ADVANCES in technology have contributed to our ability to better care for critically ill patients. New methods of ventilatory support (such as pressure-support, pressure-controlled inverse ratio, and airway pressure-release ventilation) in the intensive care unit (ICU) have allowed for easier weaning and a decrease in barotrauma.1The study by Jaber et al. 2compares the pressure-support ventilatory (PSV) mode in ICU ventilators with the recently introduced pressure support in ventilators used in operating room (OR) anesthesia workstations.
The initial method of mechanical ventilation in the OR was pressure-controlled ventilation. In this mode, the ventilator is set to deliver a certain pressure to the airway and maintain that pressure through the period of inspiration. As gas flows through the pressurized system to the patient’s lungs, a tidal volume is generated (inspiration). The duration of inspiration depends on the respiratory rate and the inspiratory-to-expiratory ratio set by the operator. In some other ventilators, the inspiratory-to-expiratory ratio is set by increasing or decreasing the gas flow rate (increasing the flow rate during inspiration shortens the inspiratory time and prolongs exhalation, whereas decreasing the flow rate results in the opposite).
The tidal volume delivered to the patient by this mode is dependent on the lung compliance and the airway resistance. In stiff lungs (pulmonary edema, interstitial fibrosis, and adult respiratory distress syndrome), the compliance of the lung is low (unable to expand), and this results in a smaller tidal volume for the given preset pressure. In cases of bronchial asthma, mucus plugging, or kinking of the endotracheal tube, the preset pressure is reached quickly, and the delivered tidal volume is also low.
The pressure-support mode of ventilation is derived from the pressure-controlled mode. In the pressure-support mode of ventilation, the ventilator senses the patient’s initiation of a breath by either a decrease in the flow rate or a decrease in the circuit pressure. When this reaches a preset threshold, usually −2 cm H2O, the ventilator is triggered and delivers a preset pressure (5–15 cm H2O) to augment the patient’s tidal volume. In the latter part of inspiration, the flow rate starts to decrease. When the inspiratory flow rate reaches another preset value, (usually a decline of approximately 25%), the ventilator ceases to deliver the preset pressure. This then allows the patient to exhale. The tidal volumes vary, as in the pressure-control mode.
In this well-designed study, the authors test the performance of the ventilators in five anesthesia work stations against the performance of four traditional ICU ventilators. The results showed that three of the five anesthesia ventilators performed close to the ICU ventilators in time delay and pressurization at different levels of pressure support. Spontaneous ventilation in critically ill patients with multiple medical problems can be quite advantageous. Therefore, the ability to ventilate these patients in the OR with the same level of ventilatory support they were receiving in the ICU may prove to be the least disruptive to their oxygenation and ventilation status, which is already compromised.
This study was performed in a laboratory setting under controlled conditions. Studies need to be performed in the clinical setting to evaluate the actual advantages and disadvantages of the PSV mode in anesthetized patients. In their study, Jaber et al. found that two of the five anesthesia ventilators that they tested did not perform up to the ICU ventilator standards. We do not know whether these differences are clinically significant. However, if they are clinically important, it would be incumbent on the anesthesiologist to ensure that new OR ventilators were functionally equivalent to their ICU counterpart before purchasing new machines.
However, we suspect that only a small number of ICU patients would actually benefit from the PSV mode in the OR. Most ICU patients who are on PSV are in the process of being weaned from mechanical ventilation. These patients are particularly vulnerable to respiratory depressant drugs. When they come to the OR, they require inhalation anesthesia and/or narcotics that cause respiratory depression. The previous settings for PSV that maintained adequate ventilation may no longer be adequate to prevent hypoxia or hypercapnia. This results in the need to increase ventilatory support. Also, the surgical procedure may require the patient to be in certain positions, (i.e. , lateral, prone, or steep Trendelenburg), which may not be well tolerated by an already compromised patient who is breathing spontaneously. Finally, if the surgical procedure required muscle relaxants, it would be impossible to continue using the PSV mode.
The PSV mode may be used in the OR in spontaneously breathing patients who are undergoing peripheral surgery with a laryngeal mask airway or an endotracheal tube. Potential advantages of this are an increase in tidal volume and a decrease in the work of breathing. Many of these patients are healthy and have a large respiratory reserve. When using the PSV mode in these patients, some prolongation in delay time, triggering, or pressurization probably will not have an adverse effect.
However, Jaber et al. did not study the ventilators in PSV mode with changes in resistance or compliance; therefore, it is not possible to know what effects, if any, this would have on ventilation. There are also issues of patient safety. Devitt et al. 3showed that in patients ventilated with a laryngeal mask airway, as the inflation pressure increased, so did the leak around the laryngeal mask airway, as well as the amount of gas entering the stomach. Currently, it is unknown how a leak around the laryngeal mask airway (using the PSV mode) would affect the tidal volume delivered or the anesthesiologist’s ability to detect a decrease in minute ventilation.
Continuing advances in ventilator technology could greatly affect the outcome of critically ill patients. By including these new modes of ventilation into anesthesia workstations, anesthesiologists may be able to minimize adverse effects that can occur when a critically ill patient comes to the OR. We see this study as an important addition to the literature because it shows that technology improvements in traditional intensive care ventilators can be transferred to the OR. Unlike the early OR ventilators, which were merely pressure generators or time cycled-flow generators with a preset maximum pressure limit, the modern OR ventilators are microprocessor-driven, sophisticated machines. However, as with any new technology, the anesthesiologist must understand its indications and limitations, know how to properly implement it, and determine its cost effectiveness.
*Department of Anesthesiology, Yale University School of Medicine, New Haven, Connecticut. jan.ehrenwerth@yale.edu
