Affect of age on HR:
- ↑ HR from birth to 1-month old logarithmic ↓ HR with ↑ age [1,2]
- ↑ HR during 1st month of life likely related to autonomic factors
- ↓ HR after 1st month of life due to slowing of intrinsic sinus node activity 
- Similar maturational HR changes in humans & smaller primates, but different absolute HR 
- Maximum HR = 220 – Age
Relationship to BSA:
- ↑ BSA logarithmic ↓ HR 
- ↑ BSA logarithmic ↑ LV volume 
- ↑ BSA logarithmic ↑ SV
- Therefore, because CO = HR x SV CO remains stable with ↑ age when indexed for BSA
Abbreviations: HR, heart rate; BSA, body surface area; LV, left ventricular; SV, stroke volume; CO, cardiac output
1. Davignon A, Rautaharju P, Boisselle E, et al. Normal ECG standards for infants and children. Ped Cardiol 1:123, 1979.
2. Onat T, Ahunbay G. Regression of right ventricular dominance in the electrocardiogram after birth: a longitudinal follow-up of a healthy cohort from birth to 3 years. PediatrCardiol 19:197, 1998.
3. Marcus B, Gillette PC, Garson A Jr. Intrinsic heart rate in children and young adults: an index of sinus node function isolated from autonomic control. Am Heart J 119:911, 1990.
4. Brady AG, Johnson WH Jr, Botchin MB, et al. Developmental changes in ECG associated with heart rate are similar in squirrel monkey and human infants [published erratum appears in Lab Anim Sci 42:214, 1992]. Lab Anim Sci 41:596, 1991.
5. Akiba T, Nakasato M, Sato S, et al. Angiographic determination of left and right ventricular volumes and left ventricular mass in normal infants and children. Tohoku J Exp Med 177:153, 1995.