Response: Commentary: Arterial blood gases in SCUBA divers at depth

In his commentary (Arieli, 2025), Dr. Arieli has pointed out that in a series of arterial blood gas measurements in divers at 15 and 42 meters of freshwater (mfw), using a prediction method based on constant arterial:alveolar (a:A) PO2 ratio (Moon et al., 1987), the predicted arterial PO2 at the greater depth was inaccurate (Paganini et al., 2024). Indeed, at predicted PO2 values in the range of 650-750 mmHg measured values were significantly lower: 427-511 mmHg. He suggests that this might be due to augmented tidal changes in perfusion as a result of the breathing cycle and greater gas density. Arieli has proposed this plausible mechanism to explain why arterial PO2 might be higher than expected during diving (Arieli, 1992). He notes that this is supported by the observations of Weaver (Weaver and Howe, 1992), where arterial PO2 was higher than predicted by the constant a:A ratio. However, in our case the measured PO2 was lower than predicted. All of the measurements in this study were obtained at 1 ATA, with predicted arterial PO2's close to atmospheric pressure (around 760 mmHg at sea level). Accurate calibration of any blood gas analyzer requires a two-point calibration below and above the unknown blood sample. During this experiment the measurements were obtained at 1 ATA, where a high point calibration is not possible for a PO2 close to or exceeding ambient pressure minus water vapor pressure (around 713 mmHg). Similarly, in Weaver's report of PO2 values in volunteers in a hyperbaric chamber measured at 1 ATA, for which an empirical correction algorithm was developed (Weaver and Howe, 1992). On the other hand, the hyperbaric measurements reported by Moon et al. up to PO2 values close to 1,700 mmHg (Moon et al., 1987) were obtained using an arterial blood gas machine calibrated inside the hyperbaric chamber. Additionally, blood samples with high PO2 are inordinately predisposed to erroneously low measurements due to diffusion of oxygen from the sample into small gas bubbles that may be present in the syringe, as may have been the case in the highest values reported by Moon (Moon et al., 1987).We believe therefore that the measurements at 42 mfw were artifactually low due to the challenge of accurate calibration of the PO2 electrode.