C Stahl

Dynamic versus static respiratory mechanics in acute lung injury and acute respiratory distress syndrome.

Objectives:It is not clear whether the mechanical properties of the respiratory system assessed under the dynamic condition of mechanical ventilation are equivalent to those assessed under static conditions. We hypothesized that the analyses of dynamic and static respiratory mechanics provide different information in acute respiratory failure. Design:Prospective multiple-center study. Setting:Intensive care units of eight German university hospitals. Patients:A total of 28 patients with acute lung injury and acute respiratory distress syndrome. Interventions:None. Measurements:Dynamic respiratory mechanics were determined during ongoing mechanical ventilation with an incremental positive end-expiratory pressure (PEEP) protocol with PEEP steps of 2 cm H2O every ten breaths. Static respiratory mechanics were determined using a low-flow inflation. Main Results:The dynamic compliance was lower than the static compliance. The difference between dynamic and static compliance was dependent on alveolar pressure. At an alveolar pressure of 25 cm H2O, dynamic compliance was 29.8 (17.1) mL/cm H2O and static compliance was 59.6 (39.8) mL/cm H2O (median [interquartile range], p < .05). End-inspiratory volumes during the incremental PEEP trial coincided with the static pressure–volume curve, whereas end-expiratory volumes significantly exceeded the static pressure–volume curve. The differences could be attributed to PEEP-related recruitment, accounting for 40.8% (10.3%) of the total volume gain of 1964 (1449) mL during the incremental PEEP trial. Recruited volume per PEEP step increased from 6.4 (46) mL at zero end-expiratory pressure to 145 (91) mL at a PEEP of 20 cm H2O (p < .001). Dynamic compliance decreased at low alveolar pressure while recruitment simultaneously increased. Static mechanics did not allow this differentiation. The decrease in static compliance occurred at higher alveolar pressures compared with the dynamic analysis. Conclusions:Exploiting dynamic respiratory mechanics during incremental PEEP, both compliance and recruitment can be assessed simultaneously. Based on these findings, application of dynamic respiratory mechanics as a diagnostic tool in ventilated patients should be more appropriate than using static pressure–volume curves.

Electrical impedance tomography to confirm correct placement of double-lumen tube: a feasibility study

BACKGROUND Double-lumen tubes (DLTs) are frequently used to establish one-lung ventilation (OLV). Their correct placement is crucial. We hypothesized that electrical impedance tomography (EIT) reliably displays distribution of ventilation between left and right lung and may thus be used to verify correct DLT placement online. METHODS Regional ventilation was studied by EIT in 40 patients requiring insertion of left-sided DLTs for OLV during thoracic surgery. EIT was recorded during two-lung ventilation before induction of anaesthesia and after DLT placement, and during OLV in the supine and subsequently in the lateral position. EIT measurements were made before and after verification of correct DLT placement by fibreoptic bronchoscopy (FOB). RESULTS EIT accurately displayed distribution of ventilation between left and right lung online. All cases (n=5) of initially misplaced DLTs in the contralateral right main bronchus were detected by EIT. However, EIT did not allow prediction of FOB-detected endobronchial cuff misplacement requiring DLT repositioning. Furthermore, after DLT repositioning, distribution of ventilation, as assessed by EIT, did not change significantly (all P>0.5). CONCLUSIONS This study demonstrates that EIT enables accurate display of left and right lung ventilation and, thus, non-invasive online recognition of misplacement of left-sided DLTs in the contralateral main bronchus. However, as distribution of ventilation did not correlate with endobronchial cuff placement, EIT cannot replace FOB in the routine control of DLT position.

Detection of partial endotracheal tube obstruction by forced pressure oscillations

Rapid airway occlusions during mechanical ventilation are followed immediately by high-frequency pressure oscillations. To answer the question if the frequency of forced pressure oscillations is an indicator for partial obstruction of the endotracheal tube (ETT) we performed mathematical simulations and studies in a ventilated physical lung model. Model-derived predictions were evaluated in seven healthy volunteers. Partial ETT obstruction was mimicked by decreasing the inner diameter (ID) of the ETT. In the physical model ETTs of different ID were used. In spontaneously breathing volunteers viscous fluid was applied into the ETTs lumen. According to the predictions derived from mathematical simulations, narrowing of the ETTs ID from 9.0 to 7.0mm decreased the frequency of the pressure oscillations by 11% while changes of the respiratory systems compliance had no effect. In volunteers, a similar reduction (10.9%) was found when 5 ml fluid were applied. We conclude that analysis of pressure oscillations after flow interruption offers a tool for non-invasive detection of partial ETT obstruction.

Flow-dependent resistance of nasal masks used for non-invasive positive pressure ventilation.

Objective and background:  Endotracheal tube resistance is known to be flow‐dependent and this understanding has improved the application of invasive ventilation. However, similar physiological studies on the interface between patients and non‐invasive positive pressure ventilation (NPPV) have not been performed. Therefore, this study was aimed at investigating the resistance of nasal masks used for NPPV.

Optimizing Perioperative Ventilation Support with Adequate Settings of Positive End-Expiratory Pressure

1.1 Mechanical ventilation Mechanical ventilation is often employed to replace spontaneous breathing of patients under general anesthesia. Even after operation, the patient still needs ventilation support until the respiratory muscles regain full function. A ventilator delivers a certain amount of air flow through a facial mask or tracheal tube to the patient whose respiratory system fails to function properly due to the effects of anesthetics or diseases. Based on the difference in breath initiation, mechanical ventilation can be divided into two categories: controlled ventilation and assisted ventilation. In this chapter, we focus on controlled mechanical ventilation, under which the patient is not able to trigger a valid breath and the ventilator overtakes all the workload of respiratory muscles. Respiratory parameters such as respiratory rate (RR), inspiratory–to-expiratory time ratio (I:E), tidal volume (Vt) (or minute volume) are controlled by the ventilator. Traditionally, controlled mechanical ventilation can either be volume controlled (VCV) or pressure controlled (PCV). Ideal respiratory signals obtained in a healthy human during VCV and PCV are shown in Fig. 1. In the VCV mode, a patient receives constant flow from the ventilator until a preset Vt is reached. A severe drawback of VCV is missing control of the peak airway pressure. Airway pressure (Paw) depends on respiratory system compliance and resistance. In patients with certain lung diseases, such as acute lung injury (ALI), the same setting of Vt as in patients with healthy lungs may lead to a higher peak Paw with the potential to further injure the lung. Therefore, VCV is often applied with a pressure limitation. Once the peak Paw rises above this limit, the ventilator will stop delivering gas even if the preset Vt is not yet reached. In the PCV mode, a maximum airway pressure (Pmax) is defined. Inspiration ends when Pmax is reached i.e. the flow driven by the pressure difference decreases to zero. PCV may be superior to VCV in patients requiring one-lung

Pressure dependency of respiratory resistance in patients with acute lung injury and acute respiratory distress syndrome

The analysis of the nonlinearity of respiratory compliance to guide ventilator settings in ALI and ARDS is well established. The pressure dependency (or volume dependency respectively) of respiratory resistance of these patients is mostly ignored. This study was performed to investigate the pressure dependency of resistance in ALI and ARDS over a wide range of pressures.

Pressure drop across neonatal endotracheal tubes during high-frequency ventilation.

High-frequency ventilation (HFV) is a concept of mechanical ventilation that is mainly used in therapy of infants. The resistance of the small neonatal endotracheal tubes (ETT) causes a noticeable difference between airway pressure (proximal end of the ETT) and tracheal pressure (distal end of the ETT). The aim of this laboratory study was to evaluate the pressure drop across the ETT during HFV and to investigate whether tracheal pressure can be calculated from airway pressure using conventional methods.