Bedside calculation of mechanical power during volume- and pressure-controlled mechanical ventilation.

BACKGROUND: Mechanical power (MP) is the energy delivered to the respiratory system over time during mechanical ventilation. Our aim was to compare the currently available methods to calculate MP during volume- and pressure-controlled ventilation, comparing different equations with the geometric reference method, to understand whether the easier to use surrogate formulas were suitable for the everyday clinical practice. This would warrant a more widespread use of mechanical power to promote lung protection. METHODS: Forty respiratory failure patients, sedated and paralyzed for clinical reasons, were ventilated in volume-controlled ventilation, at two inspiratory flows (30 and 60 L/min), and pressure-controlled ventilation with a similar tidal volume. Mechanical power was computed both with the geometric method, as the area between the inspiratory limb of the airway pressure and the volume, and with two algebraic methods, a comprehensive and a surrogate formula. RESULTS: The bias between the MP computed by the geometric method and by the comprehensive algebraic method during volume-controlled ventilation was respectively 0.053 (0.77, - 0.81) J/min and - 0.4 (0.70, - 1.50) J/min at low and high flows (r2 = 0.96 and 0.97, p < 0.01). The MP measured and computed by the two methods were highly correlated (r2 = 0.95 and 0.94, p < 0.01) with a bias of - 0.0074 (0.91, - 0.93) and - 1.0 (0.45, - 2.52) J/min at high-low flows. During pressure-controlled ventilation, the bias between the MP measured and the one calculated with the comprehensive and simplified methods was correlated (r2 = 0.81, 0.94, p < 0.01) with mean differences of - 0.001 (2.05, - 2.05) and - 0.81 (2.11, - 0.48) J/min. CONCLUSIONS: Both for volume-controlled and pressure-controlled ventilation, the surrogate formulas approximate the reference method well enough to warrant their use in the everyday clinical practice. Given that these formulas require nothing more than the variables already displayed by the intensive care ventilator, a more widespread use of mechanical power should be encouraged to promote lung protection against ventilator-induced lung injury.

Oesophageal pressure and respiratory muscle ultrasonographic measurements indicate inspiratory effort during pressure support ventilation.

BACKGROUND: Bedside measures of patient effort are essential to properly titrate the level of pressure support ventilation. We investigated whether the tidal swing in oesophageal (ΔPes) and transdiaphragmatic pressure (ΔPdi), and ultrasonographic changes in diaphragm (TFdi) and parasternal intercostal (TFic) thickening are reliable estimates of respiratory effort. The effect of diaphragm dysfunction was also considered. METHODS: Twenty-one critically ill patients were enrolled: age 73 (14) yr, BMI 27 (7) kg m-2, and Pao2/Fio2 33.3 (9.2) kPa. A three-level pressure support trial was performed: baseline, 25% (PS-medium), and 50% reduction (PS-low). We recorded the oesophageal and transdiaphragmatic pressure-time products (PTPs), work of breathing (WOB), and diaphragm and intercostal ultrasonography. Diaphragm dysfunction was defined by the Gilbert index. RESULTS: Pressure support was 9.0 (1.6) cm H2O at baseline, 6.7 (1.3) (PS-medium), and 4.4 (1.0) (PS-low). ΔPes was significantly associated with the oesophageal PTP (R2=0.868; P<0.001) and the WOB (R2=0.683; P<0.001). ΔPdi was significantly associated with the transdiaphragmatic PTP (R2=0.820; P<0.001). TFdi was only weakly correlated with the oesophageal PTP (R2=0.326; P<0.001), and the correlation improved after excluding patients with diaphragm dysfunction (R2=0.887; P<0.001). TFdi was higher and TFic lower in patients without diaphragm dysfunction: 33.6 (18.2)% vs 13.2 (9.2)% and 2.1 (1.7)% vs 12.7 (9.1)%; P<0.0001. CONCLUSIONS: ΔPes and ΔPdi are adequate estimates of inspiratory effort. Diaphragm ultrasonography is a reliable indicator of inspiratory effort in the absence of diaphragm dysfunction. Additional measurement of parasternal intercostal thickening may discriminate a low inspiratory effort or a high effort in the presence of a dysfunctional diaphragm.

Mechanical Power and Development of Ventilator-induced Lung Injury.

BACKGROUND: The ventilator works mechanically on the lung parenchyma. The authors set out to obtain the proof of concept that ventilator-induced lung injury (VILI) depends on the mechanical power applied to the lung. METHODS: Mechanical power was defined as the function of transpulmonary pressure, tidal volume (TV), and respiratory rate. Three piglets were ventilated with a mechanical power known to be lethal (TV, 38 ml/kg; plateau pressure, 27 cm H2O; and respiratory rate, 15 breaths/min). Other groups (three piglets each) were ventilated with the same TV per kilogram and transpulmonary pressure but at the respiratory rates of 12, 9, 6, and 3 breaths/min. The authors identified a mechanical power threshold for VILI and did nine additional experiments at the respiratory rate of 35 breaths/min and mechanical power below (TV 11 ml/kg) and above (TV 22 ml/kg) the threshold. RESULTS: In the 15 experiments to detect the threshold for VILI, up to a mechanical power of approximately 12 J/min (respiratory rate, 9 breaths/min), the computed tomography scans showed mostly isolated densities, whereas at the mechanical power above approximately 12 J/min, all piglets developed whole-lung edema. In the nine confirmatory experiments, the five piglets ventilated above the power threshold developed VILI, but the four piglets ventilated below did not. By grouping all 24 piglets, the authors found a significant relationship between the mechanical power applied to the lung and the increase in lung weight (r = 0.41, P = 0.001) and lung elastance (r = 0.33, P < 0.01) and decrease in PaO2/FIO2 (r = 0.40, P < 0.001) at the end of the study. CONCLUSION: In piglets, VILI develops if a mechanical power threshold is exceeded.

Inflammation and primary graft dysfunction after lung transplantation: CT-PET findings.

BACKGROUND: The leading cause of early mortality after lung transplantation is Primary graft dysfunction (PGD). We assessed the lung inflammation, inflation status and inhomogeneities after lung transplantation. Our purpose was to investigate the possible differences between patients who did or did not develop PGD. METHODS: We designed a prospective observational study enrolling patients who underwent a CT-PET study within 1 week after lung transplantation. Twenty-four patients (10 after double- and 14 after single-lung) were enrolled. Respiratory and hemodynamic data were collected before, during and after lung transplantation. Each patient underwent computed tomography-positron emission tomography (CT-PET) scan early after surgery. Broncho-alveolar lavage (BAL) fluid collection was performed to analyze inflammatory mediators. RESULTS: The grafts showed a [18F]fluoro-2-deoxy-D-glucose ([18F]FDG) uptake rate of 26[18-33]*10-4 mLblood/mLtissue/min (reference values 11[7-15]*10-4). Three double- and six single-lung recipients developed PGD. The grafts of patients who developed PGD had similar [18F]FDG uptake than grafts of patients who did not (28[18-26]*10-4 versus 26[22-31]*10-4, P=0.79). Not-inflated tissue fraction was significantly higher (28[20-38]% versus 14[7-21]%, P=0.01) while well-inflated fraction was significantly lower (29[25-41]% versus 53[39-65]%, P<0.01). Inhomogeneity extent was higher in patients who developed PGD (23[18-26]% versus 14[10-20]%, P=0.01)The lung weight was 650[591-820]g versus 597[480-650]g (P=0.09)). BAL fluid analysis for inflammatory mediators did not detect a difference between the study groups. CONCLUSIONS: Compared to healthy lungs, all the grafts showed increased [18F]FDG uptake rate, but there were no differences between patients who developed PGD and patients who did not. Of note, the PGD patients showed a worse inflation status of lungs and a higher inhomogeneity extent.

Opening pressures and atelectrauma in acute respiratory distress syndrome.

PURPOSE: Open lung strategy during ARDS aims to decrease the ventilator-induced lung injury by minimizing the atelectrauma and stress/strain maldistribution. We aim to assess how much of the lung is opened and kept open within the limits of mechanical ventilation considered safe (i.e., plateau pressure 30 cmH2O, PEEP 15 cmH2O). METHODS: Prospective study from two university hospitals. Thirty-three ARDS patients (5 mild, 10 moderate, 9 severe without extracorporeal support, ECMO, and 9 severe with it) underwent two low-dose end-expiratory CT scans at PEEP 5 and 15 cmH2O and four end-inspiratory CT scans (from 19 to 40 cmH2O). Recruitment was defined as the fraction of lung tissue which regained inflation. The atelectrauma was estimated as the difference between the intratidal tissue collapse at 5 and 15 cmH2O PEEP. Lung ventilation inhomogeneities were estimated as the ratio of inflation between neighboring lung units. RESULTS: The lung tissue which is opened between 30 and 45 cmH2O (i.e., always closed at plateau 30 cmH2O) was 10 ± 29, 54 ± 86, 162 ± 92, and 185 ± 134 g in mild, moderate, and severe ARDS without and with ECMO, respectively (p < 0.05 mild versus severe without or with ECMO). The intratidal collapses were similar at PEEP 5 and 15 cmH2O (63 ± 26 vs 39 ± 32 g in mild ARDS, p = 0.23; 92 ± 53 vs 78 ± 142 g in moderate ARDS, p = 0.76; 110 ± 91 vs 89 ± 93, p = 0.57 in severe ARDS without ECMO; 135 ± 100 vs 104 ± 80, p = 0.32 in severe ARDS with ECMO). Increasing the applied airway pressure up to 45 cmH2O decreased the lung inhomogeneity slightly (but significantly) in mild and moderate ARDS, but not in severe ARDS. CONCLUSIONS: Data show that the prerequisites of the open lung strategy are not satisfied using PEEP up to 15 cmH2O and plateau pressure up to 30 cmH2O. For an effective open lung strategy, higher pressures are required. Therefore, risks of atelectrauma must be weighted versus risks of volutrauma. TRIAL REGISTRATION: Clinicaltrials.gov identifier: NCT01670747 ( www.clinicaltrials.gov ).