Clinical Aspects

Bhavani Shankar Kodali MD

Hypermetabolic states

Dangerous hypermetabolic conditions such as malignant hyperthermia, thyrotoxic crisis, and severe sepsis, can be detected by C02 monitoring. Increased metabolic rates cause greater C02 production, which can cause PETCO2 to increase. An increasing PETCO2 may, therefore, be an early warning sign of an impending hypermetabolic crisis.1

Cardiopulmonary resuscitation

End-tidal C02 monitoring during closed chest compression is one of the most exciting recent developments in CPR. It holds the promise of making available information about the effectiveness of resuscitative efforts, that, heretofore, have been unavailable. It is non-invasive, easy to apply to the intubated patient and the theory of its use during CPR is relatively simple. During closed chest compression the blood flow to the lungs is low so that relatively few alveoli are perfused. Since tidal volumes delivered with a resuscitation bag tend to be large, many alveoli are ventilated that are not perfused and consequently, the PETCO2 is low. If the blood flow to the lungs improves, more alveoli are perfused and PETCO2 will increase. Under these circumstances the C02 presentation to the lungs is the major limiting determinant of PETCO2 and it has been found that PETCO2 correlates well with measured cardiac output during resuscitation.2-8 Therefore PETCO2 can be used to judge the effectiveness of resuscitative attempts and thus lead to changes in technique that could improve the outcome.9 Further, the PETCO2 may have a prognostic significance. It has been observed that non-survivors had lower PETCO2 than survivors and no patient with PETCO2 <10 mmHg could be successfully resuscitated.10,11

High frequency jet ventilation (HFJV)

Assessment of the adequacy of HFJV is usually done by a series of arterial blood gas measurements. Monitoring PETCO2 can be used successfully to determine PaC02 levels during HFJV. This is done by delivering a single breath of large tidal volume and measuring PETCO2 during brief interruption of HFJV. If PaC02 can be measured simultaneously by arterial puncture, then (a-ET)PCO2 can be determined and subsequent monitoring of HFJV can be done by measuring PETCO2 in periodically given single large breaths.12

(a-ET)PC02 and PEEP

Positive end expiratory pressure (PEEP) can be applied to improve oxygenation, when hypoxemia is caused by acute alveolar oedema, or in early adult respiratory distress syndrome (ARDS). Certain levels of PEEP (the inflection pressure on pressure volume compliance curve) must be reached in any particular patient before improvement in oxygenation is achieved. When oxygenation is at its best (optimum PEEP) the (a-ET)PCO2 is least. As the level of PEEP is increased beyond this the (a-ET)PC02 increases again and oxygenation worsens. Therefore it has been suggested that (a-ET)PC02 can be used as a sensitive indicator in order to titrate PEEP in patients with early ARDS or with alveolar oedema.13

Incidental Applications

Most of the capnographs measure and display inspiratory and end-expiratory oxygen, nitrous oxide and anesthetic agent concentrations or partial pressures in addition to CO2 measurements. Furthermore, capnographs using RAMAN or Mass Spectrography measure and display nitrogen concentration. These additional attributes bring the benefits of monitoring oxygen, nitrous oxide, nitrogen (detection of air embolism) and anesthetic agents concentration (vaporizer function, inadvertent administration of anesthetic vapor, information of uptake and elimination of anesthetic agents, monitoring low flow anesthesia and information on depth of anesthesia).

References:

1. Baudendistel L, Goudsouzian N, Cote C, Strafford M. End-tidal CO2 monitoring: its use in the diagnosis and management of malignant hyperpyrexia. Anaesthesia 1984;39:1000-3.

2. Falk JL, Rackow EC, Weil MH. End-tidal carbon dioxide concentration during cardiopulmonary resuscitation. N Engl J Med 1988;318:607-11.

3. Jin X, Weil MH, Tang W, Povoas H, Pernat A, Xie Jm Bisera J. End-tidal carbon dioxide as a noninvasive indicator of cardiac index during circulatory shock. Crit Care Med 2000;28(7):2415-9.

4. Isseries SA, Breen PH. Can changes in end-tidal PCO2 measure changes in cardiac output? Anesth Analg 1991;73(6):808-14.

5. Idris AH, Staples ED, ‘Brien DJ, Melker RJ, Rush WJ, Del Duca KD, Falk JL. End-tidal carbon dioxide during extremely low cardiac output. Ann Emerg Med 1994;23(3):568-72.

6. Lewis LM, Stothert J, Standevan J, Chandel B, Kurtz M, Fortney J. Correlation of end-tidal CO2 to cerebral perfusion during CPR. Ann Emerg Med 1992;21(9):1131-4.

7. Varno AJ, Morrina J, Civetta JM. Clinical utility of calorimetric end-tidal CO2 detector in cardiopulmonary resuscitation and emergency intubation. J Clin Monit 1991;7(4):289-93.

8.Steedman DJ, Robertson CE. Measurement of end-tidal carbon dioxide concentration during cardiopulmonary resuscitation. Arch Emerg Med 1990;7(3);129-34.

9. Orliaguet GA, Carli PA, Rozenberg A, Janniere D, Sauval P, Delpech P. End-tidal carbon dioxide during out-of-hospital cardiac arrest resuscitation: Comparison of active compression-decompression and standard CPR. Ann Emerg Med 1993;25(1):48-51.

10. Sanders AB, Karen KB, Otto CW, Milander MM, Ewy GA. End-tidal carbon dioxide monitoring during cardiopulmonary resuscitation. A prognostic indicator for survival JAMA 1989;262:1347-51.

11. Levine RL, Wayne MA, Miller CC. End-tidal carbon dioxide and outcome of out-of-hospital cardiac arrest. N Engl J Med 1997;337(5):301-6.

12. Mason CJ. Single breath end-tidal PCO2 measurements during high frequency jet ventilation in critical care patients. Anaesthesia 1986;41:1251-4.

13. Blanch L, Fernandez R, Benito S Mancebo J, Net A. Effects of PEEP on the arterial minus end-tidal carbon dioxide gradient. Chest 1987;92:451-4.