Arterial blood gases How does it work?
The pH, PO2 and pCO2 of the sample are measured with specific electrodes. By equilibrating the sample against different CO2, mixtures the bicarbonate concentration is calculated.
What does it tell us?
Arterial blood gas analysis is the gold standard for assessing respiratory function. The gas exchange capability of the lung can be directly measured. The three main measurements made by the blood gas analyzer are used together to obtain a detailed picture of the state of the respiratory system.
Oxygen: the PaO2 is the standard for measuring blood oxygenation. Decreases in PaO2 are due to hypoventilation, inspiration of hypoxic gas mixtures or impairment of gas exchange. Further information can be gained when the inspired oxygen level is changed from 21% to 100% and the PaO2 re-measured.
Carbon dioxide: PaCO2 is set by the balance between CO2 production and CO2 elimination. During anesthesia, CO2 production is fairly constant so the arterial level is determined by elimination. If the lungs are healthy the etCO2 is a good substitute for PaCO2 but in pulmonary dysfunction the difference between them increases which is useful diagnostically.
HCO3-, base excess, anion gap, pH: These all measure different aspects of acid-base balance. Animals with metabolic diseases may have acid-base disturbances and blood gas analysis can be used to monitor and treat them.
A blood gas analyzer is not a routine piece of monitoring equipment. Interpretation of the results requires knowledge of pulmonary and renal physiology, and because the arterial sample has to be transported to the machine for analysis there is a delay in obtaining the results. However, a blood gas analyzer is invaluable for critical care patients, pulmonary research and advanced procedures such as cardiopulmonary bypass and transplantation.
Temperature How does it work?
A probe containing a thermocouple or thermistor is placed in the esophagus or rectum. The probe is connected to a small amplifier box which displays the core temperature. Infrared sensors that are placed in the ear are also available but best used for clinical examination rather than anesthesia.
What does it tell us?
Core body temperature. It is very important with laboratory animals to maintain body temperature. Hypothermia is the most common cause of delayed recovery from anesthesia. Body temperature can be maintained with heating lamps, electrical pads, warm air blowers or circulating water blankets. Water blankets are best because there is no risk of burning the animal.
Hyperthermia is very rare in veterinary anesthesia. Horses and cows anesthetized outdoors in the summer can overheat. Malignant hyperpyrexia (MH) is a genetic disorder that is best known in pigs but also occurs in dogs. An MH attack in susceptible animals is triggered by some anesthetic drugs, the classic ones being halothane and suxamethomum. MH is usually fatal unless treated promptly with dantrolene. MH susceptible animals can be safely anesthetized by using a total intravenous technique.