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:
1. Anatomic: the upper portion of the airway that does not participate in ventilation e.g. mouth, bronchial tree etc.
2. Alveolar: alveoli that do not participate in CO2 or O2 exchange
3. Mechanical: includes endotracheal tube adaptors, "Y" and elbow adaptors on anesthetic circuits, respiratory & ETCO2 adaptors. Exhausted soda lime or non functioning one way valves can also cause increases in 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 (¡Ö 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 ¨C 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 ¨C 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.
Normal Capnograph (Figure 1): A to B = CO2 level at inspiration. This should be 0 if on 100% oxygen.
Normal Capnograph (Figure 2): C to D is the expiratory phase. This is the phase in which most alveolar gases are exhaled e.g. ETCO2. ETCO2 = 35-45 mmHg
Hypoventilation (Figure 3) is caused by a decrease in respiration rate or tidal volume (depth). It will eventually result in hypercapnia e.g. ETCO2>60mmHg and hence respiratory acidosis. Hypercapnia should be treated early. It commonly occurs when fentanyl CRIs are used to decrease MAC or to treat introperative pain.
Hyperventilation (Figure 4) is caused by an increase in the respiration rate or tidal volume (depth). Hyperventilation will eventually result in hypocapnia e.g. ECTCO2<35mmHg. It can be used to treat hypercapnia (increased CO2). Hyperventilation can occur if a patient is painful while under anesthesia.
Rebreathing CO2 (Figure 5) can be caused by a faulty expiratory valve, inadequate inspiratory flow rate or expired soda lime. Small increases in inspiratory CO2 (4-5 mmHg) are seen when the soda lime has expired. Larger inspiratory CO2 values (10mmHg or higher) occur with a faulty expiratory valve or inadequate fresh flow rate.
Inadequately inflated ET tube (Figure 6): CO2 is escaping around an inadequately inflated endotracheal tube. The endotracheal tube cuff should be checked for leakage (usually this can be heard as a "hiss" upon expiration as well as an inhalant "smell" coming from pharyngeal area). Replace the endotracheal tube if reinflation of the cuff does not solve the problem.
Obstructions (Figure 7) result when the endotracheal tube becomes occluded (FB, kinked, secretion build up etc.) or obstruction of the expiratory limb of the breathing circuit occurs. Replace the endotracheal tube if it is occluded by secretions or a FB is suspected.
How Do We Know When to Ventilate? Respiration rate is NOT always a good indicator of ventilation nor is respiratory depth. So, if we can monitor ETCO2 as well as interrupt the capnograph this will give us vital information as to when to manually or mechanically assist ventilation.
Manual or mechanical ventilation can be used to assist patients during the anesthetic period. When manual IPPV is being utilized, adequate ventilation is achieved by reading the peak inspiratory pressure (PIP) on the anesthetic manometer. Manual IPPV is labor intensive and the pop off valve can accidently be left closed creating barotrauma. However, pop off valve "safety adaptors" are available to prevent pop off valve accidents. When performing manual IPPV it is important to remember to reduce the anesthetic setting on the vaporizer to prevent accidental patient overdosing of gas inhalants. Mechanical ventilation is achieved by placing a patient on a ventilator. Tidal volume and respiration rate are set so that a normal ETCO2 is achieved. Mechanical ventilators provide a more accurate and timed delivery of O2 and anesthetic and it allows other tasks to be performed by the RVT during surgery. Again, reduce the anesthetic setting to prevent accidental patient overdosing of inhalant anesthetics.
When performing manual ventilation, PIP of 10-15cm of H20 should be achieved by monitoring the anesthetic machine manometer. Higher PIP will result in increased tidal volume. A respiration rate of 6-14 breaths/minute is recommended. Remember: IPPV is more efficient than a patient ventilating itself so vaporizer settings should be decreased. The inspiratory (I): expiratory ratio (E) on a mechanical ventilator should be set at 1:2 ¨C 1:4 to allow for adequate patient expiration i.e. expiratory times should be longer than inspiratory time. If inspiratory time or peak inspiratory pressure is too long or too high BP can drop due to increases in intrathoracic pressure that will decrease venous return to the heart. Most mechanical ventilators have an I:E preset at 1:2. Adjust minute volume (RR ¡Á TV) or PIP if necessary to correct intrathoracic pressure, hypocapnia or hypercapnia.