Oxygen Therapy in Cardiac Patients (Acute Coronary Syndrome)

Viewpoint:

Revisiting the role of oxygen therapy in cardiac patients

JACC 2010;56:1013-6

Rationale for oxygen therapy in ACS:

1) Increasing oxygen tension decreases ischemic injury and the infarct size

2) Some patients have hypoxia due to ventilation-perfusion mismatch with pulmonary edema

Coronary vasoconstriction has been demonstrated in a Doppler-based study by measurement of coronary flow velocity (Am J Physiol Heart Circ Physiol 2009;296:H854-61). Potential mechanisms of this coronary vasoconstriction to Hyperoxia:

1) Hyperoxia -> Production of reactive oxygen species ->Decreases bioavailability of nitric oxide -> causes vasoconstriction

2) K _ATP channels: Hypoxia → Depletion of ATP → Opening of K_ATP channel → Hyperpolarization of vacular smooth muscles → Vasodilation. The opposite happens with hyperoxia.

3) Oxygen-sensitive L-type Calcium channel in vascular smooth muscles. Hyperoxia causes vasoconstriction via these channels.

4) Hyperoxia → Angiotensin II → Endothelin-1 release → Vasoconstriction

5) Hyperoxia facilitates release of a potent vasoconstrictor 20-HETE (an archdonic acid metabolite).

6) High flow oxygen, despite increasing overall oxygen delivery, may not improve organ-specific oxygen delivery. In critically ill patients, high-flow oxygen causes a misdistribution of microcirculatory blood flow and reduce oxygen consumption by organs - heart (Reinhart et al. Reversible decrease of oxygen consumption by hyperoxia Chest 1991;99:690-4).

Conclusion:

Although use of oxygen is clearly appropriate and advisable to treat hypoxia, we - the authors - hypothesize that excessive use of supplemental oxygen in normoxic cardiac patients could potentially lead to worse outcomes. Further studies are needed to delineate the role of oxygen n these conditions.

My comment: Interesting view point. Not to be taken on its face value. Such, seemingly rational, claims have been made in the past - supported by seemingly adequate data. Many of them have proven wrong or disappeared without firm legs to stand on. Examples from the past that I have followed are (i) Vit K at birth causes cancer in later life, (ii) Milrinone will not work in neonates because the PDE isoform that is present in neonates is the kind that Milrinone does not work on (iii) Dopamine has no effect when given below 2 or 3 mcg/kg/min especially in newborn, etc.