Capnography: Clinical Aspects
Authors: C M. Harper
Journal: Journal of the Royal Society of Medicine (2005)

As a subject capnography is neither as boring nor as irrelevant as you might think. The range of clinical applications for the measurement of carbon dioxide is remarkable, encompassing not just ventilation but metabolism, circulation and others besides. The term describes 'the continuous recording of CO2 partial pressure [pp] in inspiratory and expiratory gases'. It is distinct from capnometry in that it provides a 'continuous [graphical] depiction of ppCO2' whereas the latter simply provides a numerical reading of inspired and expired ppCO2. In capnography the ppCO2 can be plotted against either time (more common and simpler) or volume (more complex to monitor and interpret but provides more information).

The development of capnography involved fascinating scientific investigation, ingenuity and resistance to change. John Scott Haldane (1860–1936) first described a CO2 analyser in the early 20th century. He built an apparatus in which a sample of gas, kept at a constant temperature and pressure, was drawn through a series of absorbents and its concentration was derived from the diminution in volume.

Even more ingenious was the discovery that sound could be used to determine the partial pressure of the gas. In so-called photoacoustic 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 by a microphone.

Raman scattering describes the effect that incident ultraviolet and visible light have on the rotational and vibrational energy of molecules that absorb it. One application for this is in measuring ppCO2. Possibly more interesting is that it also explains why the sky is blue (the shorter blue wavelengths in sunlight are preferentially scattered, thereby increasing the intensity of that colour as it reaches the Earth).

Despite this long history, clinical capnography was introduced into the United States as recently as 1978. Five anaesthetists attended the launch meeting at the world congress of intensive care medicine and two of them concluded that it would prove to be 'of little value'. Yet now a Canadian malpractice insurer offers massive discounts for anaesthetists who utilize capnography. This is illuminating not only because it indicates how important the technique is now considered, but more so in that it implies, worryingly, that there are anaesthetists who still do not measure end-tidal carbon dioxide.

The reason why malpractice insurers consider capnography to be so important is its ability to distinguish between tracheal and oesophageal intubation. Oesophageal intubation still causes death and expensive morbidity. A report of the American Society of Anaesthesiologists committee on professional liability found that the presence of breath sounds on auscultation had been documented in 18 out of 29 cases of unrecognized oesophageal intubation that caused injury. With improving technology, this potentially life-saving measurement tool should be available outside the operating suite. This may be in the form of miniaturized electronics or even semi-quantitative colorimetric devices (glorified litmus paper). The latter are impressively accurate in the prehospital setting.

Another area where capnography might be employed away from theatres is that of sedation—notoriously badly done by non-anaesthetists. An audit of upper GI endoscopy by Quine and co-workers (Gut 1995;36:462–7) showed frightening quantities of benzodiazepines and opioids being given and shocking figures for morbidity and mortality. Very little monitoring was employed despite the use of strong respiratory depressants, and even the more widespread availability of pulse oximetry is unlikely to completely resolve the situation. The problem rests in the poorly understood distinction between oxygenation and ventilation. This can be illustrated by brainstem death testing. Ventilation is stopped to allow the ppCO2 to rise above 6.65 kPa, but oxygenation is maintained by delivering a continuous flow of oxygen to the lungs via a suction catheter.

With sedation the problems are threefold. First, despite adequate oxygenation the CO2 level can rise leaving the patient with adverse consequences that range from narcosis to raised intracranial pressure. Second, a little oxygen goes a long way in masking imminent respiratory decompensation. With capnography this should be picked up (and therefore prevented) much earlier. Third is the issue of late complications. The available opioid and benzodiazepine antagonists have much shorter half-lives than the drugs they are supposed to counteract. It would be better to titrate the effect of the depressant drugs against end-tidal CO2, thereby using 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.

As I indicated earlier, the list of anaesthetic and non-anaesthetic uses of capnography is long, encompassing determination of respiratory physiology, metabolism and the adequacy of cardiopulmonary resuscitation, the investigation of panic disorders and asthma and even in conjunction with carbon dioxide therapy (a treatment modality for certain sleep disorders with abnormal patterns of breathing). Some of these are more deserving of a wider appreciation than others. As to whether this book is the best means to spread awareness of capnography beyond anaesthesia, I am doubtful. Perhaps necessarily, it is rather dense and, if you read it from cover to cover, there is excessive repetition. The repetition can, admittedly, be an advantage for readers who wish to consult individual chapters, so this volume would not be wholly out of place in a multidisciplinary library.