| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Guest Access | Sign In via User Name/Password
|
|
|
|||||||
Guest Access | Sign In via User Name/Password |
|||||||||
Dr. Epstein is Director, Medical Intensive Care Unit, Division of Pulmonary, Critical Care, and Sleep Medicine, Tufts-New England Medical Center, and Associate Professor of Medicine, Tufts University School of Medicine.
Correspondence to: Scott K. Epstein, MD, FCCP, Division of Pulmonary, Critical Care, and Sleep Medicine, Box 369, New England Medical Center, 750 Washington St, Boston, MA 02111; e-mail: SEpstein{at}lifespan.org
Direct administration of fresh gas or oxygen into the trachea has been employed in several settings. In outpatients in stable condition with severe COPD or pulmonary fibrosis, transtracheal oxygen (TTO) improves exercise tolerance, reduces inspired minute ventilation (without increasing PaCO2), decreases dyspnea, and lessens the work of breathing.1 2 3 With this technique, oxygen flow continues during expiration, allowing the trachea to serve as an effective oxygen reservoir. This results in oxygen conservation as lower flow rates are required to maintain a given level of oxygenation. In addition to these physiologic benefits, TTO systems may result in improved comfort and a more favorable cosmetic effect when compared to nasal cannulae. In the setting of acute on chronic respiratory failure, TTO administered via a percutaneously inserted minitracheostomy maintained improvements in oxygen for 1 week and was ultimately successful (survival) in 65% of the patients.4
In critically ill patients receiving mechanical ventilation, tracheal gas insufflation (TGI) has been under investigation for several decades. Pioneering work by Slutsky et al5 6 showed that TGI of oxygen to paralyzed dogs maintained oxygen and a stable level of hypercapnia. Ultimately, investigators began to study TGI as an adjunct to mechanical ventilation, principally in patients with acute lung injury. More recently, TGI has been examined as an aid to weaning from mechanical ventilation and as a stand-alone mode of full ventilatory support.
TGI can be delivered by a thin catheter placed through the endotracheal tube (terminating within 1 to 2 cm of the main carina) or via a modified endotracheal tube with channels embedded in the walls of the tube. TGI flow can be forward (toward the alveoli) or reversed in direction toward the proximal end of the endotracheal tube. During expiration, TGI reduces dead space by washing carbon dioxide out of the trachea, bronchi, and the endotracheal tube so that with the next breath less carbon dioxide is rebreathed. Continuous forward-flow TGI may also decrease PaCO2 by enhancing distal gas mixing. During volume-cycled ventilation, continuous TGI augments tidal volume and will increase alveolar distending pressure and the risk for volutrauma in ARDS. The effect can be diminished by using pressure control ventilation, by downward adjusting machine-delivered tidal volume during volume-cycled ventilation, or by using TGI timed to occur only during expiration (expiratory TGI). Even when using the latter strategy, TGI can impede expiration, resulting in the development of intrinsic positive end-expiratory pressure. Using reverse flow or end-expiratory (rather than pan-expiratory) TGI, or the addition of tracheal gas exsufflation, may help alleviate this problem. A number of additional safety issues with TGI have been raised including concerns about ensuring adequate humidification, increased risk of airway mucosal injury, and adverse effects on secretion clearance (especially if desiccation occurs).7 The importance of these complications, especially with long-term use of TGI, remains to be defined.
A large number of animal investigations with experimental lung injury8 and clinical studies in patients with ARDS managed with a strategy of permissive hypercapnia9 10 11 12 13 demonstrate that TGI can be used as an adjunct to either volume- or pressure-cycled ventilation. In this context, TGI can allow for a reduction in tidal volume and alveolar distending pressure without further rise in PaCO2. Alternatively, if tidal volume is held constant, TGI can result in a reduction in PaCO2, a potentially important maneuver in a patient with ARDS and intracranial hypertension14 or severe metabolic acidosis.
A pattern of rapid, shallow breathing and inefficient carbon dioxide clearance characterizes patients with COPD who cannot be weaned from mechanical ventilation.15 Therefore, TGI could facilitate liberation from ventilatory support by enhancing carbon dioxide clearance. In a sheep model of lung injury, Cereda et al16 combined TGI with continuous positive airway pressure during spontaneous breathing and demonstrated a reduction in the inspiratory work of breathing. In a flow-dependent manner, TGI decreased tidal volume, minute ventilation, dead space, and PaCO2 in 12 patients with COPD undergoing weaning trials.10 Yet, in a bench lung model, Hoyt et al17 found that TGI may increase the work needed to open the demand valve and trigger the ventilator, a problem that may be surmounted by a system that stops TGI flow prior to the end of expiration.
Intratracheal pulmonary ventilation (ITPV) is an adaptation of continuous TGI that allows for complete ventilatory support without the concomitant use of conventional ventilation. In this mode, a small catheter is placed through the endotracheal tube and positioned close to the carina. Inspiration and expiration occur as an expiratory valve is closed and opened, respectively. Expiration is further aided by a reverse-thrust catheter that entrains gas from the distal airways. In a sheep model using pressure control for comparison, ITPV reduced tidal volume, peak airway pressure, and dead space, at a constant PaCO2.18
In this issue of CHEST (see page 1742), Tagaito and colleagues extend these applications by examining TGI, with and without periodic tracheal occlusions (PTOs), in a chronic tracheostomized dog model. As with previous studies, increasing TGI flow rates decreased minute ventilation without increasing PaCO2 (dead space is decreased). The addition of PTOs led to increased minute ventilation and a fall in PaCO2, demonstrating that this technique can be used to fully support spontaneous breathing. The degree of ventilatory support was determined by the number and duration of tracheal occlusions and the TGI flow rate. Although these physiologic changes are impressive, it is important to ask whether this "minimally invasive" technique offers tangible therapeutic advantages over currently available modes of invasive and noninvasive ventilation. At this point, there is no definitive evidence that TGI-PTO improves patient-ventilator interaction, facilitates weaning from mechanical ventilation, or otherwise provides advantages over currently available ventilatory support systems. Importantly, patient-ventilator synchrony using currently available modes and machines can be improved by setting the ventilator properly, by reducing trigger sensitivity to 0.5 to 1.0 cm H2O or using flow triggering, by providing adequate tidal volume and minute ventilation, and by matching inspiratory flow rates to patient ventilatory demand.19 Similarly, the advantages of avoiding invasive airways and using mask interfaces (noninvasive ventilation) have been increasingly documented. As an example, when compared to invasive mechanical ventilation, noninvasive ventilation is associated with a lower risk for nosocomial infections, especially ventilator-associated pneumonia.20 In addition, noninvasive ventilation can be used, in select patients, to facilitate liberation from mechanical ventilation and improve overall outcome.21
In conclusion, TGI is a modality with the potential to improve important pathophysiologic manifestations of acute respiratory failure. Newer applications, such as the addition of PTOs to provide complete ventilatory support, are of great clinical interest. However, at present, approved TGI systems are not commercially available. Even when this occurs, high-quality randomized controlled trials demonstrating the efficacy of this approach are required before widespread application of TGI can be recommended.
References
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
