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 Time-based capnography or end-tidal CO2 (PETCO2) monitoring has long been established as a standard of care in the operating room. It not only helps clinicians to confirm correct endotracheal tube placement but also to monitor the integrity of the breathing circuit and airway.1 Successful experience with these applications has led to capnography's increased use beyond the operating room doors.
For example, capnography has become more commonplace for emergent procedures such as intubation and resuscitation. During resuscitation, PETCO2 assesses the adequacy of chest compressions by noninvasively monitoring pulmonary blood flow. Low PETCO2 suggests inadequate pulmonary blood flow or poor chest compressions, whereas higher PETCO2 can reflect improved pulmonary blood flow and/or the return of spontaneous circulation.2-4
In the critical care setting, capnography has become more accessible to clinicians as more ventilators come equipped with time- and volume-based CO2 monitoring capabilities. In addition, advances in mainstream technology have made measuring capnography during mechanical ventilation less cumbersome. Once bulky, finicky adaptors now have become smaller and more reliable, improving accessibility and increasing use of capnography. It may reduce the need for frequent blood gas sampling in some patients, which can decrease costs, unnecessary testing, and patient discomfort.
Education is paramount to integrate such monitors into routine clinical assessment. With a better understanding of capnography's functions, limitations, and relationship to human physiology, clinicians can use the information from these devices to apply calculated science to the art of managing the critically ill patient.
Long-standing controversy
Capnography can play an important role in assessing cardiopulmonary interaction and overall ventilation when applied in the critical care setting. When a patient is ventilated for an extended period of time, gas exchange will alter with changes in disease state, positioning, sedation levels, and suctioning procedures. Continuous capnography allows clinicians to monitor these alterations, adjust treatment accordingly, and anticipate a patient's response to future therapies and procedures. It also may alert clinicians to any sudden changes in ventilation such as circuit disconnects, unplanned extubations, and cardiopulmonary arrest.
However, capnography's continuous use during extended mechanical ventilation is the point of contention for some clinicians. This debate largely centers around the inability to use capnography as a surrogate to blood gas sampling due to its inaccuracy in the face of a large, unpredictable arterial to PETCO2 gradient (P(a-ET)CO2). Large P(a-ET)CO2 gradients make blood gas sampling necessary in many clinical scenarios. To gain the most clinical value from capnography, the clinician must understand the interaction between PETCO2 technology and human physiology.
Capnography and ventilator manipulation
In normal lungs, the P(a-ET)CO2 gradient is 4 to 5 mm Hg, due to ventilation-perfusion (V/Q) matching. This gradient increases in an irregular manner as the V/Q ratio changes with worsening lung disease or dead-space ventilation. On the other hand, a narrowing of this gradient may indicate improved lung function or less dead-space ventilation. While clinicians cannot avoid drawing blood gases when a large gradient is present, capnography can still trend ventilation, monitor acute changes, and most importantly, guide ventilator manipulation.
Volumetric CO2 monitoring makes ventilator manipulation more predictable in the face of a large P(a-ET)CO2. By combining capnography with tidal volume monitoring, volumetric capnography has proven to indicate changes in gas exchange following manipulation of ventilator settings.5 Therefore, volumetric data helps to eliminate the uncertainty associated with time-based P(a-ET)CO2 gradients because the volume of CO2 is measured per minute, otherwise known as CO2 elimination (VCO2).
Assuming the patient has been in a relative steady state of CO2 production, during ventilator manipulation with VCO2 monitoring, the clinician either removes more or less CO2 on a breath-to-breath basis. VCO2 can help the clinician titrate positive-end expiratory pressure by providing information on ventilation and pulmonary capillary blood flow.6,7 Using volumetric capnography, the clinician is immediately aware of favorable or unfavorable changes during PEEP titration.
When titrating PEEP, improved VCO2 suggests lung recruitment and improved ventilation. Conversely, decreases in VCO2 may indicate over-distention leading to impedance of pulmonary capillary blood flow. In some cases, a decrease in VCO2 is brief because open lung units may need to be over-distended in order to recruit atelectatic units. Once alveoli are recruited, clinicians can see an increase in VCO2. With PEEP optimized and lung recruitment improved, patients may endure less ventilator-induced lung injury from the shearing forces of continual collapse and re-opening of alveoli.
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