Heidi L. Shafford, DVM, PhD, DACVA PO Box 418, Clackamas, OR 97015 Ph: 503.805.5515 Fax: 503.656.2762 www.vetanesthesiaspecialists.com

Veterinary Anesthesia Specialists, LLC 2012 HL Shafford

Capnography is Cool

Capnography is a true respiratory function monitor and can allow anesthetists to recognize apnea, hypoventilation, equipment malfunction and endotracheal tube obstruction. While pulse oximetry aids in detection of hypoxemia, these monitors are of limited usefulness in anesthetized animals because patients are receiving 100% oxygen.

Impaired respiratory function is a common anesthetic complication because general anesthetics are potent respiratory depressants. As anesthetists, we are responsible for supporting homeostasis, or optimal organ function, in our anesthetized patients. Thus, routine monitoring of respiratory function with a capnograph can enhance anesthetic management in pets. Capnography & hypoventilation: Monitoring CO2 is currently the most valuable technique for evaluating our anesthetized patients respiratory function. Normal arterial CO2 is 35 45 mmHg. Hypoventilation is defined as an arterial CO2 greater than 45 mmHg, meaning that CO2 is accumulating in the patients blood due to impaired removal or increased production. Because most anesthetics are respiratory depressants, hypoventilation due to impaired removal of CO2 is common in anesthetized patients. Mild elevations in CO2 (in the range of 45 55 mmHg) are associated with sympathetic stimulation and increased blood pressure. However, as CO2 rises, blood pH decreases resulting in respiratory acidosis. Other adverse physiologic consequences of abnormally high levels of CO2 include tachycardia, vasodilation, increased intracranial pressure, as well as respiratory, cardiovascular and central nervous system depression. Capnographs measure CO2 in exhaled and inhaled gases and display a waveform of the partial pressure of CO2 throughout the breath cycle. End-tidal CO2 monitoring is a non-invasive means of estimating arterial CO2. Under most circumstances (healthy pet, no chest surgery) end-tidal CO2 is typically 5 10 mmHg less than arterial CO2. The CO2 waveform is a valuable tool for detecting leaks in the anesthetic system, rebreathing of CO2 and endotracheal tube obstruction. Because peripheral blood laden with byproducts of metabolism has to be pumped to the lungs in order for CO2 to diffuse across the alveoli and be exhaled, capnography is also useful for evaluating cardiovascular function. There are two different types of capnographs. Side-stream capnographs withdraw a continuous sample of respired gases from the (side of the) anesthetic circuit and transport it to a monitor for analysis. Mainstream capnographs have a sensor that measures CO2 in respired gases in the (main stream of the) anesthetic circuit. Regardless of monitor type, purchase one that displays a waveform! Because it takes time for exhaled gases to reach a side-stream monitor, the measurements and display are delayed. Often 100 200 mls/min are withdrawn from the anesthetic circuit by side-stream capnographs making these monitors less appropriate for very small patients (i.e., kittens). In addition, side-stream capnographs require scavenging of anesthetic gases from the monitor, regular calibration and a moisture trap to remove water vapor from respired gases. On the plus side, side-stream capnographs and their adaptors are often less expensive to purchase.

In mainstream CO2 monitors, the measurements are performed in the sensor at the end of the endotracheal tube and, thus, have a rapid response time are more accurate for rapid respiratory rates. No moisture trap is required for mainstream monitors and some sensors add very little dead space to the anesthetic circuit making them more appropriate for small patients. It should be noted that the expensive working end of the mainstream monitor is at the end of the ETT and should be protected.

Veterinary Anesthesia Specialists, LLC 2012 HL Shafford

Treatment for apnea:

Manual or mechanical ventilation.

Breathing for an anesthetized patient will not make the patient dependent upon further ventilation.

Treatment for hypoventilation (End-tidal CO2 greater than 55 mmHg):

Manual or mechanical ventilation is the way to treat hypoventilation. Treatment for rebreathing of CO2 (Inspired CO2 greater than 10 mmHg): Anesthesia machines and circuits are designed to prevent patients from rebreathing CO2 IF THEY ARE USED APPROPRIATELY. Therefore, if InspCO2 is elevated, something has gone awry with the anesthesia delivery system. The most likely reasons patients are rebreathing CO2:

Inadequate oxygen flow rates if patient on a Non-Rebreathing circuit o Relatively high O2 flow rates in a Non-Rebreathing circuit prevent patients from

rebreathing CO2

Non-functional one-way valves o Watch the valve leaflets as the patient breathes in and out o There should be a valve leaflet present in the inspiratory and expiratory limbs of the

circuit o Both should flutter and NOT stick to the domes (plastic covering) o If the valves are not working

Trade out the anesthesia machine OR put the patient on a Non-Rebreathing circuit with Oxygen flow rate at 200

ml/kg/min

Depleted CO2 absorbent granules o The best thing to do is change anesthesia machines o OR put the patient on a Non-Rebreathing circuit with Oxygen flow rate at 200 ml/kg/min o DO NOT attempt to change the CO2 absorbent granules while anesthetizing a patient

Excessive equipment dead space (adaptors & excessive length at the end of endotracheal tube) o Minimize excessive length of endotracheal tubes extending out beyond patients mouths o Keep to a minimum the number of adaptors attached to the endotracheal tube

Definition of Dead Space:

Space where there is mixing of inspired and expired gases in the absence of gas exchange. o Anatomic dead space: trachea, main stem bronchi, conducting airways o Equipment dead space: length (and volume) of ETT that extends beyond the nose, the

end of the anesthesia breathing circuit, elbow adapters, ETCO2 and other monitoring devices connected to the ETT

Capnography waveform interpretation (see Common capnograph waveforms below) The waveform of a capnograph is really cool, and can help you identify apnea, hypoventilation, hyperventilation, equipment malfunction and endotracheal tube obstruction.

Heidi L. Shafford, DVM, PhD, DACVA PO Box 418, Clackamas, OR 97015 Ph: 503.805.5515 Fax: 503.656.2762 www.vetanesthesiaspecialists.com

Veterinary Anesthesia Specialists, LLC 2012 HL Shafford

Inspiration

The normal capnograph is characterized by a square waveform that plateaus at 30 40 mmHg during expiration and returns to 0 mmHg during inspiration.

Expiration

0

40

CO

2m

mH

g

20

60

Hypoventilation is characterized by higher than normal

end-tidal CO2, indicating respiratory depression and

inefficient gas exchange. Implement treatment for

hypoventilation when ETCO2 is greater than 55 60 mmHg: decrease inhalant and/or initiate mechanical

ventilation.

Rebreathing of CO2 is occurring when the baseline is

greater than 0 mmHg. This indicates 1) malfunctioning

one-way valves, 2) exhausted soda-sorb, or 3) low

oxygen flow rates in a non-rebreathing circuit.

Rebreathing of CO2 leads to acidosis and increased

anesthetic depth.

A leak in the anesthetic circuit is indicated when the

down-slope of the expiratory wave is flattened.

Common sites for leaks are 1) endotracheal tube cuff,

2) junction of Y-piece & endotracheal tube, 3) junction of

Y-piece & anesthetic machine, and 4) soda-sorb

canister seal.

Occlusion of the endotracheal tube is characterized

by a flattening of the upslope of the expiratory wave

creating the appearance of shark fins. Check to see that the endotracheal tube is not kinked and then

aspirate the endotracheal tube or remove tube and re-

intubate.

Spontaneous breathing during mechanical

ventilation (i.e., patient is breathing around, or

bucking the ventilator). This occurs if ETCO2 is high and/or if patient is at a light plane of anesthesia.

Increase anesthetic depth or increase volume/rate of

ventilation to prevent spikes in pulmonary pressure from

occurring.

A) Cardiac oscillations may be noted in large patients.

The beating heart pushes on the lungs and displaces

small amounts of air, resulting in small peaks in the CO2waveform coincident with heart rate. This is a normal

finding. B) Dilution of CO2 sample due to high O2 flow,

rapid respiratory rate or slow sampling rate.

A B

Common capnography waveforms

Veterinary Anesthesia Specialists, LLC 2012 HL Shafford

Pulse oximetry & hypoxemia: The pulse oximeter measures oxygen saturation by detecting the differential absorption of light by oxy- and deoxy-hemoglobin. Many pulse oximeters also display a waveform, or plethysmograph, that shows pulse quality and pulse rate. In awake and anesthetized patients, oxygen saturation should remain greater than 95%. A pulse oximeter reading of less than 95% is associated with hypoxemia. Inadequate oxygenation can lead to impaired organ function, ischemia and tissue damage. In anesthetized patients, lower than normal pulse oximetry readings may indicate inadequate/exhausted oxygen supply, endo-bronchial intubation, airway obstruction, closed pop-off valves, aspiration of gastric contents or hypoventilation in patients breathing room air. Treatment for hypoxemia:

Correcting the cause of hypoxemia and providing supplemental oxygen are the most important means of treating hypoxemia. Most common causes of hypoxemia:

o Inadequate/exhausted oxygen supply o Endo-bronchial intubation o Airway obstruction o Closed pop-off valves o Aspiration of gastric contents or o Hypoventilation in patients who are breathing room air.

Even mild to moderate hypoventilation can lead to hypoxemia in the absence of supplemental oxygen. For this reason, hypoxemia is most likely to occur when patients are not receiving supplemental oxygen: following pre-anesthetic drug administration, during injectable anesthesia, and while recovering from inhalant anesthesia. The use of pulse oximetry to monitor oxygenation in patients during these susceptible times will allow hypoxemia to be detected and facilitate appropriate intervention. The majority of peri-anesthetic deaths occur during recovery a time when patients no longer benefit from intensive monitoring and supplemental oxygen. Continued monitoring of patients recovering from anesthesia, particularly with pulse oximetry, has great potential to reduce peri-anesthetic mortality. Limitations of pulse oximetry:

Anesthetized patients receiving supplemental oxygen can be apneic for several minutes before the pulse oximeter will indicate that oxygen saturation has fallen

Patients receiving supplemental oxygen can hypoventilate (even dramatically) and still have normal pulse oximeter readings

Hypotensive patients can have normal pulse oximeter readings

Anemic patients can have normal pulse oximeter readings yet have compromised oxygen delivery because they lack hemoglobin to carry the oxygen

References, resources and supplemental reading:

Capnography.com

Brock N. Veterinary anesthesia update, 2nd edition. AAHA Press, 2007. Available through www.nancybrockvetservices.com or AAHA or VIN

Gaynor JS & Muir WW. Handbook of Veterinary Pain Management, 2nd Edition. Mosby, Inc., 2008.