What is the role of arterial blood gases (ABG) in the workup of persistent pulmonary hypertension of the newborn (PPHN)?

Updated: Sep 03, 2019
  • Author: Kate A Tauber, MD; Chief Editor: Howard S Weber, MD, FSCAI  more...
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The studies discussed below are indicated in persistent pulmonary hypertension of the newborn (PPHN).

Arterial blood gas

Check arterial blood gases (ABGs) initially and frequently, ideally through an indwelling line (eg, umbilical artery catheter or peripheral artery such as the radial or posterior tibial artery). Assess the pH, the partial pressure of carbon dioxide (PaCO2), and the partial pressure of oxygen (PaO2), which may be higher in the arterial line. Using these values you can calculate the alveolar-arterial (A-a) gradient, as follows:

A-a Gradient = [FiO× (PB – PH2O) – (PaCO2/0.8)] – PaO2, where FiO2 is the fraction of inspired oxygen, PB is the local barometric pressure, PH20 is the water vapor pressure, and 0.8 is the respiratory quotient.

Be aware that the choice of sampling site can affect the ABG results. Newborns with PPHN frequently have right-to-left shunting across the patent ductus arteriosus. Therefore, preductal PaO2 values may be elevated when compared with postductal sites, unless there is significant right-to-left shunting at the level of the foramen ovale.

Oxygenation is often assessed by using the oxygenation index (OI). The OI is calculated as the mean airway pressure multiplied by the FiO2, and this product is divided by the postductal PaO2 (see below). An OI of 40 typically prompts consideration of extracorporeal membrane oxygenation (ECMO) support.

OI = (FiO× MAP × 100) / PaO2, where MAP is the mean arterial pressure.

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