Evaluation of the physiological properties of ventilatory ratio in a computational cardiopulmonary model and its clinical application in an acute respiratory distress syndrome population.
BACKGROUND
Owing to complexities of measuring dead space, ventilatory failure is difficult to quantify in critical care. A simple, novel index called ventilatory ratio (VR) can quantify ventilatory efficiency at the bedside. The study objectives were to evaluate physiological properties of VR and examine its clinical applicability in acute respiratory distress syndrome (ARDS) patients.


METHODS
A validated computational model of cardiopulmonary physiology [Nottingham Physiology Simulator (NPS)] was used to evaluate VR ex vivo in three virtual patients with varying degrees of gas exchange defects. Arterial P(CO₂) and mixed expired P(CO₂) were obtained from the simulator while either dead space or CO₂ production was altered in isolation. VR and deadspace fraction was calculated using these values. A retrospective analysis of a previously presented prospective ARDS database was then used to evaluate the clinical utility of VR. Basic characteristics of VR and its association with mortality were examined.


RESULTS
The NPS showed that VR behaved in an intuitive manner as would be predicted by its physiological properties. When CO₂ production was constant, there was strong positive correlation between dead space and VR (modified Pearson's r 0.98, P<0.01). The ARDS database had a mean VR of 1.47 (standard deviation 0.58). Non-survivors had a significantly higher VR compared with survivors [1.70 vs 1.34, mean difference 0.35, 95% confidence interval (CI) 0.16-0.56, P<0.01]. VR was an independent predictor of mortality (odds ratio 3.05, CI 1.35-6.91, P<0.01).


CONCLUSIONS
VR is influenced by dead space and CO₂ production. In ARDS, high VR was associated with increased mortality.
Magill Department of Anaesthesia, Intensive Care Medicine and Pain Management Chelsea and Westminster Hospital, 369 Fulham Road, London, UK.
P
P
Sinha
S
S
Singh
J G
JG
Hardman
A D
AD
Bersten
N
N
Soni
D000208:Acute Disease; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002245:Carbon Dioxide / Q000378:metabolism*; D003198:Computer Simulation; D006801:Humans; D008875:Middle Aged; D012126:Respiratory Dead Space; D012128:Respiratory Distress Syndrome / Q000503:physiopathology*; D012189:Retrospective Studies
D002245:Carbon Dioxide
ARDS; dead space; definition; ventilatory failure