It is the non-invasive measurement of exhaled carbon dioxide (CO2) that is displayed as a waveform concentration of CO2 over time.
During the 1980s, capnography became a standard for anesthesia care in the United States.
While pulse oximetry reflects oxygenation status, capnography reflects ventilation, perfusion, and metabolism.
Available to both intubated and spontaneously breathing patients, and now as part of the EMT curriculum, capnography is an incredibly valuable and trusted tool.
Role of capnography
Understanding the meaning and value of capnography begins with an appreciation of how carbon dioxide is generated in the body . Simply put, our bodies use oxygen and water to generate energy; carbon dioxide is the waste product of that process.
In a steady state, CO2 production remains relatively constant, therefore the exhaled CO2 measurement directly reflects ventilation.
If perfusion decreases for any reason, such as a drop in cardiac output or the onset of shock, the amount of CO2 that is returned to the lungs will decrease. Capnography will reflect that change. Nowhere is this more powerfully illustrated than during CPR.
An EtCO2 of less than 10 mmHg during an arrest suggests that chest compressions are not deep or fast enough, that the rescuer is tired (hence why we need to change the compressor every two minutes), or that excellent CPR is not produces not even the slightest perfusion.
There is a large body of data suggesting that failure to achieve peak expiratory CO2 above 10 mmHg within the first 20 minutes of CPR is never associated with ROSC (return of spontaneous circulation) and could reasonably be used to terminate resuscitation. .
Finally, capnography reflects metabolism and appears to be one of the first indicators of alterations in metabolic activity.
In cardiac arrest , for example, a sharp rise in EtCO2 was found to be the earliest indicator of return of spontaneous circulation (ROSC) in two prehospital studies.
This increase in exhaled CO2 occurred significantly earlier than palpable pulses or blood pressure. The same is true in altered metabolic states, such as diabetic ketoacidosis.
As the aerobic metabolism slows and the patient becomes more acidotic, the exhaled CO2 will also decrease.
Capnography is available for both spontaneously breathing patients and patients receiving positive pressure ventilation.
For spontaneously breathing patients, exhaled CO2 can be measured using a nasal cannula type device where one of the tips of the cannula (and sometimes an additional manifold placed over the mouth) samples the respiratory gases.
For patients receiving positive pressure ventilation, CO2 is measured with an adapter attached to the connection between the ventilation device or circuit and the advanced airway or mask worn on the patient.
In patients receiving non-invasive ventilation (NIV) such as CPAP or BiPAP, the CO2 washout caused by the high fluxes associated with NIV easily interferes with accurate CO2 measurement when sampling from anywhere other than the nasal oral site (using a cannula-type device mentioned above).
This would not be the case with bag-valve-mask ventilation in an apneic or deeply distressed patient.
Recently, an online miniature capnography device that includes a waveform has received FDA approval and could serve as an ideal device for EtCO2 measurement by both BLS and ALS providers when ventilating apneic patients or assisting in ventilations in patients with respiratory failure.
The EtCO2 waveform
The components of the capnography waveform are worth reviewing as an understanding of how the components of a normal capnography help providers to recognize and deepen the physiological changes that alter CO2 waveforms.
The height of the waveform corresponds to the amount of CO2 exhaled and the length of the waveform represents time; the higher the waveform, the higher the exhaled CO2 value; the shorter the duration of the waveforms, the faster the respiratory rate.
Note that inspiration is associated with a rapid drop in CO2 concentration back to a zero baseline.
Expiration initially continues along this zero baseline as the 50 to 150 cc of dead space is expelled from the airway. This is followed by a rapid increase in exhaled CO2 as alveolar air exhales.
The peak or point immediately preceding inspiration is the point where the CO2 value is obtained at the end of the tide (or the end of the breath).
The alveolar plateau (the relatively minimal upward slope of the expiratory phase) represents the alveolar gases that are exhaled.
Several pathological conditions are revealed in abnormalities of the normally relatively square capnogram. Inspiration is observed by a sharp decrease in the waveform.
The exhalation phase of the capnogram shows a marked increase in CO2 concentration, eventually reaching a plateau.
On normal exhalation, the alveolar plateau will vary from a relatively flat angle to a slightly upward angle.
In patients with bronchoconstriction, exhalation is impaired, resulting in a steeper angle rather than the normally seen alveolar plateau. The degree of this upward angle on the capnogram correlates with the severity of the bronchoconstriction or air trapping.
This waveform is often described as “shark fin,” as in appearance. With proper treatment, providers will see the return of the alveolar plateau on the capnogram.
EtCO2 in cardiac arrest
One of the most obvious uses for capnography is cardiac arrest. The American Heart Association recommends the use of continuous waveform capnography during any cardiac arrest for three different reasons:
- To ensure that the endotracheal tube remains in the trachea or other advanced airways it remains in place.
- To assess the quality of CPR.
- To provide an early indicator of ROSC.
The use of capnography to monitor CPR quality and detect ROSC should be done initially by connecting an EtCO2 monitoring device to the bag-valve mask device.
Once an advanced airway is placed, capnography has the additional purpose of ensuring that the airway remains in place.
Resuscitation involves maximum movement of the patient and offers considerable opportunity for airway displacement.
In fact, the degree of patient movement observed in any patient transport, both in and out of the hospital, is significant enough to cause unrecognized airway displacement.
Continuous waveform capnography is a standard of care in any patient transport environment.
The utility of capnography in detecting ROSC serves an additional purpose of monitoring perfusion and metabolism after arrest. Once ROSC is achieved, capnography provides the earliest indicator of impaired perfusion.
A drop in EtCO2 indicates to the resuscitation team that the patient is resuming.
TBI and EtCO2
Capnography has the additional role of allowing providers to monitor assisted ventilation. For post-arrest patients and head injury patients with suspected increased intracranial pressure (ICP), ventilation control can critically affect outcomes.
In patients with head injuries, for example, sustained arterial CO2 levels of 50 mmHg or more increase blood flow to the brain, thereby raising ICP. Sustained low CO2 levels of 30 or less also worsen neurological outcomes.
As EtCO2 reflects arterial CO2 in patients with reasonable perfusion, capnography is a valuable tool to avoid inadvertent hyperventilation or hypoventilation.
When are ventilations attended?
Discussions of capnography often overlook the many valuable uses this technology offers when treating awake and spontaneously breathing patients.
In fact, there are frequent opportunities to affect patient outcomes through the routine use of capnography on the wide variety of EMS patients we encounter every day.
One of the most obvious and common uses of capnography is the evaluation of patients who appear to be seriously ill. The waveform and EtCO2 values provide information not only on respiration but also on perfusion.
Normal EtCO2 values range from 35 to 45 mmHg. A critically ill patient with a normal capnography waveform and an EtCO2 value has a patent airway, is breathing adequately, and is reasonably perfused.
In hypoperfused patients with shock metabolic acidosis, EtCO2 decreases due to a compensatory increase in minute volume resulting from a decrease in serum bicarbonate (HCO3).
The more acidotic the patient becomes, the lower the serum HCO3, the higher the respiratory rate, and the lower the EtCO2.
Interestingly, although not well documented, capnography waveforms often blunt, taking on a more rounded appearance, in low cardiac output states.
The visualization of this waveform, accompanied by a low EtCO2 value, strongly suggests an element of cardiogenic compromise.
Other uses for EtCO2
Capnography then becomes a stellar tool for detecting metabolic acidosis, diabetic ketoacidosis, lactic acidosis seen in sepsis or cyanide poisoning, pulmonary embolism, and simple hypoperfusion as a result of any state of shock .
Capnography also allows a provider to differentiate between disease (metabolic acidosis resulting in increased respiratory rate and decreased EtCO2) versus not so sick (non-addictive with normal respiratory rate and normal EtCO2).
This can be particularly useful in evaluating pediatric patients with gastroenteritis, where the degree of dehydration is often very difficult to determine.
Seizure patients often confuse the assessment. Capnography can easily determine if a patient is apnea or breathing and if their breathing is effective or ineffective.
In patients with acute respiratory distress, waveform capnography helps assess the degree of airway flow obstruction and (numerically) illustrates the effectiveness of ventilation.
The response to treatment can be seen by the return of the capnographic waveform to a more normal form, while deterioration and fatigue are evidenced by an EtCO2 escalation – values higher than 70 mmHg in patients without COPD strongly suggest respiratory failure, needing assisted ventilation.
Anytime you sedate a patient (for cardioversion, aggressive behavior management, rhythm, etc.) or administer narcotics for pain, capnography should be used to manage respiratory depression.
The decrease in oxygen saturation is too late as an indicator that the patient is not breathing properly.
Finally, capnography is not subject to interference from excessive movement or low perfusion. We have all seen patients with such poor peripheral perfusion or continuous movement that no pulse oximeter can obtain a reliable signal.
Capnography will work in these situations and can provide as much, if not more, information about perfusion, ventilation, and metabolism than you would get from a pulse oximeter. This could explain why many anesthesia providers today believe that capnography is the best vital sign.