spell is controversial

**a.** By increasing SVR – by knee chest position, phenylephrine, ketamine

saturation is proportional to the degree of pulmonary blood flow.

**7.2. Pathophysiology of Tetralogy of Fallot**

valvar, supravalvar or at branch PA level.

Termination of a spell can be achieved by

respiratory stimulation, ketamine **c.** Decreasing venous return – squatting

role in preventing RVOT spasm is controversial.

of R-> L shunt.

Endocardial cushion defects present with features of both ASD and VSD. The QRS axis is abnormal between -20 to -150 degree, which is due to the disposition of the His bundle and its branches intrinsic to the pathology and not due to hemodynamic consequence.

Obligatory shunts are those where the degree of shunting does not depend on the PVR, examples of which is Gerbode defect where is there is LV to RA shunt or ruptured sinus of Valsalva [14]
