What is the expected physiologic change with the initiation of a phenylephrine infusion in a 4-month-old infant with tetralogy of Fallot and recalcitrant cyanosis?

Physiologic Changes with Phenylephrine Infusion in Tetralogy of Fallot

Phenylephrine infusion in a 4-month-old infant with tetralogy of Fallot and recalcitrant cyanosis will primarily cause increased systemic vascular resistance (SVR), which decreases right-to-left shunting and increases pulmonary blood flow, resulting in improved oxygenation. 1

Mechanism of Action and Hemodynamic Effects

• Phenylephrine is an α-1 adrenergic receptor agonist that causes vasoconstriction in most vascular beds, including renal, splanchnic, and hepatic, leading to increased systemic vascular resistance 2
• When administered to patients with tetralogy of Fallot, phenylephrine increases systolic, diastolic, and mean arterial blood pressure 2, 1
• The increased systemic vascular resistance alters the balance between systemic and pulmonary circulation, reducing right-to-left shunting across the ventricular septal defect 1

Specific Physiologic Changes in Tetralogy of Fallot

Effects on Pulmonary Blood Flow

• Phenylephrine increases pulmonary blood flow in children with tetralogy of Fallot by increasing systemic vascular resistance relative to right ventricular outflow tract obstruction 1, 3
• Studies have demonstrated a significant correlation between increased pulmonary venous flow and improved arterial oxygen saturation following phenylephrine administration 3

Effects on Oxygenation

• Phenylephrine administration has been shown to increase arterial oxygen tension by 4-28 mmHg (mean 14 mmHg) in patients with tetralogy of Fallot 1
• The net right-to-left shunt decreases by 10-40% (mean 25%) with phenylephrine infusion compared to baseline conditions 1

Effects on Ventricular Dynamics

• Increased systemic vascular resistance from phenylephrine leads to increased left ventricular end-diastolic pressure as more blood is directed to the pulmonary circulation and subsequently returns to the left heart 1, 3
• The increased afterload from phenylephrine challenges the left ventricle, resulting in increased left ventricular filling pressures 3

Clinical Implications

• Phenylephrine is an appropriate intervention for hypercyanotic spells in tetralogy of Fallot when morphine and fluid therapy have failed 4
• The improvement in oxygenation makes phenylephrine a valuable rescue therapy for protracted hypoxic spells in these patients 1, 5
• The increased systemic vascular resistance from phenylephrine forces more blood through the pulmonary circulation, which is particularly beneficial in patients with infundibular spasm 4, 1

Common Pitfalls and Caveats

• While phenylephrine increases pulmonary blood flow, it does not decrease pulmonary vascular resistance (PVR) - in fact, it may have minimal effects on PVR or slightly increase it 3
• Excessive increases in systemic vascular resistance could potentially overload the left ventricle, so careful titration and monitoring are essential 2, 3
• The reflex bradycardia that can occur with phenylephrine due to increased vagal activity should be monitored, especially in pediatric patients 2

In summary, the expected physiologic change with phenylephrine infusion in this infant with tetralogy of Fallot is increased left ventricular end-diastolic pressure due to the altered balance between systemic and pulmonary circulation, with subsequent improvement in oxygenation.