Phosphate metabolism in intensive care patients with acute respiratory failure.

OBJECTIVE To determine the variables predicting the change of plasma phosphate over the first 24 hr period in intensive care in patients with acute respiratory failure. METHODS Fifty-seven patients were studied prospectively in a university teaching hospital intensive care unit (ICU). Thirty two patients were classified as having acute respiratory failure and a primary respiratory system diagnosis (group I), 10 were classified as having acute cardiogenic pulmonary oedema (group II) and 15 were general ICU patients (group III). Arterial blood specimens at intensive care unit admission (T0) and at 24 hr post-admission (T24) were assayed for multiple plasma biochemical parameters including phosphate (PO4) and red blood cell 2,3-diphosphoglycerate (2,3-DPG). Timed urine collections were used to determine 24 hr urine phosphate loss and renal phosphate threshold concentration (RTP). During the measurement period glucose-free fluids only were infused. RESULTS Fifty seven patients had a mean (+/- SD) age of 67 +/- 12 years and Apache II score of 22 +/- 6. The plasma PO4 at T0 was 1.55 +/- 0.71 mmol/L and showed a significant 24 hr decrease of 0.55 mmol/L (p < 0.0001) at T24. Hypophosphataemia at T0 was observed in 26% of patients. Red blood cell 2,3-DPG was not elevated at T0 (13.5 +/- 3.3 umol/gHb) and showed a non-significant increment over 24 hr. Urine phosphate loss over the 24 hr period was 21.8 +/- 14.0 mmol with RTP being reduced below the lower reference range limit in groups I (0.65 +/- 0.29 mmol/L) and II (0.57 +/- 0.29 mmol/L). The naive form of phosphate change (PO4T24-PO4T0) was significantly related to initial plasma PO4 and was subject to regression to the mean, which was estimated to have inflated the relationship by 25%. The appropriate form of phosphate change was found to be log ratio T24/T0 phosphate. Independent predictors of log ratio T24/T0 phosphate were 24 hr change (T24-T0) in both 2,3-DPG and arterial pH, RTP, prescription of aminophylline (categorical factor) and the interaction of aminophylline and RTP (R2 = 0.65, ordinary least squares regression). CONCLUSIONS Twenty-four hour plasma phosphate decrement in intensive care unit patients was multi-factorial and was attended by a lowered renal threshold phosphate concentration.