Oxygen transport and doubts about PEEP
Three factors in particular govern the transport of oxygen to the tissues, namely cardiac output (probably the most important), the hemoglobin content of the blood and hemoglobin saturation, as expressed in the formulas1
oxygen transport (TO2)=cardiac output×arterial oxygen contentarterial oxygen contents = hemoglobinconcentration percent × saturation × 1.34 +arterial oxygen tension × 0.003
These three factors may be termed the “coarse adjustment” for oxygen transport and delivery, as opposed to “fine adjustments” such as pH, temperature, 2, 3 dpg and individual variations in the oxyhemoglobin dissociation curve, though these may be significant factors in disease. There is a linear relationship between oxygen transport and cardiac output (Fig. 1), and its range is not as restricted by physiologic limits as are hemoglobin concentration and saturation.
Severe impairment of oxygen transport is commonly a major problem in patients with lung failure after cardiac surgery. Positive end-expiratory pressure (PEEP) has become an established technique for improving arterial oxygenation in this situation and in similar conditions that belong to the family of lesions termed the adult respiratory distress syndrome (ARDS). But despite current enthusiasm for PEEP and its various degrees and a widespread belief in its therapeutic value, there have been few controlled studies and there is little or no evidence that it is ultimately beneficial in terms of morbidity and mortality in ARDS, however dramatically it improves a low arterial oxygen tension (PaO2), and even though it can delay a fatal outcome.2, 3 The apparent . . .