Intrathoracic blood volume is not a sensitive indicator for hypervolaemia in healthy, mechanically ventilated pigs

Objectives

Intrathoracic blood volume (ITBV) measured by the transpulmonary technique has been suggested as an alternative parameter predicting cardiac preload in critically ill patients [1]. So far, it has never been shown that by measuring ITBV it is possible to detect hypervolaemia.

Design

Prospective animal laboratory study.

Setting

University animal laboratory.

Measurements

Healthy, anaesthetized, and mechanically ventilated pigs (n = 4) with a median weight of 24.5 kg (23.3–26.5 kg) were investigated. A pulmonary artery catheter (831HF75, 7.5 F Edwards Lifesciences Ltd) inserted via the right jugular vein and an arterial catheter for transpulmonary thermodilution (4 F PV 2014L16, Pulsion Medical Systems) was inserted into the right carotid artery. Measurements of haemodynamic parameters included heart rate (HR), mean arterial pressure (MAP), cardiac output (CO), pulmonary arterial occlusion pressure (PAOP), central venous pressure (CVP), extravascular lung water (EVLW) and ITBV. After baseline measurement, hypervolaemia was induced by stepwise (10% per step) cumulative administration of 50% of the estimated blood volume of the animals (80 ml/kg) using 6% hydroxyethylstarch 200/0.5. The interval between each of the five steps was 10 min. Haemodynamic parameters, EVLW and ITBV before and after hypervolaemia were compared by Friedman's test. Data are presented as the median and interquartile range. P < 0.05 was considered significant.

Results

After establishment of hypervolaemia the following parameters increased significantly: HR from 85 beats/min (68–103) to 130 beats/min (124–134), MAP from 98 mmHg (96–101) to 116 mmHg (107–128), CO from 120 ml/min per kg (100–138) to 190 ml/min per kg (156–233), EVLW from 3.9 ml/kg (3.5–8.5) to 19.0 ml/kg (14.8–24.2), CVP from 9 mmHg (8–9) to 27 mmHg (25–30), and PAOP from 10 mmHg (10–11) to 27 mmHg (25–27), whereas ITBV did not change significantly from 21.9 ml/kg (19.0–24.7) to 23.1 ml/kg (20.8–26.3).

Conclusion

Intrathoracic blood volume is not a sensitive indicator for hypervolaemia in healthy, mechanically ventilated pigs in the presence of severely increased EVLW.