Atlas of Procedures in Neonatology, 4th Edition

Physiologic Monitoring

10

Capnography

  1. Kabir Abubakar
  2. Definitions

Capnography refers to the continuous analysis and recording of carbon dioxide concentrations in respiratory gases (PetCO2), whereas capnometry refers to measurement or analysis alone of carbon dioxide concentrations without a continuous written record or waveform. A device that measures and displays the breath-to-breath numeric values of CO2 is referred to as capnometer, whereas a device that also displays the waveform of CO2 during the respiratory cycle is called a capnograph.

  1. Purpose
  2. Noninvasive continuous analysis and recording of CO2during tidal breathing (1,2 and 3)
  3. End-tidal CO2-level monitoring (PetCO2)
  4. Respiratory rate monitoring
  5. Background
  6. Most capnography employs infrared technology, which is based on the infrared absorption of CO2.
  7. Capnographic devices incorporate one of two types of analyzers: mainstream and sidestream.
  8. With a mainstream analyzer, the sensor is attached directly to an optical adapter that is inline with the endotracheal tube (Fig. 10.1).
  9. With a sidestream analyzer, a low-dead-space adapter is placed inline with the endotracheal tube and gas is aspirated continuously to the analyzer for measurement (Fig. 10.2).
  10. For both mainstream and sidestream capnography, the only adapters that should be used for newborns are ones specifically designed for neonatal application.
  11. Indications
  12. Evaluation of the exhaled CO2, specifically end-tidal CO2, which is the maximum partial pressure of CO2exhaled during a tidal breath just prior to the beginning of inspiration (designated PetCO2) (4,5) (Fig. 10.2)
  13. Monitoring the severity of pulmonary disease and evaluating response to therapy, particularly therapy intended to change the ratio of dead space to tidal volume (6) or to improve the matching of ventilation to perfusion (V/Q)(7)
  14. Determining that tracheal rather than esophageal intubation has taken place (low or absent cardiac output may negate its use for this indication)
  15. Continued monitoring of the integrity of the ventilatory circuit (8)
  16. More accurate and continuous reflection of CO2elimination (9,10)
  17. Graphic evaluation (11)
  18. Use of capnography in combination with pulse oximetry can allow for additional monitoring to detect airway obstruction or subclinical degrees of respiratory depression in the sedated patient (12).
  19. Evaluation of surfactant efficacy can be augmented by use of dead-space and capnography measurements (13).
  20. Sidestream technology can also be used with nasal prongs in spontaneously breathing patients.
  21. Contraindications

There are no absolute contraindications to capnography in the mechanically ventilated infant.

  1. Limitations (10,14 15,16 and 17)
  2. The composition of the respiratory gas mixture may affect the capnogram; the infrared spectrum of CO2has some similarities to the spectra for both oxygen and nitrous oxide (most available capnographs have a correction factor already incorporated into the calibration).
  3. Rapid changes in respiratory rate and tidal volume may lead to measurement error, depending on the frequency response of the capnograph; different capnographs may have different frequency responses.
  4. Contamination of either the monitor or the sampling system by secretions, blood, or condensation may lead to inaccurate results.

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  1. Large dead space affects PetCO2measurements. The difference between PetCO2and PaCO2 increases as dead-space volume increases and may vary within the same patient over time.
  2. The end-tidal CO2adapter can add to the dead space and resistance of the respiratory circuit, particularly in small infants.
  3. PetCO2measurements may not provide an accurate correlation with PaCO2in small preterm infants with nonhomogenous lung disease and therefore cannot be substituted for PaCO2 analyses in preterm infants during this critical period (18,19).
 

FIG. 10.1. Mainstream capnographic monitor unit and clip-on sensor assembly. (Courtesy of Respironics, Inc., Murrysville, PA, USA.)

 

FIG. 10.2. Sidestream capnographic monitor unit with sample line. Note end-tidal CO2 value and wave. (Courtesy of Oridion Capnography, Needham, MA, USA.)

  1. Equipment
  2. For mainstream capnography, an airway adapter is needed, along with a reusable sensor attachment.
  3. For sidestream capnography, an airway adapter with sampling tube is used (Fig. 10.3).
  4. Capnogram
 

FIG. 10.3. Infant sidestream low—dead-space adapter with sample tubing. (Courtesy of Oridion Capnography, Needham, MA, USA.)

  1. Precautions
  2. In the mainstream adapter, prevent condensation in the airway adapter.
  3. In the sidestream adapter, prevent fluid (water) buildup in the sample tube.
  4. For both mainstream and sidestream, when adding bulk to the endotracheal tube, more attention should be given to properly securing the position of the tube.

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  1. Tidal volume measurements may be affected if the end-tidal CO2adapter is placed between the endotracheal tube and the ventilator flow sensor.
 

FIG. 10.4. Infant sidestream low—dead-space adapter (see arrow) inline with endotracheal tube.

  1. Technique
  2. Familiarize yourself with the system before proceeding.
  3. Attach the adapter inline with the endotracheal tube and the ventilator T piece (both sidestream and mainstream) (Fig. 10.4).
  4. For mainstream capnography, connect the sensor to the airway adapter.
  5. For sidestream capnography, connect the sampling tube to the analyzer.
  6. Complications

Capnography is a safe, noninvasive test, with some limitations, depending on the type of capnography that is employed (2).

  1. With mainstream analyzers, the use of too large an airway tube adapter together with the weight of the probe may introduce an excessive amount of bulk and weight to the endotracheal tube.
  2. With sidestream capnography, a low—dead-space adapter allows for less bulk and weight; however, care must be taken not to pull excessively on the sample line that is going to the measurement instrument.

References

  1. Campbell RS, Branson RD, Burke W, et al. Capnography/capnometry during mechanical ventilation. Respir Care.1995; 40:1321.
  2. Hess DR, Branson RD.Noninvasive respiratory equipment. In: Branson RD, Hess DR, Chatburn RL, eds. Respiratory Care Equipment.Philadelphia: Lippincott; 1994:184.
  3. Block FE, McDonald JS.Sidestream versus mainstream carbon dioxide analyzers. J Clin Monit. 1992;8:139.
  4. Carlon GC, Ray C, Miodownik S, et al. Capnography in mechanically ventilated patients. Crit Care Med.1988;16:550.
  5. Gravenstein N, Good ML.Noninvasive assessment of cardiopulmonary function. In: Civetta JM, Taylor RW, Kirby RR, eds. Critical Care. Philadelphia: Lippincott; 1988:291.
  6. Yamanaka MK, Sue DY.Comparison of arterial-end-tidal PCO2 difference and dead space/tidal volume ratio in respiratory failure.Chest. 1987;92:254.
  7. Burrows FA.Physiologic dead space, venous admixture, and the arterial to end-tidal carbon dioxide difference in infants and children undergoing cardiac surgery. Anesthesiology. 1989;70:219.
  8. Eichhorn JH, Cooper JB, Cullen DJ, et al. Standards for patient monitoring during anesthesia at Harvard Medical School. JAMA.1986;256:1017.
  9. Morley TF, Giamo J, Maroszan E, et al. Use of capnography for assessment of the adequacy of alveolar ventilation during weaning from mechanical ventilation. Am Rev Respir Dis.1993;148:339.
  10. Rozycki HJ, Sysyn GD, Marshall MK, et al. Mainstream end-tidal carbon dioxide monitoring in the neonatal intensive care unit.Pediatrics.1998;101:648.
  11. Bhavani-Shankar K, Moseley H, Kumar AY, et al. Capnography and anesthesia. Can J Anesth.1992;39:617.
  12. Tobias J.End-tidal carbon dioxide monitoring during sedation with a combination of midazolam and ketamine for children undergoing painful, invasive procedures. Pediatr Emerg Care. 1999;15:173.
  13. Wenzel U, Rudiger M, Wagner M, et al. Utility of deadspace and capnometry measurements in determination of surfactant efficacy in surfactant-depleted lungs. Crit Care Med.1999;27:946.
  14. Graybeal JM, Russell GB.Capnometry in the surgical ICU: an analysis of the arterial-to-end-tidal carbon dioxide difference. Respir Care. 1993;38:923.
  15. Hess D.Capnometry and capnography: technical aspects, physiologic aspects, and clinical applications. Respir Care. 1990;35:557.
  16. Isert P.Control of carbon dioxide levels during neuroanesthesia: current practice and an appraisal of our reliance upon capnography.Anaesth Intensive Care. 1994;22:435.
  17. Saili N, Dutta AK.End tidal carbon dioxide monitoring—its reliability in neonates. Indian J Pediatr. 1997;64:389.
  18. Aliwalas LL, Noble L, Nesbitt K, et al. Agreement of carbon dioxide levels measured by arterial, transcutaneous and end tidal methods in preterm infants < or = 28 weeks gestation. J Perinatol.2005;25(1):26.
  19. Tingay DG, Stewart MJ, Morley CJ.Monitoring of end tidal carbon dioxide and transcutaneous carbon dioxide during neonatal transport. Arch Dis Child Fetal Neonatal Ed. 2005;90(6):F523.