![]() ![]() Manually ventilating too often in order to achieve a “normal” capnograph reading can cause the patient to rely on the anesthetist for ventilation. Some degree of hypercapnia during anesthesia may be tolerated in a spontaneously breathing patient without underlying disease (up to of 60mmHg). As patients emerge from anesthesia, the respiratory drive becomes even more important because if it remains suppressed the elimination of inhalant is delayed. Patients are often unable to compensate for anesthetic related hypoventilation for example. Many of the drugs that we use in anesthesia suppress this phenomenon. Normal respiration is driven by CO2 levels in the blood stimulating respiratory centers in the brain. Mainstream monitors are larger and more cumbersome than sidestream and may not be appropriate for all patients. These monitors provide quicker information as the gas is analyzed directly at end exhalation, and they do not require scavenging of the sampled gases. Mainstream monitors have an infrared sensor that measures ETCO2 directly at the end of the endotracheal tube. In the presence of normal vitals and blood pressure, trends and capnogram waveforms are monitored more reliably than actual numbers in very small pets. In very small patients, some of the exhaled gas may become diluted with fresh gas resulting in a lower and inaccurate ETCO2 reading. Another drawback is that moisture can accumulate within the tubing causing an occlusion. Due to the diverting of gases, there is a slight delay in readings. Sidestream technology diverts sampled gases up a length of tubing to the monitor for infrared absorption analysis. There are two classifications of capnographs. The capnogram can help to provide descriptive information about an abnormality. Many capnographs also display a capnogram, or waveform which diagrams inspiration and exhalation over time. We know that elevated ETCO2 (hypercapnia) occurs during hypoventilation, and a decrease in ETCO2 (hypocapnia) occurs with hyperventilation. Normal range is 35-45mmHg, and roughly correlates with the partial pressure of CO2 in arterial blood (remember that PaCO2 is usually slightly higher than ETCO2 by 2-5mmHg). This gives us a means of estimating ventilation and how well the lungs are removing CO2 from the body. CapnometryĬapnometry is the measurement of carbon dioxide (CO2) in exhaled gas (ETCO2). Mechanical dead space will result in discrepancies between ETCO2 and PaCO2 as well. We also need to be cognizant of mechanical dead space (ETT length, monitor or positional adaptors, and anywhere that bidirectional gas flow occurs). If serial measurements are taken, a decrease in this gradient indicates that the patient’s status is improving. Poor perfusion (or increased alveolar dead space ventilation) is one common reason for this. As the gradient increases (PaCO2 is increasing while ETCO2 is decreasing or staying the same), this indicates that CO2 is not effectively being eliminated and is increasing at tissue levels. The gradient can be reflected as P(a-ET)CO2, and the normal range for this is 2-5mmHg. ![]() In a normal, healthy patient this gradient, or difference, is minimal and the two are closely matched. We can evaluate this relationship using the gradient between ETCO2 and arterial CO2 levels (PaCO2) measured via arterial blood gas. Atelectasis, bronchial intubation or lung pathologies will result in lack of alveolar ventilation although alveoli are perfused. Significant hypotension, for example, will cause inadequate perfusion although alveoli are still being ventilated. Alveoli must be adequately ventilated but also adequately perfused in order for effective gas exchange to take place. We often talk about the relationship or ratio between ventilation (V) and perfusion (Q) in regards to anesthesia (V/Q). Anything that causes changes in circulation, tissue perfusion, metabolism, or ventilation can cause changes in CO2 production and elimination. CO2 will then diffuse into venous circulation, be transported to the lungs and eliminated via exhalation. ![]() This non-invasive monitor can give valuable information about cardiac output, perfusion, and ventilation.Īerobic cellular metabolism produces CO2 as a by-product. It is the measurement of CO2 at the completion of exhalation and roughly correlates to the CO2 present in arterial blood. Massachusetts Veterinary Referral Hospital, Woburn, MAĮnd-tidal carbon dioxide monitoring (ETCO2) has clinical uses far beyond solely determining hypo- or hyperventilation. Jaime Maher, CVT, VTS (ECC, Anesthesia/Analgesia) ![]()
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