Monitoring during mechanical ventilation

Monitoring is the continuous, or nearly continuous, evaluation of the physiologic function of a patient in real time to guide diagnosis and management decisions - including when to make therapeutic interventions and assessment of those interventions. Many physiologic parameters can be monitored during mechanical ventilation, including pulse oximetry, capnography, and ventilator graphics. Although its impact on patient outcomes has not been well studied, monitoring has become an integral part of the care of mechanically ventilated patients. Continuous pulse oximetry has become a standard of care for critically ill mechanically ventilated patients. Two wavelengths of light (660 nm and 940 nm) are passed through a pulsating vascular bed using two light-emitting diodes and a photodetector. This is translated into a plethysmographic waveform and the ratio of the amplitudes of these two plethysmographic waveforms is translated into a display of oxygen saturation. A number of limitations of pulse oximetry should be recognized, appreciated, and understood by everyone who uses pulse oximetry data. Most pulse oximeter errors can be explained as too little signal (e.g., low perfusion, improper probe placement) or too much noise (e.g., motion, ambient light). The newest generation of pulse oximeters is affected less by these potential errors. At saturations >80%, the accuracy of pulse oximetry is about ±4-5%. Below 80%, the accuracy is worse, but the clinical importance of this is questionable. New pulse oximeters also measure carboxyhemoglobin and methemoglobin in addition to oxygen saturation. Capnometry is the measurement of CO2 at the airway opening during the ventilatory cycle. The relationship between the PaCO2 and end-tidal PCO2 will vary depending upon the relative contributions of various V/Q units of the lungs. Thus, caution should be exercised when extrapolating end-tidal PCO2 to PaCO2. End-tidal PCO2 is a standard of care to determine proper endotracheal tube position (no exhaled CO2 with esophageal placement). The slope of the capnogram is increased in patients with airway obstruction. Volume-based capnography can be used to assess carbon dioxide production (metabolic rate) and dead space ventilation. Using volume-based capnography, it is also possible to nonnvasively measure cardiac output with the partial CO2 rebreathing technique. Pulmonary mechanics is the expression of lung function through measures of pressure and flow. From these measurements, a variety of derived indices can be determined such as volume, compliance, resistance, and work-of-breathing. Pulmonary graphics are derived when one of the parameters of pulmonary mechanics is plotted as a function of time or as a function of one of the other parameters. This produces scalar pressure-time, flow-time, and volume-time graphics as well as flow-volume and pressure-volume loops. Current generation ventilators provide monitoring of pulmonary mechanics and graphics in real time. Use of esophageal pressure allows assessment of pleural pressure changes during the respiratory cycle. The monitors readily available during mechanical ventilation provide important insights into the pathophysiology respiratory failure and are helpful to direct the care of mechanically ventilated patients. The entire January and February issues of Respiratory Care journal relate to assessment of respiratory mechanics and graphical waveforms during mechanical ventilation. © 2006 Elsevier Ltd. All rights reserved.