Monitoring respired gases - Anesthesia Clinical management

Monitoring respired gases

Capnography

The delivery of carbon dioxide to the lungs depends first on the metabolic production of carbon dioxide. Capnography, therefore, says something about metabolism which may be depressed by cold or fired up during hyperthermia. Capnography depends on blood flow to the lungs. It therefore says something about circulation, specifically that regarding pulmonary blood flow. The delivery of carbon dioxide in the expired gas requires ventilation of alveoli and trans-port of alveolar gas to the outside. Capnography therefore says something about ventilation. Because the ambient air is free of carbon dioxide (well, not com-pletely free with only about 0.03% in air), the appearance of carbon dioxide in the inspired gas must mean that carbon dioxide is being added to the gas or that the patient is re-inhaling the carbon dioxide he just exhaled, for exam-ple from a breathing circuit with a defective valve that causes the dead space in the circuit to increase. Thus, capnography, the measurement of carbon di-oxide in the respired gas, really offers rich information that is relatively easily acquired.

The respired gases can be sampled for analysis by aspirating gas from the breathing circuit or from the nose – should the patient be breathing spontaneously – and then delivering it to an analyzer. This is called “side-stream” sampling. We can also clamp an analyzing cuvette directly on the breathing tube so that all the respired gas passes through a system measuring the carbon dioxide, the so-called “on-airway” or “main stream” capnogram.

There are several methods that enable us to measure carbon dioxide. Clin-ically most often used are infra-red spectroscopy and chemical analysis. Because the infra-red method responds rapidly, it is possible to generate a tracing of the changing carbon dioxide concentration in the respired gases. A capnogram results (Fig. 7.1).

The chemical method is slow but can record approximate ranges of carbon dioxide in gas, which is good enough if you are only interested whether CO₂ is present, for example after intubating the trachea (instead of the esophagus) in an emergency.

One clever method, the volume-based capnogram, plots carbon dioxide over the volume of gas exhaled (Fig. 7.2). It not only lets us estimate the end-tidal concentration of carbon dioxide but it also provides an estimate of deadspace.

Oxygen

When we connect a patient to an atmosphere other than room air, we assume full responsibility for the patient’s oxygen supply. The patient might require only 21% oxygen at ambient pressure at sea level, or he might need much more, depending on clinical circumstances. Uncounted patients have died because that seemingly simple requirement was not met either because gases were mixed such that less than 21% oxygen was present in the inspired gas or because a gas other than oxygen came out of the cylinder or pipeline as happens when cylinders are misfilled or pipes delivering gases are switched by mistake. Monitoring oxygen in the inspired gas, therefore, has become mandatory when patients depend on us to prepare their respired gases.

Several methods are available. Ideally, we would like to have a rapidly respond-ing analyzer that generates “oxygrams” as shown in Fig. 7.1. The technology for that relies on mass spectrometry or paramagnetic devices. Many current anes-thesia machines incorporate a fairly slowly responding fuel cell. However, even an instrument with a response time of many seconds suffices.

Anesthetic gases

With side-stream gas monitors, it becomes possible to use the technology incorporated into capnography to analyze nitrous oxide and the halogenated inhalation anesthetics. The response time of these analyzers enables us to mon-itor both inspired and expired gas concentrations. We can thus watch what con-centration the patient inhales. This frequently differs from the concentration set at the vaporizer which delivers gas to the breathing circuit where the fresh gases are diluted by the gases the patient re-inhales (see Anesthesia machine chapter).

Temperature

The body of an adult patient can absorb many calories before becoming notice-ably warmer or, conversely, will cool only relatively slowly when losing heat by radiation (which accounts for most of the heat loss), evaporation (next in impor-tance), convection, and conduction (least important).² However, monitoring the temperature, regardless how slowly it changes, becomes important in babies and small children and in patients exposed to large heat losses as occur with lengthy intraabdominal or intrathoracic operations. In patients whose temperature drifts down to 35 °C, wound infections may be more common. Other side effects of hypothermia include reduced enzyme activity and shivering (which increases oxygen consumption potentially contributing to myocardial ischemia), as well as the patient’s discomfort.

Central blood in the vena cava or pulmonary artery gives the most representa-tive “core temperature.” Tympanic membrane, esophagus, under the tongue, and the rectum offer other sites. During endotracheal anesthesia, esophageal temper-atures can be measured easily with the help of an esophageal stethoscope that carries a temperature probe (thermistor) at its tip.

Skin temperatures can be measured in the axilla and on the forehead. For the latter site, temperature sensing adhesives are available that change their color with changing temperatures. Their accuracy is limited not only by the fact that ambient temperatures affect skin temperature but also because the temperature-sensitive liquid crystals do not offer good resolution.