Pulse oximetry versus transcutaneous pO₂ in sick newborn infants

Abstract

A pulse oximeter (Ohmeda Biox 3700) and two transcutaneous systems (Radiometer TCM3) were applied simultaneously to 18 newborn infants with respiratory insufficiency. All infants had either an umbilical catheter placed in the mid thoracic aorta or a radial artery catheter. The average monitoring time was 2 hours. Arterial blood pO₂, pCO₂ and pH (Radiometer ABL300), arterial sO₂, HbCO and metHb (Radiometer OSM3), erythrocyte 2,3 DPG concentration, and fetal hemoglobin fraction (alkali denaturation kinetic method) were measured.

Using arterial sO₂ and pO₂ as reference, the analytical bias of pulse oximetry (-0.5 ±1.0%, mean ± 1 SD) corresponded in magnitude, when coverted to pO₂, to that of transcutaneous -pO₂ (0.6 ±1.4 kPa for combined O₂-CO₂ electrode and -0.1 ±2.3 kPa for single O₂ electrode). Transcutaneous pCO₂ showed the smallest bias (0.3 ± 0.3 kPa). Both pulse oximetry and transcutaneous pO₂ electrodes were good as trend monitors detecting rapid changes in the infants' oxygenation status. The pulse oximeter offers certain advantages in not requiring calibration or heating. The variations in the levels of fetal hemoglobin fraction (44 to 97 %), pH (7.27 to 7.49), pCO₂ (3.3 to 6.8 kPa) and 2,3 diphosphoglycerate concentration (1.6 to 5.9 mmol/1) between the infants studied, resulted in a variable pO₂-sO₂ relation (p₅₀ 2.5 to 3.5 kPa). This presents difficulties in interpreting sO₂ values in sick newborn infants, and we therefore recommend caution in using apulse oximeterto apply strict limits foravoiding hypoxia and hyperoxia in this population.

Key-words:

• Blood gases
• oxygen saturation
• hemoglobin-oxygen dissociation curve