Question

A 2-week-old neonate undergoes an ECG during evaluation for a murmur. ECG shows a heart rate of 150 bpm, right axis deviation, tall R wave in V1, and inverted T wave in V6. Which of the following best explains these findings?

• A. ECG is abnormal, suggesting right ventricular hypertrophy
• B. Findings are normal for age due to physiological right ventricular dominance
• C. The child has tricuspid atresia
• D. ECG suggests myocardial ischemia

Hint:

Normal Neonatal ECG Rules:
$\bullet$ Right Axis Deviation ($+110^\circ$) is NORMAL.
$\bullet$ Tall R in $V_1$ is NORMAL (RV dominance).
$\bullet$ T wave in $V_1$ MUST BE INVERTED between Day 7 and $\approx 10\text{ years}$ of age! (Upright $T$ in $V_1$ = RVH).

Answer 1

The Correct Option is B

Concept:
The neonatal electrocardiogram undergoes rapid developmental evolution following the transition from fetal to postnatal circulation.
In utero, the fetal right ventricle handles approximately $55\text{ to }60\%$ of total cardiac output against high pulmonary vascular resistance, resulting in marked physiological right ventricular dominance at birth.
Explanation:
• Normal neonatal ECG features reflecting physiological right ventricular dominance include:
1. Right Axis Deviation (RAD): Mean QRS frontal axis in healthy 2-week-old neonates normally ranges between $+60^\circ\text{ and }+160^\circ$ (mean $+110^\circ$).
2. Tall R waves in right precordial leads ($V_1$, $V_{3R}$, $V_4R$) and deep S waves in left precordial leads ($V_5$, $V_6$), with an $R/S$ ratio in $V_1 > 1$.
3. Resting heart rate of $120\text{--}160\text{ bpm}$ is entirely physiological for age.
4. T-wave changes: T waves in lead $V_1$ are upright at birth, invert within the first $3\text{ to }7\text{ days}$ of life, and remain normally inverted throughout childhood until adolescence (upright $T$ in $V_1$ beyond day 7 indicates pathological RVH), while lateral leads ($V_5, V_6$) show normal upright or transiently variable T waves.

• These findings represent the normal physiological baseline for a 2-week-old infant and do not indicate pathology or ischemia.

• Tricuspid atresia is characterized by left axis deviation and left ventricular hypertrophy, which is distinctly abnormal in a neonate.
Final Answer:
These electrocardiographic features reflect normal physiological right ventricular dominance expected in a healthy 2-week-old neonate.