A Practical Approach, Second Edition=Ronald D. Ho.pdf

POSTNATAL DEVELOPMENTAL MILESTONES 1069course over the ventricles in adults, compared to infant and juvenile animals, might explain thisincreasing excitation period.In mice, the entire J junction-S-T segment-T wave complex, R-R interval and Q-T intervalswere shorter during early postnatal development and matured by postnatal days 7 to 14 (Wang etal., 2000). Age-dependent ECG responses to K + channel blockers were also reported in mice.Postnatal Cardiac Output and HemodynamicsAt birth, the cardiovascular system changes dramatically; arterial blood pressure, heart rate, andcardiac output increase, and blood flow distribution undergoes regional changes. During the earlypostnatal period, increasing heart rate, rather than stroke volume, is the primary mechanism bywhich cardiac output is increased during the neonatal period.HumanThe mean basal heart rate at birth is approximately 120 to 130 beats/minute. A steep increase inheart rate occurs between the 5 th and 10 th postnatal day, reaching maximum levels of ~140-150beats/minute, which then gradually decrease to ~120 beats/minute over the first 100 postnatal daysas parasympathetic restraints develop. Adult heart beat rates (55-85 beats/min) are achieved around12 years of age (Oberhansli et al., 1981; Mrowka et al., 1996).Mean Values for Heart Functional Parameters in Human Infants (Oberhansli et al., 1981)Heart Parameter1day3days6daysAge1month2month6–11months12–14monthsHeart Rate (beats/min) 133 129 135 155 150 140 124RV Pre-ejection Period (msec) 71 63 59 51 55 55 61RV Ejection Time (msec) 199 203 203 193 204 232 243LV Pre-ejection Period (msec) 65 61 59 55 59 59 65LV Ejection Time (msec) 197 193 192 184 192 200 228Mean Velocity 1.67 1.72 1.75 1.79 1.73 1.97 1.51Ejection Fraction 70 70 71 70 70 77 71RV- right ventricle; LV- left ventricleInfants and young children are reported to have significantly smaller ventricular end-diastolicvolumes and stroke indexes than adults, slightly smaller ejection fractions, but no differences incardiac index (Graham et al., 1971; Mathew et al., 1976). Oh et al. (1966) reported marked increasesin the distribution of cardiac output to the kidney during the first days after birth, but still a smallfraction of output, when compared to adults.The average blood pressure values (systolic/diastolic) increased rapidly from 62.1/39.7 mm Hgat age day one to up to 72.7/46.9 mm Hg at age one week and up to 85.0/47.4 mmHg at age twomonths (Hwang and Chu, 1995). Blood pressure changes relatively little between the ages of 6months and 10 years. Systolic blood pressure rose from a mean of 88.5 mm Hg at age 6 monthsto 96.2 mm Hg at 8 years, and diastolic blood pressure rose from 57.8 mm Hg at 5 years to 61.8mm Hg at 10 years (de Swiet et al., 1992). Blood pressure was correlated with weight, weightadjusted for height, height, and arm circumference, at all ages studied. Blood pressure values(systolic/diastolic) increased rapidly from 68.0/43.7 mm Hg in children weighing less than 5 kgup to 87.6/41.8 mm Hg in those weighing 5 to 10 kg. Subsequently, these values increased graduallywith body weight (Hwang and Chu, 1995).© 2006 by Taylor & Francis Group, LLC