Monica Chierichetti

Lung stress and strain during mechanical ventilation: any safe threshold?

RATIONALE Unphysiologic strain (the ratio between tidal volume and functional residual capacity) and stress (the transpulmonary pressure) can cause ventilator-induced lung damage. OBJECTIVES To identify a strain-stress threshold (if any) above which ventilator-induced lung damage can occur. METHODS Twenty-nine healthy pigs were mechanically ventilated for 54 hours with a tidal volume producing a strain between 0.45 and 3.30. Ventilator-induced lung damage was defined as net increase in lung weight. MEASUREMENTS AND MAIN RESULTS Initial lung weight and functional residual capacity were measured with computed tomography. Final lung weight was measured using a balance. After setting tidal volume, data collection included respiratory system mechanics, gas exchange and hemodynamics (every 6 h); cytokine levels in serum (every 12 h) and bronchoalveolar lavage fluid (end of the experiment); and blood laboratory examination (start and end of the experiment). Two clusters of animals could be clearly identified: animals that increased their lung weight (n = 14) and those that did not (n = 15). Tidal volume was 38 ± 9 ml/kg in the former and 22 ± 8 ml/kg in the latter group, corresponding to a strain of 2.16 ± 0.58 and 1.29 ± 0.57 and a stress of 13 ± 5 and 8 ± 3 cm H(2)O, respectively. Lung weight gain was associated with deterioration in respiratory system mechanics, gas exchange, and hemodynamics, pulmonary and systemic inflammation and multiple organ dysfunction. CONCLUSIONS In healthy pigs, ventilator-induced lung damage develops only when a strain greater than 1.5-2 is reached or overcome. Because of differences in intrinsic lung properties, caution is warranted in translating these findings to humans.

Nitrogen washout/washin, helium dilution and computed tomography in the assessment of end expiratory lung volume.

IntroductionEnd expiratory lung volume (EELV) measurement in the clinical setting is routinely performed using the helium dilution technique. A ventilator that implements a simplified version of the nitrogen washout/washin technique is now available. We compared the EELV measured by spiral computed tomography (CT) taken as gold standard with the lung volume measured with the modified nitrogen washout/washin and with the helium dilution technique.MethodsPatients admitted to the general intensive care unit of Ospedale Maggiore Policlinico Mangiagalli Regina Elena requiring ventilatory support and, for clinical reasons, thoracic CT scanning were enrolled in this study. We performed two EELV measurements with the modified nitrogen washout/washin technique (increasing and decreasing inspired oxygen fraction (FiO2) by 10%), one EELV measurement with the helium dilution technique and a CT scan. All measurements were taken at 5 cmH2O airway pressure. Each CT scan slice was manually delineated and gas volume was computed with custom-made software.ResultsThirty patients were enrolled (age = 66 +/- 10 years, body mass index = 26 +/- 18 Kg/m2, male/female ratio = 21/9, partial arterial pressure of carbon dioxide (PaO2)/FiO2 = 190 +/- 71). The EELV measured with the modified nitrogen washout/washin technique showed a very good correlation (r2 = 0.89) with the data computed from the CT with a bias of 94 +/- 143 ml (15 +/- 18%, p = 0.001), within the limits of accuracy declared by the manufacturer (20%). The bias was shown to be highly reproducible, either decreasing or increasing the FiO2 being 117+/-170 and 70+/-160 ml (p = 0.27), respectively. The EELV measured with the helium dilution method showed a good correlation with the CT scan data (r2 = 0.91) with a negative bias of 136 +/- 133 ml, and appeared to be more correct at low lung volumes.ConclusionsThe EELV measurement with the helium dilution technique (at low volumes) and modified nitrogen washout/washin technique (at all lung volumes) correlates well with CT scanning and may be easily used in clinical practice.Trial RegistrationCurrent Controlled Trials NCT00405002.

Effect of different cycling-off criteria and positive end-expiratory pressure during pressure support ventilation in patients with chronic obstructive pulmonary disease

Objective:During pressure support ventilation, ventilator inspiration ends when inspiratory flow drops to a given percentage of the peak inspiratory flow cycling-off criteria. This study evaluated the effect of two different cycling-off criteria on breathing pattern, respiratory effort, and gas exchange in patients with chronic obstructive pulmonary disease. Design:Clinical study. Patients:Thirteen mechanically ventilated patients with acute exacerbation of chronic obstructive pulmonary disease primarily due to pneumonia (Pao2/Fio2 291 ± 114 mm Hg, Paco2 53 ± 19 mm Hg). Interventions:Two cycling-off criteria (5% and 40% of the peak inspiratory flow) at two levels of pressure support (5 and 15 cm H2O) with and without the application of an external positive end-expiratory pressure (6 and 0 cm H2O) were applied. Measurement:Patient–ventilator time delay of cycling-off was computed as the difference between the end of inspiratory flow and the lowest value of inspiratory esophageal pressure. Inspiratory effort was estimated by computing the work of breathing, the pressure time product partitioned into the total pressure time product, and the pressure time product due to the dynamic intrinsic positive end-expiratory pressure. Results:At 5 and 15 cm H2O of pressure support ventilation, the cycling-off criteria 40% significantly reduced the patient–ventilator time delay of cycling-off from 0.40 ± 0.20 secs to 0.29 ± 0.16 secs and from 0.93 ± 0.50 secs to 0.52 ± 0.25 secs, respectively; the dynamic intrinsic positive end-expiratory pressure from 3.9 ± 1.8 cm H2O to 3.1 ± 2.1 cm H2O and from 2.4 ± 2.0 cm H2O to 1.7 ± 1.4 cm H2O, respectively; and the pressure time product due to the dynamic intrinsic positive end-expiratory pressure. At 5 cm H2O of pressure support, the cycling-off criteria 40% significantly reduced the tidal volume and the inspiratory effort. The modification of cycling-off criteria did not affect the gas exchange. Conclusion:The modification of cycling-off criteria may have a beneficial effect on reducing the dynamic hyperinflation and inspiratory effort in chronic obstructive pulmonary disease patients, especially at low levels of pressure support.

The effect of different volumes and temperatures of saline on the bladder pressure measurement in critically ill patients

IntroductionIntra-abdominal hypertension is common in critically ill patients and is associated with increased severity of organ failure and mortality. The techniques most commonly used to estimate intra-abdominal pressure are measurements of bladder and gastric pressures. The bladder technique requires that the bladder be infused with a certain amount of saline, to ensure that there is a conductive fluid column between the bladder and the transducer. The aim of this study was to evaluate the effect of different volumes and temperatures of infused saline on bladder pressure measurements in comparison with gastric pressure.MethodsThirteen mechanically ventilated critically ill patients (11 male; body mass index 25.5 ± 4.6 kg/m2; arterial oxygen tension/fractional inspired oxygen ratio 225 ± 48 mmHg) were enrolled. Bladder pressure was measured using volumes of saline from 50 to 200 ml at body temperature (35 to 37°C) and room temperature (18 to 20°C).ResultsBladder pressure was no different between 50 ml and 100 ml saline (9.5 ± 3.7 mmHg and 13.7 ± 5.6 mmHg), but it significantly increased with 150 and 200 ml (21.1 ± 10.4 mmHg and 27.1 ± 15.5 mmHg). Infusion of saline at room temperature caused a significantly greater bladder pressure compared with saline at body temperature. The lowest difference between bladder and gastric pressure was obtained with a volume of 50 ml.ConclusionThe bladder acts as a passive structure, transmitting intra-abdominal pressure only with saline volumes between 50 ml and 100 ml. Infusion of a saline at room temperature caused a higher bladder pressure, probably because of contraction of the detrusor bladder muscle.

Continuous negative abdominal distension augments recruitment of atelectatic lung.

Rationale:In acute lung injury, atelectasis is common and frequently develops in the dependent and diaphragmatic regions. Attempts to recruit lung with positive pressure constitute a major aim in the management of acute respiratory distress syndrome but are associated with overdistension and injury in nonatelectatic regions. Objective:To test the hypothesis that continuous negative abdominal pressure using an iron lung would augment positive end-expiratory pressure in recruiting atelectatic lung. Methods and Main Results:An in vivo rabbit model of ventilator-induced lung injury was used in which a recruitment maneuver followed by positive end-expiratory pressure (110 cm H2O) had no effect on oxygenation. Addition of sustained continuous negative abdominal pressure (−5 cm H2O) to the positive end-expiratory pressure significantly increased the end-expired lung volume and PaO2 but impaired ventricular preload and cardiac output (suggested by echocardiography). Addition of transient (15 mins) continuous negative abdominal pressure resulted in comparable and lasting (60 mins) increases in PaO2. Sustained, but not transient, continuous negative abdominal pressure was associated with hemodynamic depression and lactic acidosis, which appeared (illustrative echocardiography, n = 2) to be caused by decreased cardiac preload. Computerized tomography (n = 2) suggested that continuous negative abdominal pressure was an effective adjunct to positive end-expiratory pressure for recruiting atelectasis in dependent and diaphragmatic regions. In surfactant-depleted but noninjured lungs, sustained continuous negative abdominal pressure augmented lung recruitment and oxygenation in the setting of higher (but not lower) levels of positive end-expiratory pressure and reduced central venous oxygenation. Conclusions:Continuous negative abdominal pressure may be a potential adjunct to positive end-expiratory pressure in the recruitment of diaphragmatic atelectasis. The approach ultimately might be useful when ceilings exist on the level of desired positive end-expiratory pressure.

Accuracy of central venous oxygen saturation with a fiberoptic catheter

Central venous oxygen saturation (ScvO2) can reflect the overall balance between the systemic oxygen delivery and supply. Several recent studies reported the importance of ScvO2 monitoring in critically ill patients. Recently, ScvO2 monitoring with fiberoptic catheters has been made available. The aim of this study was to evaluate the correlation between the ScvO2 values obtained by a fiberoptic catheter (CeVOX; Seda, Milan, Italy) and those measured with a CO-oximeter (GEM 4000; Instrumentation Laboratory Milan, Italy).