Evolving Management of Respiratory Failure - May 2006
Over the past ten years, management of children with acute respiratory failure has evolved, offering new and improved therapies. The following is a brief discussion of current options.
Conventional Mechanical
When children develop acute respiratory failure, they may be tracheally intubated and placed on conventional mechanical ventilation. Roughly, conventional mechanical ventilation imitates normal breathing. Depending on the age of the child and medical condition, a conventional ventilator delivers ten to 60 breaths per minute (the respiratory rate), with each mechanical breath (the tidal volume) measured at five to ten milliliters per kilogram. That tidal volume is delivered over a finite time period ranging from about one-half second to more than one second (the inspiratory time). For example, a 10 kilogram child might receive 30 breaths per minute with a tidal volume of 60 milliliters delivered over 0.6 second. If the child has bad lung disease, typically accompanied with decreased lung compliance and increased airway resistance, large amounts of pressure may be required. Casual observation of a conventionally ventilated child reveals the chest rising and falling regularly, almost as though the child were breathing without assistance.
High Frequency Oscillatory Ventilation (HFOV)
High frequency oscillatory ventilation (sometimes referred to as ‘jet ventilation’) might be used when conventional mechanical ventilation fails. Originally developed in intensive care nurseries, HFOV has spread to the older pediatric population and more recently to the adult world as well, particularly in patients with diffuse alveolar disease (e.g. ARDS). As its name implies, HFOV does not try to mimic a normal breathing pattern. Instead, HFOV moves gas into and out of patients at an extremely rapid rate, typically three to fifteen Hz (cycles per second), or 180 to 900 breaths per minute. Rather than the normal-appearing regular chest rise and fall of conventional ventilation, HFOV produces a forceful vibration of the patient’s torso.
The time spent on inhalation (the inspiratory time) is only a miniscule fraction of a second before exhalation begins. For example, at 10 Hz (600 breaths per minute) and with a typical inspiratory fraction of 33%, the inspiratory time is only 0.033 second. With such brief inspiratory times, high frequency ventilation HFOV cannot move the large volume of gas into and out of the lung with each breath that is seen with conventional mechanical ventilation. The tidal volume is not directly measured, but is estimated to be in the range of 1 to 1.5 milliliters per kilogram.
For our 10 kilogram patient, each breath would be 10 to 15 milliliters or only two or three teaspoons of air with each breath. And yet, HFOV can oxygenate patients and excrete carbon dioxide efficiently. The mechanism of gas movement and exchange in HFOV is not well understood, but probably combines several physiologic mechanisms. Without going into detail, these proposed mechanisms include bulk convection, asymmetric velocity profiles, pendelluft, cardiogenic mixing, Taylor dispersion and turbulence, molecular diffusion, and collateral ventilation. It is likely that all of these mechanisms play some role in HFOV, enhancing both gas transport and gas exchange, often more successfully than conventional ventilation. HFOV may have the additional advantage of causing less ventilator-induced lung injury (barotrauma), with reduced risk of air leaks (pneumothorax, pneumomediastinum, and subcutaneous emphysema).
Permissive Hypercapnia
Importantly, the goal of ventilator therapy is always adequate oxygenation but not necessarily normal carbon dioxide tensions. Large adult studies emphasize that patient outcomes are superior if ventilator support manages to avoid high distending pressures. Using this strategy, carbon dioxide levels may rise to markedly abnormal levels. This is acceptable, as long as reasonable oxygenation can be maintained. This approach is called permissive hypercapnia.
At what point should we convert a patient from conventional mechanical ventilation to high frequency ventilation? In intensive care nurseries, with the most experience with HFOV, and where barotrauma is a major concern in immature lungs, there appears to be a trend toward earlier and earlier use of HFOV. A similar movement may follow in the older pediatric population as well. At this point, different institutions use different criteria, many using the oxygenation index, a mathematical derivation of the pO2, the FiO2, and the mean airway pressure, to help decide when to change ventilator strategies.
Prone Positioning
Many ventilated Pediatric Intensive Care Unit (PICU) patients will be turned intermittently from supine to prone. Prone positioning appears to reduce the risk of atelectasis and nosocomial pneumonia in mechanically ventilated patients. Anatomically, the major bronchi slope from dorsal to ventral (“back to front”), so that prone positioning allows more natural drainage of secretions from those major airways. In addition, prone positioning appears to make both ventilation and perfusion of the lung more homogeneous, producing better V/Q matching and better oxygenation. Improved lung expansion may result from the anatomic position of the diaphragm; because the diaphragm is angled, normal lung volumes are larger in the dorsal part of the lung than the ventral. Prone position places the largest lung volume in the least dependent position, permitting better overall expansion. Improved lung expansion may even result from reducing the weight of the heart on dorsal lung segments. In prone position, the weight of the heart rests against the sternum, permitting better expansion of the lung dorsal to the heart.
Regardless of the exact mechanism, prone positioning appears to improve oxygenation over the supine position, improvement that persists even after the patients are turned supine.
This information provided by John Tsukahara, M.D., Pediatric Intensive Care Unit, California Pacific Medical Center