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

INTRODUCTION: End 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. METHODS: Patients 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. RESULTS: Thirty 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. CONCLUSIONS: The 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 REGISTRATION: Current Controlled Trials NCT00405002.

Lung stress and strain during mechanical ventilation for acute respiratory distress syndrome.

RATIONALE: Lung injury caused by a ventilator results from nonphysiologic lung stress (transpulmonary pressure) and strain (inflated volume to functional residual capacity ratio). OBJECTIVES: To determine whether plateau pressure and tidal volume are adequate surrogates for stress and strain, and to quantify the stress to strain relationship in patients and control subjects. METHODS: Nineteen postsurgical healthy patients (group 1), 11 patients with medical diseases (group 2), 26 patients with acute lung injury (group 3), and 24 patients with acute respiratory distress syndrome (group 4) underwent a positive end-expiratory pressure (PEEP) trial (5 and 15 cm H2O) with 6, 8, 10, and 12 ml/kg tidal volume. MEASUREMENTS AND MAIN RESULTS: Plateau airway pressure, lung and chest wall elastances, and lung stress and strain significantly increased from groups 1 to 4 and with increasing PEEP and tidal volume. Within each group, a given applied airway pressure produced largely variable stress due to the variability of the lung elastance to respiratory system elastance ratio (range, 0.33-0.95). Analogously, for the same applied tidal volume, the strain variability within subgroups was remarkable, due to the functional residual capacity variability. Therefore, low or high tidal volume, such as 6 and 12 ml/kg, respectively, could produce similar stress and strain in a remarkable fraction of patients in each subgroup. In contrast, the stress to strain ratio-that is, specific lung elastance-was similar throughout the subgroups (13.4 +/- 3.4, 12.6 +/- 3.0, 14.4 +/- 3.6, and 13.5 +/- 4.1 cm H2O for groups 1 through 4, respectively; P = 0.58) and did not change with PEEP and tidal volume. CONCLUSIONS: Plateau pressure and tidal volume are inadequate surrogates for lung stress and strain. Clinical trial registered with www.clinicaltrials.gov (NCT 00143468).

Effect of a heated humidifier during continuous positive airway pressure delivered by a helmet.

INTRODUCTION: The helmet may be an effective interface for the delivery of noninvasive positive pressure ventilation. The high internal gas volume of the helmet can act as a 'mixing chamber', in which the humidity of the patient's expired alveolar gases increases the humidity of the dry medical gases, thus avoiding the need for active humidification. We evaluated the temperature and humidity of respiratory gases inside the helmet, with and without a heated humidifier, during continuous positive airway pressure (CPAP) delivered with a helmet. METHODS: Nine patients with acute respiratory failure (arterial oxygen tension/fractional inspired oxygen ratio 209 +/- 52 mmHg) and 10 healthy individuals were subjected to CPAP. The CPAP was delivered either through a mechanical ventilator or by continuous low (40 l/min) or high flow (80 l/min). Humidity was measured inside the helmet using a capacitive hygrometer. The level of patient comfort was evaluated using a continuous scale. RESULTS: In patients with acute respiratory failure, the heated humidifier significantly increased the absolute humidity from 18.4 +/- 5.5 mgH2O/l to 34.1 +/- 2.8 mgH2O/l during ventilator CPAP, from 11.4 +/- 4.8 mgH2O/l to 33.9 +/- 1.9 mgH2O/l during continuous low-flow CPAP, and from 6.4 +/- 1.8 mgH2O/l to 24.2 +/- 5.4 mgH2O/l during continuous high-flow CPAP. Without the heated humidifier, the absolute humidity was significantly higher with ventilator CPAP than with continuous low-flow and high-flow CPAP. The level of comfort was similar for all the three modes of ventilation and with or without the heated humidifier. The findings in healthy individuals were similar to those in the patients with acute respiratory failure. CONCLUSION: The fresh gas flowing through the helmet with continuous flow CPAP systems limited the possibility to increase the humidity. We suggest that a heated humidifier should be employed with continuous flow CPAP systems.

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.

INTRODUCTION: Intra-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. METHODS: Thirteen 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 degrees C) and room temperature (18 to 20 degrees C). RESULTS: Bladder 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. CONCLUSION: The 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.