Blood pressure and heart rate response to head-up position in full-term newborns

The reactions of heart rate (HR), systolic (sBP) and diastolic (dBP) blood pressure were studied in response to passive head-up tilting (successively to +45 degrees and +90 degrees) in a group of 83 full-term, 1- to 7-day-old newborns who were quiet and awake. A significant mean increase of HR was noted for the whole group, from 121 +/- 14 beats/min to 124 +/- 15 beats/min at +45 degrees and to 127 +/- 17 beats/min at +90 degrees, while sBP rose from 65 +/- 9 mmHg to 67 +/- 12 mmHg at +45 degrees and to 70 +/- 13 mmHg at +90 degrees tilts, respectively (5th post-tilt min). A negative correlation (r = -0.41, P < 0.01) was found between basal supine values of HR and their post-tilt increments after +45 degrees tilting. After +90 degrees tilting dBP correlated negatively (r = -0.33, P < 0.003) with the supine values, as well. as HR (r = -0.30, P < 0.01). Breaking down the whole group according to age has shown that in 1-day-old babies (N = 11), the only one significant mean change was an increase of HR by 7 +/- 10 beats/min after the +45 degrees tilt. At the age of 2 days (N = 36), the group mean values showed a rise of sBP only by 5 +/- 12 mmHg after the +90 degrees tilt. In 3-day-old babies (N = 18), increased values were noted in both the HR (by 6 +/- 18 beats/min) and sBP (by 5 +/- 15 mmHg) even after the +45 degrees tilt. The increase of all the variables: HR by 7 +/- 12 beats/min, sBP by 6 +/- 8 mmHg, dBP by 2 +/- 4 mmHg after 5 min of +90 degrees tilt was present only in 4- to 7-day-old neonates (N = 18). It seems that the evolution of orthostatic circulatory regulation is a component of the evolution in the overall responsiveness of the cardiovascular system to external stimuli and becomes apparent after a relative stabilization of the neonatal blood volume on the 2nd-3rd postnatal day.