Monitoring anesthetized patients-capnography (Proceedings)

The capnograph is a noninvasive monitoring device that can provide information on ventilation (pulmonary function), blood flow, and equipment function. Knowledge of respiratory and cardiovascular physiology is essential to the understanding of capnography. Application of this knowledge will allow the use of the capnograph to assess:

     • Ventilation

     • Cardiovascular function

     • Anesthetic machine function

Carbon Dioxide: Review of Physiology:

Carbon dioxide is produced in mitochondria during aerobic metabolism. This metabolic product eventually travels to the lungs via the circulatory system. The lungs eliminate carbon dioxide into the atmosphere when an animal ventilates. Thus, the amount of carbon dioxide left in the blood after it has traveled through the lungs (i.e., the arterial carbon dioxide tension; PaCO2) is influenced by: CO2 production (metabolism) and CO2 elimination (pulmonary ventilation). Many things can affect metabolic rate: Body temperature, physical activity, shivering, endocrine alterations (ie, hyperthyroidism, catecholamine release), malignant hyperthermia, and parenteral nutrition with solutions containing high glucose concentrations can all affect CO2 production. HOWEVER, in most clinical situations y, CO2 production is relatively constant-Thus, PaCO2 is considered to change inversely with alveolar ventilation).

Normal CO2 tension ranges between 35-45 mm Hg. Hypocapnea (hyperventilation) is generally defined by PaCO2 < 35 mmHg, and may be the result of voluntary hyperventilation (ie, pain, anxiety), iatrogenic hyperventilation (a consequence of overzealous mechanical ventilation), hypoxemia (and stimulation of ventilation via peripheral chemoreceptor response to low arterial oxygen tension), hypotension, metabolic acidosis (respiratory compensation), mechanical stimulation of pulmonary J receptors, stimulation of proprioreceptors located in the joint capsule, or hyperthermia.

When PaCO2 is greater than 45 mmHg, an animal is said to be hypoventilating. Central nervous system depression (ie, Anesthetic drugs, CNS trauma), paralysis or damage of the muscles of ventilation (ie, neuromuscular blocking drugs), disruption of thoracic wall integrity (ie, flail chest), malignant hyperthermia, respiratory failure or fatigue of the respiratory muscles, and increased deadspace ventilation may all lead to hypoventilation.

Capnography:

Carbon dioxide tension is frequently measured in the blood (blood-gas analysis), and in the expired gas (capnometry/capnography). Normally, the maximal exhaled carbon dioxide tension (end-tidal CO2; PETCO2) correlates with PaCO2 and is about 5-6 mm Hg lower. Changes in ventilatory status, anesthetic machine function, or cardiovascular performance may all affect PETCO2. PETCO2 may not be predictive of PaCO2 during periods of hemodynamic instability or during thoracotomy or thoracoscopy. Factors which affect the capnograph include: airway obstruction or disconnection, decreased cardiac output, hyperventilation, increased dead space ventilation, and rebreathing:

Capnographs work by measuring the absorbance of infrared light through a chamber of exhaled gas. The absorbance of light increases as the concentration (and partial pressure) of CO2 increases. Gas may be measured directly by inserting a heated sensor and airway adapter between the endotracheal tube and the patient-end of breathing system. This is called mainstream or flow-through sampling. Alternatively, an aspiration system can be used to withdraw gas from the breathing system through an adapter and sampling tube placed at the end of the endotracheal tube. Ideally, the small sampling tube should extend down into the lumen of the endotracheal tube of the patient. This type of sampling is called side-stream sampling, and the sampled gas is withdrawn to the capnograph for measurement.

Side-stream sampling devices can be used in patients that are not intubated, and the airway adapter is light and small and is convenient to use. However, withdrawal of gas to the capnograph requires effective moisture scavenging system and a gas aspiration rate that is appropriate for the patient. Calibration of these units may also require the regular purchase of calibration gases to ensure accuracy. In addition, gas withdrawn from the anesthetic circuit should be disposed of through a scavenging system or returned to the patient circuit. Mainstream devices, on the other hand, do not require an aspiration pump, and may be simpler and less expensive to calibrate and operate. However, the heated sensor is bulky and may be easily dropped and damaged because it must be handled with every use. Teixeira and co-workers evaluated a side-stream and mainstream capnograph and found the accuracy of both to be acceptable in dogs.

In summary, the capnograph is useful monitoring tool in anesthetized or critical patients that can provide information on pulmonary ventilation, blood flow, and anesthetic machine function. However, an understanding of respiratory physiology, cardiovascular physiology, and anesthetic machine function is essential for interpretation of capnograph measurements.