Measurement of skeletal muscle tissue oxygenation in the critically ill

Shock is a state of acutely reduced tissue oxygenation. In cardiogenic shock oxygen delivery (DO2) is reduced, but oxygen extraction is preserved. In septic shock DO2 is preserved, but oxygen extraction is decreased because of microvascular changes and disturbed metabolism. Global assessment of DO2 and oxygen consumption does not tell us enough about adequacy of regional perfusion. The aim of this study was to assess the value of near infrared spectroscopy (NIRS) in detecting skeletal muscle tissue oxygenation (StO2) in critically ill patients. Patients in cardiogenic shock (n=17), septic shock (n=14), without shock but with localized infection (n=14) and healthy volunteers (n=15) were included. Thenar StO2 was measured with NIRS before (baseline StO2, %), between (downward StO2 slope, %/min) and after 90 seconds of upper arm stagnant ischemia (hyperemic StO2, %). Muscle oxygen extraction (mOER) was calculated as follows: mOER (%) = (1-baselineStO2/hyperemic StO2)*100. Repeatability was assessed using the Bland Altman method (95 % of values within limits of agreement), comparing 55 pairs of measurements performed in 5-minute intervals.Repeatability of measurements was clinically acceptable. Compared to septic shock patients, cardiogenic shock patients had lower baseline StO2 (68.9 ± 10.0 % vs. 84.3 ± 10.4 %; p < 0.05) and hyperemic StO2 (80.8 ± 7.8 % vs. 91.8 ± 8.3 %; p < 0.05), and a higher downward StO2 slope (-17.4 ± 31.7 %/min vs. -9.1 ± 2.6 %/min; p < 0.05). mOER was higher in healthy volunteers (11.9 ± 3.8 %) and volunteers with cardiogenic shock (14.8 ± 7.3 %) compared to septic shock patients (8.1 ± 7.8 %) and those with localized infection (7.6 ± 5.4 %) (p < 0.05).Repeatability of baseline StO2 and hyperemic StO2 is clinically acceptable. Results support the hypothesis that skeletal muscle oxygen extraction capability is preserved and extraction is increased in cardiogenic shock compared to septic shock.