Capnography: monitoring CO2

Carbon dioxide in the body

The content of the air we breathe is a mixture of gases, with each gas exerting a pressure dependent on its proportional size. This is called its partial pressure and is abbreviated as the letter ‘P’, e.g. PO₂—partial pressure of oxygen. The total of the gas pressures gives an atmospheric pressure measured in millimetres of mercury (mmHg). At sea level, atmospheric air would be around 760 mmHg with the PO₂ around 159 mmHg or 20.8% of the total gases (Tortora and Derrickson, 2014).

Within a person's blood vessels is a mixture of gases, each with a partial pressure. To ensure effective ventilation and perfusion, the levels of each gas need to be within a certain parameter. The exhalation of CO₂ from the lungs is dependent on a partial pressure difference between the CO₂ levels in the pulmonary capillary and the CO₂ levels in the alveolar air (PACO₂) creating a mechanism for diffusion. The breathing rate of the person is calculated by the respiratory centre in the brainstem from an analysis of the blood pH levels. A higher partial pressure of CO₂ in the blood would indicate a need to increase ventilation.

Measuring a sample of gases within arterial blood is an invasive procedure which gives a single reading of the level of patients' arterial blood gases (ABG), including CO₂ (PaCO₂) (Manifold et al, 2013). This determines the efficacy of a person's ventilation through a comparison of measured gases against normal values (Long et al, 2017). This shows the efficiency of gas exchange and is used to inform treatment (Hunter et al, 2015).

CO₂ as a by-product of cellular metabolism, is transported to the lungs within the blood vessels where it crosses the alveolar membrane and is exhaled. The body needs to maintain CO₂ levels in the blood within strict parameters as it is an acidic chemical which can change the pH of the blood and affect homeostasis (Tortora and Derrickson, 2014).

Medical devices to capture expired carbon dioxide

There are different ways of measuring CO₂ levels. Many devices use a light sensor to register the levels of gases within the air. This could be in the form of a side stream or main stream capture device (Figure 1). The difference is in whether the analysis chamber is within the breathing circuit of the patient or in a distinct side chamber (Deacon and Pratt, 2017). Because this method is not invasive, it is ideal for the pre-hospital environment and for situations where frequent readings are needed (Hunter et al, 2015).

The capnograph provides this information in wave and numerical format, while a capnometer will give the CO₂ level in numerical form only (Zuver et al, 2011). A colourmetric device may also be used and this reacts to the pH of the air and changes colour accordingly (Deacon and Pratt, 2017). As mentioned, the level of CO₂ in the air is almost zero, so any changes in a medical device show a measurement of the patient's expired CO₂ (Long et al, 2017).

Capnography is the measurement of the percentage of CO₂ in exhaled air. Studies have shown a strong correlation between the levels of CO₂ in the end tidal breath of a person in normal expiration and the levels of CO₂ in arterial blood. Capnography is useful in the pre-hospital environment where there is a need to continually monitor the level of CO₂ gases in the blood.

Capnograph wave

Figure 2 on the next page shows a normal capnography wave. Repetitions of this wave would show the rate at which a person was breathing, as well as the depth and efficacy of ventilation. The wave is divided into phases. It is useful to remember that the wave begins to change with exhalation followed by inspiration. The initial baseline (Phase 0) indicates the dead space levels of CO₂ with no CO₂.

Physiological changes would affect the formation of the wave; production of CO₂; transport of the gas; perfusion; and ventilation. The wave rises (Phase II) as the percentage of CO₂ increases with exhalation. This plateaus in Phase III as the CO₂ is exhaled.

The calculation for the end tidal CO₂ (EtCO₂) is measured at the end of Phase III, as shown on the capnograph. This figure represents the peak levels of CO₂ during the ventilatory cycle, at the end of the expiration. It is normally within the range of 35–40 mmHg. As the person breathes in, the capnograph chamber is filled with air and the CO₂ levels reduce (0), returning to a baseline as shown in Phase I.

The difference between the capnography measurement and the arterial CO₂ levels is about 3–5 mmHg in a healthy patient (Long et al, 2017). This difference is owing to the presence of dead space CO₂ and alveolar CO₂. Where there is normal perfusion and ventilation, the end tidal measurement in capnography has close parallels with the levels of CO₂ in arterial blood (Sanders, 1989).

Clinical indications

Capnography is a measurement which is distinct from pulse oximetry. It shows the percentage of oxygen within arterial blood (SpO₂) (Deacon and Pratt, 2017), and therefore the oxygenation of tissues.

Capnography may provide a wider gauge of a patient's physiological status (Manifold et al, 2013). For example, in the pathophysiology of a pulmonary embolism, there is decreased perfusion for an area of the lung (Hemnes et al, 2010). Dependent on the affected area, ventilation may remain normal and would not register significantly on the pulse oximetry levels (Ward and Yealy, 1997). The increased alveolar dead space would lower the expired CO₂. On the capnography waveform, this would alter or flatten the slope in Phase III (Figure 2) (Long et al, 2017).

EtCO² has been shown to aid pre-hospital diagnosis of congestive cardiac failure (CCF) and in instances of chronic obstructive pulmonary disease (COPD) (Hunter et al, 2015). Decreased CO² levels have also been shown to inversely mirror lactate levels in patients with sepsis (Puskarich, 2012; Hunter et al, 2016). Further research has also identified how capnography interpretation may assist patients with traumatic brain injuries (Nassar and Schmidt, 2016).

The use of capnography is indicated in all pre-hospital intubations (Resuscitation Council UK, 2015) as the most accurate means of confirming ETT (Li, 2001), although it is not the sole means of identifying a correct intubation (Grmec and Malley, 2004).

In the pre-hospital environment, other influences have been shown to affect the assessment of ETT placement. Some markers for ETT placement may be mirrored in oesophageal intubation (Anderson and Hald, 1989) with fogging of the tube occurring (Kelly et al, 1998) and discernible movement of the chest wall still visible (Birmingham et al, 1986).

There is also a strong margin of error in reliance on breath sound auscultation in a noisy or difficult environment (Jones et al, 2003). Capnography waves for an oesophageal intubation will drop from the normal waveform because of the lack of CO² in the expiratory measurement (Long et al, 2017).

Placing the tube in the right main bronchus will produce a similar waveform to accurate intubation, though an initial wave will tail off if it has not passed the vocal cords (Long et al, 2017). The primary confirmation should be visual confirmation of tube placement through the vocal cords (Resuscitation Council, 2015), though in a pre-hospital intubation, the presence of blood and vomit may preclude this (Zuver et al, 2011).

Summary

Capnography is a non-invasive and inexpensive rapid assessment tool (Manifold et al, 2013), which allows the paramedic to accurately assess CO₂ levels in patients. In research, this information is linked to the diagnosis and treatment of many acute conditions found within the pre-hospital environment (Percival, 2011). Capnography provides information on ventilation, perfusion and metabolism (Long et al, 2017)—providing a wider diagnostic range than is possible with pulse oximetry for the assessment and treatment of patients.

Learning Points