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POINT-COUNTERPOINT
Our opponent referenced the theoretical analysis of Barnea et al. (1). Despite touting an "ideal" blood gas to achieve maximal DO2, their model could not demonstrate how or if manipulation of FIO2 or FiCO2 would alter the Qp:Qs in the whole organism.
A deeper analysis of Reddy et al.'s (6) sheep data indicates that although the hypoxic gas mixture improved the Qp:Qs ratio, DO2 fell by 38%. Granted, they used an FiO2 of 0.10, which would be considered extreme in the treatment of humans. Importantly, 5% CO2 improved Qp:Qs and increased DO2, similar to Tabutt et al.'s (7) findings in humans.
Some of Dr. Ebenroth's references pertain to the postoperative single ventricle heart, in which Qp is limited by a restrictive shunt (3, 8). We do not feel those studies apply to this discussion.
Our opponent suggests that hypoxia allows recapitulation of stable fetal physiology in the newborn. Unfortunately, we have not yet been able to replicate the in utero environment postnatally, and it is unlikely that the transitioning newborn undergoing fluctuations in PVR can achieve stability comparable to the fetus in whom gas exchange is performed by the placenta.
"Hypoxia is well tolerated in the newborn." We counter that the long-term effects of postnatal hypoxia are poorly understood. In fact, recent data suggest that postnatal hypoxia, as seen in the preoperative patient with transposition of the great arteries, is associated with brain injury (periventricular leukomalacia) (5). Conversely, hypercapnic acidosis has been shown to have cerebral and pulmonary protective effects in term and preterm infants at risk for hypoxic ischemic encephalopathy and chronic lung disease (2, 4, 9).
We concede the point that hypoxic gas therapy is simpler to administer than hypercarbia, and cautiously advocate its role in the minimally symptomatic neonate in whom surgery is not readily available, with concomitant cerebral near infrared spectroscopy monitoring to assure appropriate DO2. We maintain, however, for infants who present critically ill in shock, hypercarbia is the preferred therapy because it not only limits pulmonary flow, but also augments DO2.
There is only one real sin and that is to persuade oneself that the second best is anything but second best.
-Doris Lessing
REFERENCES
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