Always Troubleshoot An Increase in Peak Airway Pressure
Adam D. Niesen MD
Juraj Sprung MD, PhD
Occasionally, the anesthesiologist needs to troubleshoot elevated peak airway pressure in an intubated patient. Although each patient has a different baseline peak airway pressure, the anesthesiologist must investigate possible causes of a sudden increase in ventilating pressure. For the most part, these pressure increases can be attributed to problems related to the patient, to airway/ventilation equipment, or even to positioning of surgical staff and equipment.
CONSEQUENCES OF ELEVATED VENTILATORY PRESSURE
Any sudden increase in peak airway pressures requires immediate attention. Barotrauma (trauma resulting from high peak pressure) or volutrauma (trauma to the lungs resulting from overstretched alveoli caused by increased tidal volume) can result from prolonged positive-pressure ventilation at high pressures and/or unrestricted lung expansions. Overstretch of alveoli may cause disruption of the alveolar-capillary membrane, thereby causing an increase in pulmonary vascular permeability and pulmonary edema. High peak inspiratory pressure (PIP) may also cause pneumothorax and pneumomediastinum. As a consequence, an acute respiratory distress syndrome-like picture, referred to as ventilator-induced lung injury, may develop. To prevent ventilator-induced lung injury, excessive PIP should be avoided and alveolar plateau pressures should be maintained below 30 cm H2O.
In addition to injuring lungs, high airway pressures may also affect hemodynamics. If excessive positive end-expiratory pressure (PEEP) is used, or if auto-PEEP is generated by breath stacking, cardiac output may be adversely affected through increase in intrathoracic pressure, which can impede venous return to the right ventricle. In addition, compression of pulmonary vessels may increase right ventricular afterload, which in turn decreases right ventricular stroke volume and causes right ventricular dilatation. This subsequently shifts the interventricular septum toward the left ventricle, further reducing left ventricular preload and stroke volume. If breath stacking is prolonged and severe, cardiovascular collapse may ensue, appearing as pulseless electrical activity.
AIRWAY/VENTILATION EQUIPMENT PROBLEMS
One of the most important strategies in preventing high airway pressure-induced lung injury is to thoroughly check the anesthesia machine and breathing circuit system (including inspiratory and expiratory valves) before each use. In this way, mechanical problems can be remedied early. However, the fact that the machine was checked does not rule it out as the source of a sudden problem. Some examples include a suddenly stuck manual PEEP valve, a PEEP valve placed incorrectly into the inspiratory limb of the breathing circuit (this rarely happens because contemporary anesthesia machines have a built-in PEEP valve), a stuck expiratory valve causing breath stacking, or a faulty pressure-relief (pop-off) valve during spontaneous breathing. Other causes of high circuit pressures have been described, such as malfunctioning of ventilator relief and control valves, scavenger system blockage, or occlusion of the muffler on the ventilator. Any of these can result in elevated peak airway pressures and should be considered in determining the underlying cause of elevated pressures in the breathing circuit.
After initiation of mechanical ventilation, the pop-off valve should be set to the “open” position. With resumption of spontaneous breathing at the end of the case, if this valve were closed and this remained unnoticed, high pressure (50 to 60 mm Hg) could develop in the breathing circuit and in the patient’s lungs. This high pressure can be further aggravated by flushing bypass oxygen into the system. The same applies when, after the circuit has been disconnected and then reconnected to a tracheal tube, in order to fill the ventilator bellows, an anesthesiologist pushes the oxygen bypass button simultaneously with mechanical inspiration. During this maneuver, all gas delivered to the circuit is directed toward the patient’s lungs, potentially causing barotrauma.