Capnography – a refresher

Respiratory function monitoring can provide valuable information for patients undergoing anaesthesia or those that are critically ill.

The primary function of the respiratory system is to allow exchange of oxygen and carbon dioxide between the body and environment.

If not functioning optimally, the patient will suffer from reduced tissue oxygen delivery, with hypoxia and a buildup of carbon dioxide with resultant acidaemia.

While a variety of monitoring tools are available that give information on respiratory system function, it is important to remember basic, hands-on monitoring gives valuable information.

Occasionally, anaesthetic monitoring equipment will give artefactual information; basic monitoring is critical to distinguish this from what is really happening.

Visual assessment of the patient’s chest movements allows evaluation of the respiratory pattern, effort and rate.

Thoracic auscultation allows identification and location of abnormal lung sounds, indicating developing problems, including pulmonary oedema or aspiration (crackles), bronchoconstriction (expiratory wheezes) or pleural space disease (muffled lung sounds).

With anaesthetised patients, manual inflation of the lungs – by squeezing the reservoir bag with the adjustable pressure‑limiting valve closed – allows a crude assessment of the pulmonary compliance and inspiratory resistance. This also gives feedback regarding the presence of upper respiratory tract (endotracheal tube) obstructions or pleural space disease.

In recent years, it has become common for veterinary practices to have access to equipment that can monitor aspects of the respiratory system in more depth. A basic understanding of their function – and the relevance of the information provided – is important, otherwise the machine is, essentially, useless.

Capnography can be an extremely useful and versatile tool.

Carbon dioxide

Carbon dioxide (CO₂) is produced in the body’s cells as a result of metabolism and is carried to the lungs by the circulation. It is then removed by alveolar ventilation.

The concentration of CO₂ that is expired is, therefore, determined by three factors (Figure 1):

• alveolar ventilation
• metabolism
• circulation

Capnography

Capnography is a “real time”, non-invasive measurement and numerical display of the CO2 concentration in the respiratory gases during breathing. It also measures the patient’s respiratory rate.

While both inspired and expired CO₂ concentrations are measured and provide useful information, it is the expired concentration we are most interested in.

The peak expired CO₂ concentration is referred to as end-tidal CO₂ (ETCO₂) and is usually displayed as a number with the measurement in mmHg. Alternative units of measurement include per cent or kilopascal.

This is taken to represent the alveolar partial pressure of CO₂; in the healthy patient, this correlates well with the arterial partial pressure of CO₂ (PaCO₂), which would be expected to be only 2mmHg to 5mmHg higher.

When two of the three factors determining expired CO₂ are stable, capnometry provides information on changes on the third. Capnometry can also give valuable information regarding our anaesthetic equipment (Langton et al, 2010).

Examples include:

• whether the endotracheal tube is correctly positioned and patent
• if leaks are present in the anaesthetic breathing system
• if the fresh gas flow is inadequate when using a non-rebreathing anaesthetic system
• whether the CO₂ absorber is exhausted, or if one-way valves are malfunctioning when using a rebreathing anaesthetic system

Principles of measurement

Capnography units can either be mainstream analysers, with the sensor placed directly between the endotracheal tube and breathing system, or sidestream analysers (Table 1).

Table 1. Comparison of the two main types of capnography units – mainstream and sidestream analysers
	Mainstream analysers	Sidestream analysers
Sensor position	Between the endotracheal tube and breathing system.	In the anaesthetic monitor, with a gas sample line drawing gas from a connector at the level of the endotracheal tube.
Advantages	• Rapid response time. • Better suited for patients with a fast respiratory rate and very small tidal volumes.	• Doesn’t require endotracheal intubation for measurement. • Less dead space. • Units are less prone to damage.
Disadvantages	• Requires endotracheal intubation for measurement. • Unit creates increased dead space. • Prone to malfunction due to water vapour or dirt entering the unit. • Units tend to be expensive.	• Slower response time; depends on the gas sampling rate (50ml/min to 200ml/min) • Requires a water trap to remove water vapour enabling a dry gas sample. • The gas sample requires scavenging or returning to the anaesthetic breathing system.

Sidestream analysers are more common and require a gas sampling line attached to a connector near the endotracheal tube. Samples are drawn to the anaesthetic monitor for analysis. Water vapour is removed with a water trap prior to measurement, to prevent interference.

Endotracheal intubation is not a prerequisite for sidestream sampling as the gas sample line can be placed at the level of the nares, attached to an intranasal catheter or inside a face mask in non-intubated patients.

Nasal capnography has been shown to provide an acceptable alternative to arterial blood gas analysis in both sedated and conscious, critically ill dogs (Pang et al, 2007; Kelmer et al, 2009).

The gas is ideally sampled as close to the patient as possible to measure alveolar gas. Gas samples are generally taken at between 50ml/min and 200ml/min, which means a delay can exist from the patient taking a breath and it appearing on the capnogram.

A higher sampling rate reduces the response time of the monitor (it shortens the time between measurement and monitor output), but also means a larger part of the fresh gas flow is “lost” each minute.

CO₂ in respiratory gases can be measured in numerous ways. The majority of capnograms rely on the absorption of infrared light by CO₂ molecules within the gases. The capnography unit produces infrared radiation, which passes through a gas sample chamber. The opposite side of the chamber has a photodetector.

The amount of radiation that reaches the photodetector depends on the concentration of CO2 within the chamber – the higher the concentration of CO₂ in the chamber, the more radiation absorbed, with less falling on the photodetector, and vice versa (Figure 2).

Capnogram interpretation

Additionally to providing discrete numerical values of CO₂ in respiratory gases, most capnography monitors also plot a capnogram. This is a diagrammatic representation of changes in CO₂ over time. The basic components of the capnogram are illustrated in Figure 3.

As aforementioned, in most healthy dogs and cats, the ETCO₂ is very similar to the concentration of gas in the arterial blood. Concentrations of CO₂ are defined as:

• hypocapnia: less than 35mmHg
• normocapnia: 35mmHg to 45 mmHg
• hypercapnia: greater than 45mmHg

Mild hypocapnia can be desirable in patients with suspected intracranial disease where raised intracranial pressure could be a concern. Hypocapnia will induce cerebral vasoconstriction and, therefore, aid with a reduction of intracranial pressure via the Monro-Kellie doctrine. However, it is difficult to achieve without manual or mechanical ventilation.

In most patients, hypocapnia is not desirable as cerebral ischaemia can result. It is common for most anaesthetised patients – especially if spontaneously breathing – to be hypercapnic.

Although a deviation from normal, mild hypercapnia can have beneficial effects in most patients (Saghaei et al, 2014). However, CO₂ concentrations greater than 60 mmHg should instigate changes in ventilation to reduce the ETCO₂.

Reasons for increased and decreased ETCO₂ are explained in Panel 1.

• Decreased ventilation – for example, due to a low respiratory rate, reduced tidal volume or an increased dead space in the breathing circuit. Patient positioning and obesity can be a caused of hypoventilation.
• Increased production of CO₂ – for example, occurring in hypermetabolic states and may indicate malignant hyperthermia, or severe fever.
• Increase in the inspired concentration of CO₂ – for example due to rebreathing, may be due to exhausted soda lime or an external source of CO₂.

Decreased ETCO₂

• Gradual decrease:
  • Increased alveolar ventilation
  • Reduced CO2 production:
    • hypothermia
    • hypometabolic state
  • Increased alveolar dead space – for example, hypotension due to low cardiac output, pulmonary embolism or high positive end-expiratory pressure during intermittent positive-pressure ventilation.
  • Sampling error – for example, water blocking the sample line, inadequate tidal volume or a leaking endotracheal tube cuff.
• Sudden or complete decrease:
  • Failure of ventilation – for example, apnoea, equipment blockage, disconnection or displacement.
  • Failure of circulation – for example, cardiac arrest.

Capnogram examples

Using knowledge of the capnogram components and reasons for increased or decreased ETCO₂, it is possible to use the capnogram trace to diagnose a variety of conditions. Common examples are illustrated in Figures 4 to 9.