Capnography clinical aspects

Capnography is not as boring or irrelevant, as you might think. The range of clinical applications for measuring carbon dioxide is remarkable that not only ventilation, but metabolism, circulation and other things. The term describes "the ongoing registration of CO2 partial pressure [PP] in inspiratory and expiratory gases. It differs from capnometry in that it provides a continuous [graphic] depiction of ppCO2 ', while the latter simply provides a numeric reading of inspired and expired ppCO2 . Capnography In ppCO2 can be plotted against either time (more common and simpler) or volume (more complicated to monitor and interpret, but gives more information).

The development of capnography involved fascinating scientific study, ingenuity and resistance to change. John Scott Haldane (1860-1936) first described by a CO2 analyzer in the early 20 century. He built an apparatus in which a sample of gas kept at a constant temperature and pressure, was dragged through a range of absorbents and the concentration was derived from the decrease in volume.

Even more ingenious was the discovery that sound can be used to determine the partial pressure of the gas. The so-called photo-acoustic measurement, a beam of infrared light is pulsed through a sample, and the frequency of pressure change (proportional to the amount of CO2 present) is detected acoustically with a microphone.

Raman scattering describes the effect that the incident ultraviolet and visible light have rotational and vibrational energy of molecules that absorb it. An application for this is the measurement ppCO2. Possibly more interesting is that it also explains why the sky is blue (the shorter blue wavelengths of sunlight are mostly dispersed, thereby increasing the intensity of the color as it reaches the Earth).

Despite this long history of clinical capnography was introduced in the U.S. as late as in 1978. Five anesthetist attended the launch at the World Congress in intensive care medicine and two of them concluded that it would prove to be "of little value." But now a Canadian malpractice insurer offers massive discounts for anesthetist who exploit capnography. It is illuminating not only because it shows how important the technology is now, but more that it is worrying that the anesthetist is still not measuring end-tidal carbon dioxide.

The reason for malpractice insurers consider capnography to be so important is its ability to distinguish between tracheal and oesophageal intubation. Oesophageal intubation is still causing animal deaths and morbidity. A report by the American Society of Anesthesiologists Committee on professional liability was that the presence of breath sounds on auscultation were documented in 18 out of 29 cases of undetected oesophageal intubation, which caused the injury. With better technology, this potentially life-saving measurement tool available outside the operating suite. This may be in the form of miniature electronics and semi-quantitative colorimetric units (glorified litmus paper). The latter is impressively accurate in Prehospital setting.

Another area where capnography may be recruited away from the theaters is that the notoriously bad-sedation by non-anesthetist. An audit of upper gastrointestinal endoscopy of Quine and employees (Gut 1995; 36:462-7) showed alarming amounts of benzodiazepines and opioids sheep and shocking figures on morbidity and mortality. Very little control was employed, despite strong respiratory inhibition, and even the more widespread availability of pulse oximetry is unlikely to completely resolve the situation. The problem lies in that it is difficult to understand the distinction between oxygenation and ventilation. This can be illustrated by brainstem death testing. Ventilation is stopped to allow ppCO2 to rise above 6.65 kPa, but oxygenation is maintained by supplying a continuous flow of oxygen to the lungs via a suction catheter.

With sedation problems are threefold. First, despite adequate oxygenation of CO2 levels may rise, leaving the patient with negative consequences, ranging from anesthesia to increased intracranial pressure. Secondly, little oxygen goes far in masking imminent respiratory decompensation. With capnography this should be picked up (and thus prevented) much earlier. Thirdly, the issue of late complications. The availability of opioid and benzodiazepine antagonists have much shorter half-life than the medicines they are supposed to counteract. It would be better to titrate the effect of sedative drugs for the treatment of end-tidal CO2, and thus use less of them. Sampling during sedation is simple. In my own practice I use 'split' nasal prongs-one line 'in' for oxygen, a second "out" for CO2 sampling.

List of anesthetic and non-anesthetic uses of capnography is long and includes the provision of respiratory physiology, metabolism, and requirements for resuscitation equipment, the study of panic disorder and asthma, and even within the context of carbon dioxide therapy (a treatment for certain sleep disorders with abnormal patterns of breathing).