Pressure & Volume Monitoring
Peak inspiratory pressure is the highest circuit pressure generated during an inspiratory cycle, and provides an indication of dynamic compli-ance. Plateau pressure is the pressure measured during an inspiratory pause (a time of no gas flow), and mirrors static compliance. During
normal ventilation of a patient without lung dis-ease, peak inspiratory pressure is equal to or only slightly greater than plateau pressure. An increase in both peak inspiratory pressure and plateau pressure implies an increase in tidal volume or a decrease in pulmonary compliance. An increase in peak inspira-tory pressure without any change in plateau pressure signals an increase in airway resistance or inspira-tory gas flow rate (Table 4–3). Thus, the shape of the breathing-circuit pressure waveform can provide important airway information. Many anesthesia machines graphically display breathing-circuit pres-sure (Figure 4–28). Airway secretions or kinking of the tracheal tube can be easily ruled out with the use of a suction catheter. Flexible fiberoptic bronchos-copy will usually provide a definitive diagnosis.
Alarms are an integral part of all modern anesthesia ventilators. Whenever a ventilator is used “disconnect alarms” must be passively activated. Anesthesia workstations should have at least three disconnect alarms: low peak inspiratory pres-sure, low exhaled tidal volume, and low exhaled car-bon dioxide. The first is always built into the ventilator whereas the latter two may be in separate modules. A small leak or partial breathing-circuit disconnection may be detected by subtle decreases in peak inspiratory pressure, exhaled volume, or end-tidal carbon dioxide before alarm thresholds are reached. Other built-in ventilator alarms include high peak inspiratory pressure, high PEEP, sustained high airway pressure, negative pressure, and low oxygen-supply pressure. Most modern anesthesia ventilators also have integrated spirometers and oxygen analyzers that provide additional alarms.