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Milan, Italy
Dr. Chiumello is affiliated with the Dipartimento di Anestesia e Rianimazione, Fondazione Istituto di Ricovero e Cura a Carattere Scientifico, "Ospedale Maggiore Mangiagalli, Regina Elena."
Correspondence to: Davide Chiumello, MD, Istituto di Anestesia e Rianimazione, Fondazione IRCCS, Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena di Milano, Via Francesco Sforza 35, 20122 Milan, Italy; e-mail: chiumello{at}libero.it
Invasive mechanical ventilation, although highly effective in improving gas exchange and reducing the work of breathing, may not only increase the risk of nosocomial infections but can also cause discomfort for the patient and barotrauma.1 Most of these complications are related to the endotracheal tube. Conversely, noninvasive positive pressure ventilation (NPPV), defined as any form of ventilatory support applied without the use of an endotracheal tube, offers the potential advantage, in selected patients, of avoiding these drawbacks.
NPPV was first applied in the 1930s when Barach and colleagues2 showed that continuous positive airway pressure could be useful in the treatment of acute pulmonary edema. Subsequently, NPPV gained popularity as first-line ventilatory treatment of respiratory failure. Two prospective epidemiologic surveys34 found that NPPV was used in 5 to 15% of patients with respiratory failure; however, the percentage of patients treated with NPPV significantly differed between centers, ranging from none in eight ICUs to 67% in one ICU.4
NPPV reduced the need for intubation and mortality in patients with acute cardiogenic pulmonary edema,5 acute exacerbation of COPD,6 and hypoxemic acute respiratory failure.7 However, during NPPV up to 40% of patients may require endotracheal intubation and invasive mechanical ventilation, primarily because of poor patient tolerance and the severity of underlying disease.4 Although the face mask is the most common interface used to deliver NPPV, it may be responsible for a certain proportion of NPPV failures. One way of reducing the need for premature termination of NPPV could be to use a different interface to limit the pressure necrosis of the skin, air leaks, and discomfort. A plastic "helmet" that covers the patient’s whole head, originally used to deliver an air mixture during hyperbaric oxygen therapy, has now been developed for NPPV. The helmet makes no contact with the head, so it should be more comfortable than the face mask. In patients with acute hypoxemic respiratory failure and cardiogenic pulmonary edema, the helmet improved gas exchange similarly to the face mask but was more comfortable and permitted longer continuous application.89
One of the most important drawbacks of the helmet, due to its larger inner volume compared to the face mask (ie, a dead space volume of 8 to 12 L), is carbon dioxide rebreathing, which could limit the efficacy of NPPV.10 Antonelli and colleagues,11 comparing NPPV using the helmet and face mask in patients with acute exacerbation of COPD, found that after 1 h of NPPV both groups had a significant reduction in arterial carbon dioxide levels. However, the decrease in arterial carbon dioxide was smaller in the helmet group, and this difference was greater in the subgroups of patients who failed NPPV.11
The helmet comprises a plastic hood with a soft collar, making for better comfort than the face mask, although it may dissipate the inspiratory pressure delivered by the ventilator. Two studies1213 in humans that evaluated the helmet and face mask in healthy subjects during pressure support ventilation found a similar breathing pattern, but the helmet required greater inspiratory muscle effort and took a longer time to reach the selected level of airway pressure.
In this issue of CHEST (see page 1424), Dr. Moerer and colleagues14 report a bench study of ventilatory performance by simulating spontaneous breathing at the beginning of an inspiratory effort, and compared a helmet and face mask during NPPV at different levels of pressure support ventilation and positive end-expiratory pressure (PEEP). They measured the time delay to activate the ventilatory trigger, the time between the initiation of an inspiratory effort until the preset PEEP level is reached, and the inspiratory pressure time product (ie, the muscle inspiratory effort) during these two periods. The helmet, although presenting a significantly longer time delay, caused a lower pressure time product compared to the face mask. In addition, by increasing the level of pressure support or PEEP, the helmet furthermore significantly reduced the delay times and pressure time product.
During NPPV, the ventilator must first pressurize the interface (face mask or helmet) and then the respiratory system.12 The actual design of the helmet (high compliance) means that it needs higher inspiratory volumes than the face mask to reach the same airway pressure, causing a longer delay times. However, the patient can use the higher gas volume inside the helmet at the beginning of an inspiration, thus reducing the initial inspiratory muscle effort.
A practical clinical message from the current14 and previous studies1213 is that the physician should set higher levels of PEEP and pressure support to reduce inspiratory muscle effort closer to that with the face mask. Although the patient tolerates the helmet better, it needs careful clinical monitoring and setting.
References
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