Journal of Applied Physiology AJP: Endocrinology and Metabolism
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J Appl Physiol 104: 1838-1839, 2008; doi:10.1152/japplphysiol.00154.2008b
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POINT-COUNTERPOINT

Rebuttal by Ebenroth

I must admit that Drs. Liske and Aschner give a concise, yet complete, summary of the physiology of single ventricle hearts and the problems and pitfalls associated with their management. I also have to agree with their closing remarks that "surgery is the optimal means of reducing blood flow in the single ventricle heart." Unfortunately, this does not adequately support their counterpoint, since their assigned task was to prove that hypoxia is not the optimal means of reducing pulmonary blood flow in the preoperative single ventricle heart.

My esteemed antagonists claim that hypoxia leads to diminished PaO2 in the steep portion of the oxygen-hemoglobin dissociation curve, resulting in significantly diminished oxygen saturation and subsequent developmental delays. This population, however, has the advantage of being newborn infants. Newborn infants possess a large percentage of residual fetal hemoglobin; up to 80% in some studies (1). This causes a shift of the oxygen-hemoglobin dissociation curve to the left, allowing for improved oxygen uptake at lower PaO2 (4), thus mitigating the potential for lower oxygen transport and subsequent developmental delay.

By their own admission, they use a hypoxic environment in their institution to manage recalcitrant patients at risk of hemodynamic decompensation. Hypercarbia requires mechanical ventilation to handle the associated elevated PCO2 and can create an acidotic milieu that patients with hypoplastic left heart syndrome will not tolerate. Nitrogen is superior to hypercarbia in that it does not alter pH and does not require mechanical ventilation (2). The added benefits of remaining extubated and bonding with one's parents should not be underestimated. Research has shown that infants who are given adequate maternal contact early after birth have better psychomotor development, especially in gross motor and language skills, than infants that experience a paucity of maternal contact (3).

I contend that hypoxia, accomplished by the addition of nitrogen, either through the ventilatory circuit of intubated patients, or via nasal cannula in spontaneously breathing patients, is the best way to reduce pulmonary blood flow in the pre-operative single ventricle heart.

REFERENCES

  1. Cook CD, Brodie HR, Allen DW. Measurement of fetal hemoglobin in newborn infants; correlation with gestational age and intrauterine hypoxia. Pediatrics 20: 272–278, 1957.[Abstract/Free Full Text]
  2. Day RW, Barton AJ, Pysher TJ, Shaddy RE. Pulmonary vascular resistance of children treated with nitrogen during early infancy. Ann Thorac Surg 65: 1400–1404, 1998.[Abstract/Free Full Text]
  3. De Chateau P. The interaction between the infant and the environment: the importance of mother-child contact after delivery. Acta Paediatr Scand Suppl 344: 21–30, 1988.[Medline]
  4. Vidal Melo M. Clinical respiratory physiology of the neonate and infant with congenital heart disease. Int Anesthesiol Clin 42: 29–43, 2004.[CrossRef][Medline]




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