The effect of total parenteral nutrition and morphine on ventilation

Total parenteral nutrition (TPN), specifically amino acid infusions, has been shown to increase the ventilatory response to inhaled CO2. The hypothesis tested was that morphine sulfate (known to depress ventilatory CO2 responsiveness) would diminish the augmented ventilatory CO2 response in patients receiving TPN. The influence of morphine on hyperoxic hypercapnic ventilatory response (assessed by the Read rebreathing technique) was therefore examined in four otherwise healthy subjects who were receiving TPN at home for long-standing nutritional support secondary to malabsorption syndrome (short-bowel syndrome), and in a control group of four healthy subjects who were not receiving TPN. The slope and intercept of the CO2 response was estimated by linear regression on the relationship between ventilation (VE) and end-tidal PCO2 (PETCO2). Administration of morphine in the non-TPN group elicited the expected decrease in the VE-PETCO2 slope. In contrast, morphine administration was associated with an increase in the VE-PETCO2 slope in the TPN group. While this investigation does not provide a direct indication of the mechanisms underlying the augmenting action of morphine on the ventilatory response to CO2 in subjects receiving TPN, it does suggest that patients on TPN who demonstrate no impairment of ventilatory control may be given normal doses of morphine sulphate (ie, as for pain control or preoperative medication) with no increased concern for an adverse ventilatory outcome.