depth of anaesthesia, drugs such as opioids, alpha2agonists, ketamine, benzodiazepines or pathological reasons such as pleural space or musculoskeletal disease). Intermittent positive pressure ventilation or mechanical ventilation can be provided to correct this but the cause should be addressed – consider why the patient is hypoventilating (e.g. If the ETCO2 is above the normal range (hypercapnia), the most likely cause is hypoventilation due to respiratory depression. What could cause the ETCO2 range to fall outside the normal limits? When the patient inspires, the trace should return sharply to zero (the inspiratory downstroke). The alveoli should all contain a similar amount of CO2, so phase 3 is seen as a plateau of between 35-45mmHg - this is the end tidal CO2 (ETCO2 ). When the alveolar gas containing CO2 from gas exchange reaches the sensor, the trace rises sharply (phase 2). It contains the dead space gas from the endotracheal tube (ETT) and the mouth/nares to the bronchioles, where no gas exchange takes place. The first part of the exhaled breath (phase 1) should contain no CO2, assuming that the patient did not inspire any during the last breath. If CO2 levels are too low (60mmHg) cause a reduction in blood pH, respiratory acidosis, compromised myocardial function and narcosis.Īs the exhaled breath passes over the sensor (or in the case of a sidestream capnograph, the sampling tube), the trace is reflected on the screen (fig. These homeostatic mechanisms become depressed in anaesthetised patients, so their CO2 levels can become abnormal. In the conscious patient, CO2 is finely controlled by central and peripheral chemoreceptors as part of homeostasis. An end-tidal carbon dioxide (ETCO2) value that is within the normal range (35-45mmHg) suggests that the processes of metabolism, perfusion to the tissues, perfusion to the alveoli, and ventilation are functioning as they should – this single parameter provides information about the respiratory, circulatory and metabolic status of the patient. Glucose and oxygen are delivered to the tissues by the blood, and likewise CO2 must be delivered back to the alveoli to be eliminated via the lungs. The basics – how is carbon dioxide generated and eliminated from the body?ĬO2 is the by-product of cellular metabolism – conversion of glucose to energy in the presence of oxygen. There are monetary savings to be made from using less volatile agent and oxygen, and if the FGF is no higher than it needs to be, heat loss and moisture loss from the patient's airways is reduced. Benefits of capnography therefore extend to using less volatile agent, which is beneficial to staff, and to the environment (isoflurane and sevoflurane are extremely potent greenhouse gases). The latter is non-invasive and provides a continuous and real-time measurement of the patient's end tidal and inspired CO2 concentration.Ĭapnography enables the fresh gas flow (FGF) to be titrated to the exact requirement of the patient rather than making an estimation using circuit factors which tend to be much higher than they need to be. The patient's CO2 concentration is arguably the most useful and informative parameter to monitor during anaesthesia, and can be assessed either by obtaining an arterial blood gas sample for analysis, or via the exhaled breath using a capnograph. Use of a capnograph is mandatory in every general anaesthetic and sedation in human medicine. Information provided by the capnograph alerts the person monitoring the anaesthetic to life-threatening problems before they can be detected clinically by even the most experienced and diligent practitioner, and before it may be too late to save the patient (2,3). The use of capnography in veterinary medicine to effectively monitor anaesthetised patients is essential (1). Why do we need to measure expired and inspired carbon dioxide (CO2)?
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