Ventilation and ETCO2—it's more than just breathing (Proceedings)
Ventilation is the act or process of supplying fresh air and moving gas in and out of the alveoli. Air that is being replaced in the lung has a higher partial pressure of O2 but a lower partial pressure of CO2. Oxygen and CO2 are moved by pressure gradients.
Dead Space is an area where mixing of inspired and expired gases occur in the absence of gas exchange. There are 3 types of dead space:
Both 1 & 2 (anatomic and alveolar) make up physiological dead space i.e. parts of the bronchial and respiratory system that do not participate in O2 and CO2 exchange.Negative dead space is a concern in patients under 6kgs (≈ 12lbs). Negative dead space occurs when NO ventilation is taking place i.e. the volume of dead space is greater than tidal volume (10-15mls/kg). This will result in CO2 build up. Negative dead space is a very REAL possibility in small patients if:
1. Flow rates are not adequate – sometimes an O2 flow rate of 1-2 L/min is not adequate in a non rebreathing system. If inspiratory CO2 (inCO2) is not 0 than the O2 flow rate is increased until inCO2=0.
2. Cracks are present in the breathing system
3. Too many adaptors are present – use only one. Pediatric ETCO2 adaptors should be utilized when available on small patients. This will decrease dead space by 5mls. These adaptors should be utilized when the diameter of the ET tube is the same or smaller than the pediatric ETCO2 adaptor.
4. ET tubes are too long.
ETCO2 is the amount of CO2 expelled at the end of a breath. ETCO2 allows a noninvasive method of measuring patient ventilation during anesthesia. Changes in ETCO2 will occur before decreases in SPO2. A relationship exists between ETCO2 and PaCO2 - ETCO2 is usually 2- 5 points LOWER than PaCO2. It is a good indicator of how well a patient is ventilating. ETCO2 can "potentially" monitor PaCO2 without having to perform arterial blood gas sampling, but ideally an arterial blood gas sample should be obtained so that all respiratory information is available to the anesthetist. Normal ETCO2 = 35-45mmHg. Usually an ETCO2 above 45mmHg requires manual or mechanical ventilation assistance. As ETCO2 levels climb they cause direct or indirect effects on the body. Indirect (shorterm) effects increase BP and contractility as well as produce tachycardia caused by endogenous catecholamine release. Direct (prolonged) effects produce vasodilation and myocardial depression caused by respiratory acidosis (lower pH and hypercapnia). CO2 can also act as an anesthetic if ETCO2 levels approach 80 mmHg or greater.
Use ETCO2 monitors on all patients whenever possible.
Mainstream vs. Sidestream
Mainstream technology utilizes infrared light technology. Rapid sample analysis occurs at the end of the ET and breathing tube. Mainstream ETCO2 adaptors have a calibration device that is incorporated into the ETCO2 line. This enables the anesthetist to check ETCO2 accuracy (if in doubt) at any time during an anesthetic procedure. The heated adaptor prevents moisture buildup. Mainstream machines do not require scavenging of sampled gases since none are produced. The warm up period usually takes a few minutes. The adaptors are expensive to replace and are sensitive to secretions blocking the adaptor "window". Sample size is influenced by an O2 flow rate of less than 50-150 ml/min.
Sidestream technology pumps the exhaled respiratory gases to the anesthetic monitor for analysis. Sample analysis has a small delay (few seconds) and no immediate calibration device is available to check accuracy. Moisture build up in the delivery tubing has been a problem. The exhaled waste gases that are pumped into the anesthetic monitor should be scavenged. No warm up period is needed and sidestream monitoring is usually more desirable when MRI testing is being performed. Sample size is influenced by low O2 flow rates as well. A 50-150 ml/min flow rate is needed for accurate sampling. However, high fresh gas flow rates in small patients can give a false low ETCO2.