Combined gas exchange and pulse wave monitoring for detecting anaerobic metabolism in critically ill patients
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KeywordsCardiac Index Anaerobic Metabolism Pulse Contour Mixed Venous Oxygen Saturation Pulse Wave Analysis
The anaerobic metabolism causes lactate to increase. This is accompanied by a reduction in bicarbonate concentration in the blood, causing CO2 production (VCO2) to accelerate, evidenced as an increased respiratory CO2 output. Over the physiological range of CO2 content the CO2 tension is linearly related to CO2 content, and the venoarterial CO2 tension difference (DPCO2) could be used as a surrogate for the difference between mixed venous and arterial CO2 contents. Under anaerobic metabolism the respiratory quotient increases. The venoarterial CO2 tension difference/arteriovenous O2 content difference ratio [DPCO2/C(a-v)O2] therefore increases. We tested the DPCO2/C(a-v)O2 ratio and mixed venous oxygen saturation (SvO2) values as potential predictors of anaerobic metabolism.
Fifty-one consecutive adult patients (head injury, subarachnoid haemorrhage, sepsis) admitted to our ICU were prospectively studied. The DPCO2/C(a-v)O2 ratio, SvO2, oxygen delivery index (DO2I), and arterial lactate (Lac) values were collected at T1 (ICU admission) and at T2 (after 24 hours). The DO2I was calculated using the cardiac index (CI) measured by the Pressure Recording Analytical Method (PRAM). PRAM is a pulse contour system for beat-to-beat quantification of the CI. It does not need calibrating factors, and allows the stroke volume (SV) to be calculated avoiding the inaccuracies derived from instant variations of arterial impedance. Radial (80%) or femoral (20%) arteries were used for the blood pressure analysis. The presence of anaerobic metabolism (e.g. hyperlactatemia, Lac+) was defined by an increase in Lac >2 mmol/l. Linear correlations and the ROC test were applied.
For a threshold value of DO2I > 330 ml/min/m2, an inverse relationship (R2 = 0.69; P < 0.05) between Lac and SvO2 at T1 and at T2 (R2 = 0.88; P < 0.05) was found. For a DO2I < 330 ml/min/m2, good direct correlations between Lac and the DPCO2/Da-vO2 ratio calculated at T1 (R2 = 0.89; P < 0.05), were found. The SvO2 did not show any significant relationship with Lac for a DO2I < 330 ml/min/m2. ROC curves to predict Lac+ were constructed. The areas under the ROC curves (AUC) were 0.78 and 0.47 for the DPCO2/Da-vO2 ratio and SvO2, respectively. The AUC for the DPCO2/Da-vO2 ratio was significantly greater that that for SvO2 (P < 0.05). From the ROC curve an optimal cutoff value of 1.5 (sensitivity = 0.78, specificity = 0.75) was determined for the DPCO2/Da-vO2 ratio predicting the presence of Lac+.
Pulse wave analysis plays a key role in the continuous monitoring of critically ill patients. It provides beat-to-beat values of the SV, and consequently allows the DO2I to be frequently calculated. The DPCO2/Da-vO2 ratio is simple and quick to calculate and would be a valuable approach in clinical practice. Our findings showed that DPCO2/Da-vO2 ratio values are directly related to Lac. For DO2I < 330 ml/min/m2 the DPCO2/Da-vO2 ratio seems a more reliable predictor of anaerobic metabolism than SvO2. Combined gas exchange and pulse wave monitoring might be a very useful approach to detect anaerobic metabolism in ICU patients.