Prognostic value of different dead space indices in mechanically ventilated patients with acute lung injury and ARDS.

STUDY OBJECTIVE: The aim of this prospective observational study was to evaluate the utility of derived dead space indexes to predict survival in mechanically ventilated patients with acute lung injury (ALI) and ARDS. STUDY POPULATION: Thirty-six patients with ALI (Murray score, > or =1; Pao(2)/fraction of inspired oxygen [Fio(2)] ratio, < 300) in critical care departments at two separate hospitals entered the study. MEASUREMENTS: At ICU admission, 24 h, and 48 h, we measured the following: simplified acute physiologic score II; Pao(2)/Fio(2) ratio; respiratory system compliance; and capnographic indexes (Bohr dead space) and physiologic dead space (Enghoff dead space [Vdphys/Vt]), expired normalized CO(2) slope, carbon dioxide output, and the alveolar ejection volume (Vae)/tidal volume fraction (Vt) ratio. RESULTS: The best predictor was the Vae/Vt ratio at ICU admission (Vae/Vt-adm) and after 48 h (Vae/Vt-48 h) [p = 0.013], with a sensitivity of 82% and a specificity of 64%. The difference between Vae/Vt-48 h and Vae/Vt-adm show a sensitivity of 73% and a specificity of 93% with a likelihood ratio (LR) of 10.2 and an area under the receiver operating characteristic (ROC) curve of 0.83. The interaction between the Pao(2)/Fio(2) ratio and Vae/Vt-adm predict survival (p = 0.003) with an area under the ROC curve of 0.84, an LR of 2.3, a sensitivity of 100%, and a specificity of 57%. The Vdphys/Vt after 48 h predicted survival (p = 0.02) with an area under the ROC curve of 0.75, an LR of 8.8, a sensitivity of 63%, and a specificity of 93%. Indexes recorded 24 h after ICU admission were not useful in explaining outcome. CONCLUSIONS: Noninvasive measures of Vae/Vt at ICU admission and after 48 h of mechanical ventilation, associated with Pao(2)/Fio(2) ratio provided useful information on outcome in critically ill patients with ALI.

Evaluation of a noninvasive method for cardiac output measurement in critical care patients.

OBJECTIVE: Thermodilution (TD) is the gold standard to monitor cardiac output (CO) in critical care. However, there is concern about the safety of right-ventricular catheterization. The CO(2) rebreathing technique allows noninvasive CO determination by means of the indirect Fick principle. Our objectives were: (a) to assess the accuracy of a new system of CO measurement using the CO(2) partial rebreathing method (PRCO); (b) to evaluate whether the PRCO itself may induce changes in CO. DESIGN AND SETTING: Prospective study in the intensive care department in a university-affiliated hospital. PATIENTS: Twenty-two mechanically ventilated critically ill patients. INTERVENTIONS: CO measured simultaneously by PRCO and TDCO. MEASUREMENTS AND RESULTS: PRCO and TDCO values were compared by concordance analysis. Stability of cardiac output during PRCO was evaluated by comparing the TDCO measurements before, during, and after the partial rebreathing period using analysis of variance. From a total of 79 valid sets of measurements, bias and precision was calculated at -0.18+/-1.39 l/min. The concordance analysis of lower and intermediate CO values (<7 l/min) yielded a bias and precision calculation of -0.07+/-0.91 l/min. No changes in hemodynamics were observed during the partial rebreathing period. CONCLUSIONS: The noninvasive partial CO(2) rebreathing technique may be an alternative method for CO determination in mechanically ventilated critically ill patients. The rebreathing maneuver alone does not induce changes in CO.

Recruitment maneuvers during lung protective ventilation in acute respiratory distress syndrome.

The objective was to analyze the physiologic effects of recruitment maneuvers (RM) in 17 patients with acute respiratory distress syndrome (ARDS) ventilated with a lung protective strategy. RM consisted of 2 min of pressure-controlled ventilation at a peak pressure of 50 cm H(2)O and a positive end-expiratory pressure (PEEP) above the upper inflection point of the respiratory pressure-volume curve obtained at zero PEEP. In eight patients, RM were repeated in the late phase of ARDS. Oxygenation did not change 15 min after RM in the early and late phase of ARDS. When Pa(O(2))/fraction of inspired oxygen (FI(O(2))) increased during RM, venous admixture (Q VA/Q T) decreased. The opposite occurred in patients in whom Pa(O(2))/FI(O(2)) decreased during RM. RM-induced changes in cardiac output were not observed. A significant correlation was found between RM-induced changes in Pa(O(2))/FI(O(2)) during the RM and changes in respiratory system compliance at 15 min (r = 0.66, p < 0.01) and RM-induced changes in Q VA/Q T (r = -0.85; p < 0.01). The correlation between RM-induced changes in Pa(O(2))/FI(O(2)) in responders (improvement in Pa(O(2))/FI(O(2)) of greater than 20% during the RM) and the inspired oxygen fraction was also significant. In ARDS patients ventilated with a lung protective strategy we conclude that RM have no short-term benefit on oxygenation, and regional alveolar overdistension capable of redistributing blood flow can occur during RM.