Impact of cardiac output and alveolar ventilation in estimating ventilation/perfusion mismatch in ARDS using electrical impedance tomography.

INTRODUCTION: Electrical impedance tomography (EIT) can be used to assess ventilation/perfusion (V/Q) mismatch within the lungs. Several methods have been proposed, some of them neglecting the absolute value of alveolar ventilation (VA) and cardiac output (QC). Whether this omission results in acceptable bias is unknown. METHODS: Pixel-level V/Q maps of 25 ARDS patients were computed once considering (absolute V/Q map) and once neglecting (relative V/Q map) the value of QC and VA. Previously published indices of V/Q mismatch were computed using absolute V/Q maps and relative V/Q maps. Indices computed with relative V/Q maps were compared to their counterparts computed using absolute V/Q maps. RESULTS: Among 21 patients with ratio of alveolar ventilation to cardiac output (VA/QC) > 1, relative shunt fraction was significantly higher than absolute shunt fraction [37% (24-66) vs 19% (11-46), respectively, p < 0.001], while relative dead space fraction was significantly lower than absolute dead space fraction [40% (22-49) vs 58% (46-84), respectively, p < 0.001]. Relative wasted ventilation was significantly lower than the absolute wasted ventilation [16% (11-27) vs 29% (19-35), respectively, p < 0.001], while relative wasted perfusion was significantly higher than absolute wasted perfusion [18% (11-23) vs 11% (7-19), respectively, p < 0.001]. The opposite findings were retrieved in the four patients with VA/QC < 1. CONCLUSION: Neglecting cardiac output and alveolar ventilation when assessing V/Q mismatch indices using EIT in ARDS patients results in significant bias, whose direction depends on the VA/QC ratio value.

Potential of Multiplex Polymerase Chain Reaction Performed on Protected Telescope Catheter Samples for Early Adaptation of Antimicrobial Therapy in ARDS Patients.

BACKGROUND: Diagnosis of co/superinfection in patients with Acute Respiratory Distress Syndrome (ARDS) is challenging. The FilmArray Pneumonia plus Panel (bioMérieux, France), a new rapid multiplex Polymerase Chain Reaction (mPCR), has never been assessed on a blinded protected telescope catheter (PTC) samples, a very common diagnostic tool in patients under mechanical ventilation. We evaluated the performance of mPCR on PTC samples compared with conventional culture and its impact on antibiotic stewardship. METHODS: Observational study in two intensive care units, conducted between March and July 2020, during the first wave of the COVID-19 pandemic in France. RESULTS: We performed 125 mPCR on blinded PTC samples of 95 ARDS patients, including 73 (77%) SARS-CoV-2 cases and 28 (29%) requiring extracorporeal membrane oxygenation. Respiratory samples were drawn from mechanically ventilated patients either just after intubation (n = 48; 38%) or later for suspected ventilator-associated pneumonia (VAP) (n = 77; 62%). The sensitivity, specificity, positive, and negative predictive values of mPCR were 93% (95% CI 84-100), 99% (95% CI 99-100), 68% (95% CI 54-83), and 100% (95% CI 100-100), respectively. The overall coefficient of agreement between mPCR and standard culture was 0.80 (95% CI 0.68-0.89). Intensivists changed empirical antimicrobial therapy in only 14% (18/125) of cases. No new antibiotic was initiated in more than half of the CAP/HAP pneumonia-suspected cases (n = 29; 60%) and in more than one-third of those suspected to have VAP without affecting or delaying their antimicrobial therapy. CONCLUSIONS: Rapid mPCR was feasible on blinded PTC with good sensitivity and specificity. New antibiotics were not initiated in more than half of patients and more than one-third of VAP-suspected cases. Further studies are needed to assess mPCR potential in improving antibiotic stewardship.