Vol. 13 •Issue 5 • Page 14
Role of Capnography Grows in the Sleep Lab
The use of capnography is an essential component in hospitals for anesthetized or critically ill patients. And in the sleep lab, capnography has become the standard of care for recording patients with pulmonary disease or pediatric studies.
Capnography is similar in technology to oximetry; both work on the principle of selective light absorption.
Exhaled air is sampled via a nasal or nasal/oral cannula connected to an analyzer. A vacuum pump draws the exhaled air into a chamber where infrared light is passed through the sample. CO2 molecules absorb a selective portion of the light, and the remaining light passes to a photodetector. The amount of light absorbed is used to calculate the amount of CO2 in the sample.
Because the amount of CO2 significantly varies with inhalation and exhalation, the varying quantity in the sample can produce a waveform that can represent airflow.
The waveform begins with an initial flat baseline, representing gases exhaled from the mechanical and alveolar dead spaces. As exhalation continues, the sample contains a mix of air from the anatomic dead spaces and the alveoli, and the waveform rapidly rises from the baseline.
Toward the end of exhalation, a slowly rising plateau is seen representing alveolar gas. The waveform reading at the end of the plateau is representative of alveolar gas or ETCO2.
Inspiration contains air that’s essentially free of CO2, so there’s a sharp, rapid decline to the baseline.
CAPNOGRAPHY WITH COPD
The benefits of using capnography in adults with chronic obstructive pulmonary disease are best understood by looking first at the effects of sleep on breathing, with or without COPD.
Sleep results in decreased ventilatory drive, increased upper airway resistance, pooling of secretions, and decreased functional residual capacity with increased ventilation-perfusion ratio (V/Q) mismatch. REM sleep, in particular, involves upper airway muscle atonia, decreased activity of accessory muscles of respiration, and even further decreases in ventilatory responses to hypercapnea and hypoxia.
These changes aren’t clinically significant for those without pulmonary disease, but they may result in profound hypoxemia in patients with COPD who have PaO2 levels on the steep portion of the oxyhemoglobin dissociation curve.
An “overlap syndrome” describes those patients with both COPD and obstructive sleep apnea syndrome. Patients with overlap syndrome or obesity hypoventilation syndrome sometimes present with hypercapnic respiratory failure.
If OSAS is recognized, diagnosed and effectively treated in these patients, the result is a reduction of daytime PCO2, improvement in oxygenation, and cor pulmonale. The use of capnography in the sleep lab may help to identify these patients before they require admission to the hospital.
The capnography waveform is often abnormal in patients with COPD with a longer expiratory slope, elevated ETCO2 values, and a rounded peak instead of a well-defined plateau and peak.
The displayed ETCO2 values may be lower than arterial CO2 in the presence of V/Q mismatch. The difference between PaCO2 and ETCO2 is approximately 2 to 3 mm Hg with normal physiology and correct sampling techniques. This value is known as a-ADCO2, and it may be increased slightly with shunt perfusion and increased significantly with dead space ventilation.
In such cases, transcutaneous monitoring of CO2 may be more accurate. Transcutaneous pCO2 monitoring (TcCO2) utilizes a small electrode, an electrolyte and a heating element. CO2 passes through the membrane of the electrode and changes the pH of the electrolyte. The purpose of the heating element is to control the value of the output and to provide vasoconstriction.
Calibration of the TcCO2 electrode requires exposure to two known values of pCO2. TcCO2 measurements aren’t affected by V/Q mismatches and may be more accurate than ETCO2 in some patients.
While a capnogram waveform isn’t provided, the trend can be very useful, especially in cases of REM-related hypoventilation. Care must be taken with exposure of the skin to the heating element over the course of the sleep study. Newer equipment provides a simple calibration process and an ability to monitor and adjust the temperature of the heating element.
APPLICATIONS FOR PEDIATRIC PSG
Children also may exhibit a variety of sleep-disordered breathing abnormalities, and calibrated ETCO2 measurements can effectively detect the CO2 retention associated with apnea or prolonged hypoventilation. In the pediatric population, demonstration of obstructive hypoventilation instead of clearly defined apneas or hypopneas can be a common occurrence.
Accurate ETCO2 measurement can reveal the elevation in ETCO2 values seen during these and other respiratory patterns as the child cycles through REM and non-REM sleep.
In general terms, normative values of ETCO2 usually range from 35 to 45 mm Hg. One definition of obstructive hypoventilation is a maximum ETCO2 > 53 mm Hg or ETCO2 > 45 mm Hg for > 60 percent of total sleep time or SaO2 < 89 percent.
Because children may not exhibit the electroencephalogram arousals common in adults with SDB, ETCO2 monitoring will reveal increased trends often associated with these respiratory abnormalities.
“ETCO2 monitoring is an invaluable tool in our pediatric population since children can often retain CO2 even during subtle respiratory impairments,” said Patrick Sorenson, RPSGT, the chief technologist at Children’s Hospital in Boston. “ETCO2 monitoring can help to assess the overall severity of apneic events, and this measure of severity can help sway a decision about whether or not a surgical procedure such as adenotonsillectomy is warranted.”
In order to better assess trends throughout the night, Sorenson suggests that ETCO2 be monitored on a strip-chart run at a speed of 3 cm/hour.
RECENT ADVANCES
Consider several factors when choosing capnogram equipment for use in the sleep lab, keeping in mind that the monitor may need to be located in the patient’s room. In order to minimize the possibility of disturbing the person’s sleep, technologists requested some modifications, and manufacturers have responded to our unique needs.
Some monitors now have an illuminated display that can be dimmed or turned off. Adding mufflers to the internal pump needed to sample exhaled air has reduced the noise level on some units.
Sampling and display of ETCO2 is increased on some units to each breath compared to an average of four breaths on standard monitors. Some units monitor and display oximetry as well as ETCO2. For oximetry, sleep lab application requires a sampling rate of several seconds, rather than the typically longer sampling rate of units designed for use in the intensive care unit or patient floor.
The unit selected should be easily calibrated to the digital acquisition system used in the sleep lab, and the calibration should be valid from recording to recording.
Lastly, the cannula size should be patient-appropriate. This is especially important in pediatric polysomnography — the use of an adult cannula may occlude a child’s nares. Some cannulas are available in a style that permits dual monitoring and application of O2.
TECHNICAL CHALLENGES
Some technical challenges exist in the use of “sidestream” monitoring of ETCO2, though.
When a nasal or nasal/oral cannula is used, room air may be mixed with the sample, potentially lowering the displayed CO2. Capturing exhaled air from the mouth is difficult, even with a properly placed nasal/oral cannula. Condensation of water vapor and respiratory secretions may lead to dampening of the signal.
As with other sensors used to represent airflow, the cannula may be dislodged by the patient’s tossing and turning in bed or inadvertent pulling at the cannula.
This is especially problematic in the pediatric population and requires the recording technologist to be vigilant. A consistently low CO2 value may indicate a displacement of the cannula.
With the growing prevalence of COPD and an increasing number of labs performing pediatric PSG on at least an occasional basis, all sleep facilities may want to consider the addition of capnography.
Malloy is the lead instructor of the Atlanta School of Sleep Medicine and Technology. He can be reached at [email protected].
This issue of Sleep Tracks is produced in conjunction with the Association of Polysom.nographic Technologists. For more information on APT activities, offerings and membership, call (708) 492-0796 or write to APT Executive Office, One Westbrook Corporate Center, Suite 920, Westchester, IL 60154. Also visit the APT online at www.aptweb.org or e-mail [email protected].