Peak Pressures During Manual Ventilation
Abstract & Commentary
David J. Pierson, MD, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, is Editor for Critical Care Alert.
Synopsis: When experienced respiratory therapists manually ventilated a lung model set to mimic several common clinical scenarios, peak airway pressures frequently exceeded 60 cm H2O and were sometimes more than 100 cm H2O.
Source: Turki M, et al. Respir Care. 2005;50:340-344.
In this study from the University of Vermont, Turki and colleagues sought to determine the tidal volumes delivered and peak airway pressures generated during manual ventilation (hand bagging). They recruited experienced, off-duty respiratory therapists who worked regularly in the ICU and thus frequently performed manual ventilation. Turki et al set up a commercial lung model so that resistance and compliance could be adjusted separately, and covered it with a sheet so that a sense of chest movement could be achieved in addition to the pressure and other tactile sensations produced during ventilation. They used 2 resistances (normal and increased, as in severe obstructive lung disease) and 2 different compliance settings (relatively normal and reduced, as in restrictive lung disease), and varied these independently.
The respiratory therapists were unaware of the specific purpose of the study. During an initial session, therapists were told the model represented a 70-kg adult patient, but were given no additional information, and resistance and compliance were varied randomly during the session. During subsequent sessions, with different therapists as subjects, Turki et al presented 3 different clinical scenarios, corresponding to different adjustments of resistance and compliance. These scenarios included patient history, current management, arterial blood gas values, and other clinical information. The therapists were then instructed to manually ventilate the patient in the context of that clinical information, and they recorded the tidal volumes and pressures generated without this information being accessible to the subjects.
The tidal volumes generated during manual ventilation did not vary significantly with the different combinations of resistance and compliance. Peak airway pressure, however, varied significantly between the four loads, and was highest with both high resistance and low compliance. There was marked variation in both tidal volume and peak airway pressure between therapists, with tidal volume varying from 300 mL to more than 1000 mL, and peak pressure varying from less than 20 cm H2O to in excess of 100 cm H2O. The tidal volumes delivered by the therapists were not different with the different clinical scenarios.
Comment by David J. Pierson, MD
Ventilator-induced lung injury and clinical barotrauma such as pneumomediastinum and pneumothorax are important complications of mechanical ventilation, and considerable effort is currently being made to reduce them through lung-protective ventilatory strategies. The ARDS Network tidal volume study,1 and others, have shown impressive improvements in patient outcomes when lung distension and airway pressures are limited through the use of smaller than traditional tidal volumes. These improvements are thought to be due, at least in part, to avoidance of ventilator-induced lung injury. No good studies are available to compare the incidence of overt barotrauma in this era of lung-protective ventilation to that observed 20 years ago when much higher tidal volumes and airway pressures were commonly used. However, many clinicians have the impression that the incidence of extra-alveolar air during mechanical ventilation has decreased, particularly among patients with acute lung injury and the acute respiratory distress syndrome.
In an era when concerted efforts are being made to avoid the use of high tidal volumes and airway pressures during mechanical ventilation, the results of this study are troublesome. In an editorial accompanying this study by Turki et al, Ricard2 discusses the risks of barotrauma in relation to manual ventilation. He points out that the present study’s findings are consistent with those of others, and so should be taken seriously by clinicians. For example, Clarke et al3 studied pressures and volumes during manual ventilation in connection with physiotherapy being administered to patients receiving mechanical ventilation. They found that the mean delivered tidal volume during manual ventilation was 170 percent of that delivered by the ventilator before physiotherapy. They also found that the average peak inflation pressure during manual ventilation was 37-74 cm H2O.
The goals of lung-protective ventilation in acute lung injury and the acute respiratory distress syndrome, which are currently being applied in the management of many other conditions, include keeping end-inspiratory plateau pressure below 30-35 cm H2O. This upper limit of safe tidal volume was routinely exceeded in both of the manual ventilation studies cited, in the case of the Turki study, dramatically so.
Does manual ventilation cause barotrauma? The answer to this question is not known. Clearly there are differences between the tidal volume delivered during a few manual breaths in association with airway suctioning and the constant tidal volume delivered throughout the 24-hour day. And, brief peaks in airway pressure occur frequently, as with coughing and turning in addition to manual ventilation, and whether these increase the likelihood of barotrauma is unclear. Nonetheless, the findings of this study should serve as a reminder that tidal volumes and pressures well in excess of those intended may be generated in patients during the mundane, everyday performance of manual ventilation.
References
- The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000;342(18):1301-1308.
- Ricard JD. Respir Care. 2005;50(3):338-339.
- Clark RC, et al. Anaesthesia. 1999;54(10):936-940.
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