Ventilatory ratio, dead space, and venous admixture in patients with acute respiratory distress syndrome.

BACKGROUND: Ventilatory ratio (VR) has been proposed as an alternative approach to estimate physiological dead space. However, the absolute value of VR, at constant dead space, might be affected by venous admixture and CO2 volume expired per minute (VCO2). METHODS: This was a retrospective, observational study of mechanically ventilated patients with acute respiratory distress syndrome (ARDS) in the UK and Italy. Venous admixture was either directly measured or estimated using the surrogate measure PaO2/FiO2 ratio. VCO2 was estimated through the resting energy expenditure derived from the Harris-Benedict formula. RESULTS: A total of 641 mechanically ventilated patients with mild (n=65), moderate (n=363), or severe (n=213) ARDS were studied. Venous admixture was measured (n=153 patients) or estimated using the PaO2/FiO2 ratio (n=448). The VR increased exponentially as a function of the dead space, and the absolute values of this relationship were a function of VCO2. At a physiological dead space of 0.6, VR was 1.1, 1.4, and 1.7 in patients with VCO2 equal to 200, 250, and 300, respectively. VR was independently associated with mortality (odds ratio [OR]=2.5; 95% confidence interval [CI], 1.8-3.5), but was not associated when adjusted for VD/VTphys, VCO2, PaO2/FiO2 (ORadj=1.2; 95% CI, 0.7-2.1). These three variables remained independent predictors of ICU mortality (VD/VTphys [ORadj=17.9; 95% CI, 1.8-185; P<0.05]; VCO2 [ORadj=0.99; 95% CI, 0.99-1.00; P<0.001]; and PaO2/FiO2 (ORadj=0.99; 95% CI, 0.99-1.00; P<0.001]). CONCLUSIONS: VR is a useful aggregate variable associated with outcome, but variables not associated with ventilation (VCO2 and venous admixture) strongly contribute to the high values of VR seen in patients with severe illness.

Anatomical and functional intrapulmonary shunt in acute respiratory distress syndrome.

OBJECTIVES: The lung-protective strategy employs positive end-expiratory pressure to keep open otherwise collapsed lung regions (anatomical recruitment). Improvement in venous admixture with positive end-expiratory pressure indicates functional recruitment to better gas exchange, which is not necessarily related to anatomical recruitment, because of possible global/regional perfusion modifications. Therefore, we aimed to assess the value of venous admixture (functional shunt) in estimating the fraction of nonaerated lung tissue (anatomical shunt compartment) and to describe their relationship. DESIGN: Retrospective analysis of a previously published study. SETTING: Intensive care units of four university hospitals. PATIENTS: Fifty-nine patients with acute lung injury/acute respiratory distress syndrome. INTERVENTIONS: Positive end-expiratory pressure trial at 5 and 15 cm H2O positive end-expiratory pressures. MEASUREMENTS AND MAIN RESULTS: Anatomical shunt compartment (whole-lung computed tomography scan) and functional shunt (blood gas analysis) were assessed at 5 and 15 cm H2O positive end-expiratory pressures. Apparent perfusion ratio (perfusion per gram of nonaerated tissue/perfusion per gram of total lung tissue) was defined as the ratio of functional shunt to anatomical shunt compartment. Functional shunt was poorly correlated to the anatomical shunt compartment (r2 = .174). The apparent perfusion ratio at 5 cm H2O positive end-expiratory pressure was widely distributed and averaged 1.25 +/- 0.80. The apparent perfusion ratios at 5 and 15 cm H2O positive end-expiratory pressures were highly correlated, with a slope close to identity (y = 1.10.x -0.03, r2 = .759), suggesting unchanged blood flow distribution toward the nonaerated lung tissue, when increasing positive end-expiratory pressure. CONCLUSIONS: Functional shunt poorly estimates the anatomical shunt compartment, due to the large variability in apparent perfusion ratio. Changes in anatomical shunt compartment with increasing positive end-expiratory pressure, in each individual patient, may be estimated from changes in functional shunt, only if the anatomical-functional shunt relationship at 5 cm H2O positive end-expiratory pressure is known.

The out-of-hospital esophageal and endobronchial intubations performed by emergency physicians.

BACKGROUND: Rapid establishment of a patent airway in ill or injured patients is a priority for prehospital rescue personnel. Out-of-hospital tracheal intubation can be challenging. Unrecognized esophageal intubation is a clinical disaster. METHODS: We performed an observational, prospective study of consecutive patients requiring transport by air and out-of-hospital tracheal intubation, performed by primary emergency physicians to quantify the number of unrecognized esophageal and endobronchial intubations. Tracheal tube placement was verified on scene by a study physician using a combination of direct visualization, end-tidal carbon dioxide detection, esophageal detection device, and physical examination. RESULTS: During the 5-yr study period 149 consecutive out-of-hospital tracheal intubations were performed by primary emergency physicians and subsequently evaluated by the study physicians. The mean patient age was 57.0 (+/-22.7) yr and 99 patients (66.4%) were men. The tracheal tube was determined by the study physician to have been placed in the right mainstem bronchus or esophagus in 16 (10.7%) and 10 (6.7%) patients, respectively. All esophageal intubations were detected and corrected by the study physician at the scene, but 7 of these 10 patients died within the first 24 h of treatment. CONCLUSION: The incidence of unrecognized esophageal intubation is frequent and is associated with a high mortality rate. Esophageal intubation can be detected with end-tidal carbon dioxide monitoring and an esophageal detection device. Out-of-hospital care providers should receive continuing training in airway management, and should be provided additional confirmatory adjuncts to aid in the determination of tracheal tube placement.