In 2008, the Joint Royal Collages Ambulance Liaison Committee (JRCALC) airway working group concluded that:
‘Tracheal intubation without the use of drugs has little value in prehospital practice’
This has led to much discussion both for and against paramedic intubation. Contributing factors informing their decision included: inadequate training and skill fade of paramedics with endotracheal tubes (ETTs); insufficient evidence supporting intubation; improving patient outcome; and increasing development, availability and ease of use of supraglottic airway devices (SADs).
Intubation should not be forgotten in prehospital practice as SADs become increasingly favoured. Medical personnel, other than paramedics and paramedics with extended roles, are increasingly attending incidents out-of-hospital to provide their expertise in airway management and resuscitation techniques. This complies with the National Confidential Enquiry into Patient Outcome and Death (NCEPOD) trauma report that recommended:
‘The prehospital response for these patients should include someone with the skill to secure the airway (including the use of rapid sequence intubation), and maintain adequate ventilation’
With the emphasis on the use of rapid sequence intubation being shifted to a broader skill-set of drug assisted airway management (DAAM)—whether the level of airway intervention may be supplementary oxygen, positioning, manoeuvres, adjuncts, secure airway (e.g. SAD) or a definitive airway (e.g. ETT)—this should not influence the clinical significance of monitoring adequate ventilation, i.e. ‘rise and fall’ of the chest and etCO2.
The JRCALC and AAGBI (The Association of anaesthetists of Great Britain and Ireland) recommend that minimum standards applied in UK anaesthetic and emergency departments be applied in the prehospital environment (AAGBI, 2009; Deakin et al, 2010).
In addition to clinical assessment, one method adopted to improve intubation success rate, establish tube placement and, most importantly, to confirm adequate ventilation, is the detection of expired carbon dioxide. End tidal CO2 (etCO2) measurement by capnography or capnometry is already mandatory in anaesthetic and emergency departments in the UK.
However, in the prehospital setting, a follow-up survey in 2003 of all ambulance services in England and Wales showed that just three (14%) had this tool widely available for use (Roberts et al, 2005). The same problem has been found further afield in the US (Delorio, 2005). In more recent times, this unavailability has been addressed with one study finding that 85% of the UK's rescue helicopters now have capnometry available (Schmid et al, 2009).
Many ambulance services now have new defibrillators (monitors) available, equipped with full waveform capnography functionality. This will assist, not only with airway management, but also with the monitoring of ventilation and circulation in patients. More recently, the introduction of a small, self contained, mainstream capnometer has created a whole new approach for monitoring etC02 in prehospital care (Figure 1). This device is lightweight, less costly than current waveform counterparts used in some ambulance services, and easy to use in the challenging context of prehospital medicine.
However, does miniaturization of the device deem it less superior than full waveform capnography advocated by guidelines? This article reviews the current available literature to investigate if the latest technological advances in etCO2 monitoring are as safe and reliable as conventional waveform capnography in the prehospital setting.
Terminology
This article has already eluded to two different modes of displaying etCO2; however, there is often little distinction made between capnometry and capnography in the literature which is fundamental to understanding their comparison. Those which follow will be applied throughout this article and are illustrated in Box 1 (D'mello and Butani, 2002).
| Capnometry | Measurement and numerical display of maximal expiratory CO2 during a respiratory cycle |
| Capnogram | CO2 waveform of concentration versus time (or less commonly volume) during a respiratory cycle |
| Capnograph | Measurement and display of CO2 in a capnogram waveform. |
Physiology of carbon dioxide
Carbon dioxide is produced during aerobic cellular metabolism and transported in the blood to the lungs where it is expelled during expiration. The partial pressure of CO2 in the alveoli (PACO2) closely matches that of etCO2 at the end of expiration in healthy individuals. This is true when there is little ventilatory deadspace where no gas transfer occurs and as long as there is adequate pulmonary perfusion. At this point, there is minimal difference between the partial pressure of CO2 in the arterial circulation (PaCO2) and PACO2, and therefore minimal difference between PaCO2 and etCO2. This makes etCO2 an accurate estimate of PaCO2 which varies approximately 35 to 45 mmHg, and is particularly useful when serial readings are used (Sanders, 1989). Conversion of mmhg to kPa is a factor of 7.5.
The physiology helps us understand how etCO2 monitoring can provide valuable information regarding:
Anything interfering with the production, transport or elimination of CO2 will quickly be shown as a change in etCO2 levels. etC02 detection is therefore a valuable tool for detecting adverse ventilatory and circulatory events, and has been shown to be more sensitive than oxygen saturation measurements in detecting these situations (Kober et al, 2004; Krause, 2006).
The ability of etCO2 detection to help determine how well the body is functioning can easily be seen providing benefit if applied in the time critical setting of prehospital medicine. It allows for early detection of problems arising mainly in the circulatory and ventilatory systems, although thought should also be given to metabolic or technical causes of change. Box 2 displays some examples for a decreasing or increasing etCO2.
Methods of etCO2 detection
The ability to detect changes in etC02 gives a fast and reliable method of determining the circulatory and ventilator status of a patient, but how are these values determined? The two main methods for measuring etCO2 are qualitative colourimetric and quantitative numerical devices, with or without a capnogram waveform.
Colourimetric devices are pH sensitive chemical indicators that change from purple to yellow when exposed to CO2. They are easy to use, but can give false positive results in oesophageal intubation if gastric CO2 is present due to carbonated drinks, gastric reflux and drugs such as epinephrine. They also have the disadvantage of requiring six breaths before accurate readings can be taken (Srinivasa and Kodali, 2007). These devices are not recommended in current UK practice (Woollard and Furber, 2010).
Quantitative methods use infrared light detection to determine etCO2. The amount of absorbed light is proportional to the concentration of absorbing gas and by comparing to a known standard the concentration of CO2 can be calculated. Traditionally, these types of monitors have been either sidestream or mainstream. Sidestream devices divert the gas away from the airway via a sample tube. Disadvantages as a consequence are that the samples are subject to water vapour, temperature and pressure changes, and there is a time delay due to gas travel; yet patients are not required to be intubated (Donald and Paterson, 2006; Kodali, 2010).
Mainstream or non-diverting devices have the infrared detector placed right at the airway. These methods are the most reliable way of measuring etCO2 in prehospital practice (Grmec, 2002). As described above, the most recent advancement is the self-contained, battery powered, mainstream device which displays a numerical reading, but no capnogram waveform.
With these types of devices available, current recommendations can be met and have etCO2 monitoring widely available in prehospital care where it has several useful applications.
etCO2 monitoring in prehospital care
etCO2 monitoring has several applications in prehospital care (Box 3). The most widely recognized use of etCO2 detection in prehospital practice is in ensuring correct placement of ETTs. One study described 100% accuracy in distinguishing tracheal from oesophageal intubation in 50 patients when used by paramedics and nursing staff (Macleod and Inglis, 1992).
A misplaced ETT can be a potentially life-threatening scenario if not detected quickly. An absent etCO2 reading from a device either means that the ETT is in the incorrect position, i.e. oesophagus, or that undetectable levels of carbon dioxide are being delivered to the lungs (as in a cardiac arrest).
Commencement of CPR (cardiopulmonary resuscitation) will produce a small rise in detectable levels as cardiac output is marginally increased and carbon dioxide is once again delivered to the lungs for exchange. As resuscitation continues, etCO2 can be monitored in order to assess the efficiency of cardiac compressions maintaining viable cardiac output, and also give one of the first clues to the return of spontaneous circulation (Kupnik and Skok, 2007). High initial etCO2 levels in cardiac arrest scenarios also correlate well with greater short-term survival, making this tool a useful prognostic indicator in CPR (Hatlestad, 2004).
Much higher initial levels of carbon dioxide detected soon after intubation can also aid the practitioner in differentiating between primary cardiac arrest or arrest secondary to asphyxia. This is understood as in asphyxia, the cardiac output is still initially available to transport CO2 to the lungs until the arrest occurs.
‘The measurement of end tidal carbon dioxide has proven to be an invaluable additional tool for monitoring patients in the prehospital environment’
The practical importance of this application is questionable since the difference between these levels disappears following one minute of CPR, which may have already been commenced in the prehospital setting before a secure airway with etCO2 detection is available (Grmec et al, 2003).
More novel applications of etCO2 measurement involve prediction of trauma outcome, assessing the probability of pulmonary embolism and guide to fluid resuscitation in haemorrhagic shock. Grmec et al (2007) assessed the differences in the prediction of trauma outcome between an established prehospital scoring system and the same system in combination with capnometry. They found that the addition of etCO2 values improved the original scoring systems ability to predict trauma outcome (Grmec et al, 2007). In pulmonary embolism, etCO2 levels are low due to the mismatch of ventilation and perfusion in the affected distribution of the lung.
One study, which aimed to determine how effective the combination of clinical suspicion and etCO2 is in predicting a pulmonary embolism (PE) in patients with positive a D-dimer, found that etCO2 had a negative predictive value of 94.2% and may be able to safely rule out a PE in these sorts of patients (Rumpf et al, 2009). This application may be limited due to the unavailability of prehospital determination of D-dimers.
In haemorrhagic shock, etCO2 decreases immediately and correlates well with the point of oxygen supply dependency (Guzman et al, 1997). Vital signs can be maintained in healthy individuals even with large blood losses and the use of etCO2 can be used to reduce delay in fluid resuscitation and in monitoring permissive hypotension.
It should be noted that etCO2 monitoring can be used in intubated and non-intubated patients via face mask, nasal cannula (sidestream devices) and SADs, allowing monitoring of patients for asthma and chronic obstructive pulmonary disease (COPD) decompensation; hypoventilation in overdose, intoxication and post-ictal states; and diabetic ketoacidosis (Krauss, 2006). The benefits of being able to use side-stream in spontaneously breathing patients without a secure or definitive airway may not warrant the other concerns discussed above of side-stream devices, as other clinical input would often be adequate.
Current technology has allowed for the design and production of portable capnometers, enabling this tool to be used in prehospital care.
Portable capnometry
The measurement of etC02 has proven to be an invaluable additional tool for monitoring patients in the prehospital environment. In this time critical setting where geography varies and the situation is uncertain, the equipment available to emergency medicine practitioners should be designed for availability, ease of use and interpretation, speed and practicability. Although modern defibrillators with built in waveform capnography are in use in the UK, an alternate device could meet practical design requirements better in many circumstances by being self contained, smaller, lighter and standard battery powered.
Devices meeting these specifications were originally of the colourimetric type designed primarily to allow continuous monitoring of ETT patency, but as explored above, have disadvantages. Petroianu et al (1996) tested the first quantitative handheld capnometer (Capnocheck, BCI, Waukesha, WI) on animals and found it to be accurate +/− 3.6 mmHg, compared with their standard anaesthetic capnography equipment.
Similarly, Wolfgang et al (1998) conducted a near identical study using pigs and showed the portable quantitative capnometer (Bruker CO2 Module) was again accurate +/− 2 mmHg. Both studies’ concluded that these devices were suited as stand alone monitoring systems for monitoring etCO2 in the prehospital setting and warranted field testing.
Moving into the millennium, Takano et al (2003) described the portable NPB- 75 handheld sidestream capnograph/pulse oximeter (Nellcor Puritan Bennett Inc) as accurate and reliable in estimating PaCO2 in non-intubated, spontaneously breathing patients. Biedler et al (2003) investigated the accuracy of six portable quantitative capnometers and capnographs (mainstream and sidestream) including the effect of ambient temperature, a variable previous studies failed to address. They concluded that portable capnometers are able to fulfill international standards, however, found that the NPB-75 device could be affected by ambient temperature.
The same device was investigated in the paediatric critical care setting by Singh et al (2001) and in the prehospital setting by Singh et al (2006) where it ‘functioned well’ in the transport of intubated children.
It is clear that these transportable devices do have a potentially crucial role to play within prehospital medicine. The devices investigated in previous studies were reasonably compact and lightweight, but would still require to be handheld. If of sidestream design, there may be practical problems such as the requirement of a breath sample volumes too high for certain paediatric patients, extra filters, and lines that may snag.
Mainstream devices overcome some of these disadvantages but, as has been shown in early models, they were frequently bulky and heavy which brought concerns of increased deadspace, accidental extubation and kinking of the ETT, especially in paediatric practice.
Another concern regarding mainstream sample cells being heated above body temperature was resultant thermal injuries (Reder et al, 1983; Bhende, 1999). Modern day devices have become smaller, lighter, use low deadspace adapters and contain failsafe features so they do not overheat (Jaffe, 2002).
Recently, a self contained mainstream capnometer has been produced for the emergency care market. This device is the EMMA™emergency capnometer (Figure 1), manufactured by PHASEIN AB in Sweden. The device displays etCO2 and respiratory rate by using the latest advances in infrared technology. It attaches via disposable adapters to airway devices such as face masks, SADs and ETTs displaying etCO2 after one breath, while requiring no warm up (Figures 2, 3 and 4) (PHASEIN AB, 2011).
Contrasting with sidestream devices, the most striking differences are the absence of cables and no requirement for calibration. This provides these types of devices with major advantages over other methods of measuring etCO2; namely, the ability to monitor a patient as soon as an airway is secured and the potential to avoid carrying the cumbersome equipment currently provided in today's practice.
A recent study evaluating EMMA™ in the hospital setting via a facemask found a statistically significant underestimation of the device, compared to a reference anaesthetic machine, but deemed it to be of limited importance in the clinical setting. It was also reported that battery power did not influence readings (Hildebrandt et al, 2010).
These technological advancements enable manufacturers to construct devices that are capable of meeting the desired specifications of a ‘perfect’ prehospital portable capnometer. These modern devices of small size and mainstream design could be connected to an airway adjunct, allowing the practitioner to continue with further management and be alerted to any problems by an alarm.
Critisism of this type of capnometer may be because it does not display a capnogram waveform (capnography). Some may describe this like having the heart rate without an electrocardiogram rhythm. Brown et al (1998) investigated whether a hand held capnometer with a digital display of etCO2 could differentiate between cardiac and obstructive causes of respiratory distress. The device could not reliably differentiate between the two causes. The ability to view the capnogram in these cases may have shown the classic ‘sharp fin’ appearance of airway obstruction in the waveform and made diagnosis easier; however, would this distinction be necessary in the prehospital environment?
Capnometry in prehospital care
The role of a prehospital practitioner is not necessarily to diagnose conditions, but to assess and, if required, stabilize as best as possible while transporting patients for definitive treatment at hospital. Although no studies directly comparing capnometry and capnography in clinical settings are available and that often capnometry is not distinguished from waveform capnography, it is the authors’ judgement that the availability of an etCO2 waveform would not dramatically alter the effect on patient management in prehospital practice, where the context and priorities differ to those in hospital. Of all example applications quoted above (Box 3), none would require a waveform to aid management.
‘End tidal CO2 monitoring has evolved as an invaluable tool throughout anaesthetic and emergency care’
One must remember that capnometry is just a single tool in the emergency practitioners’ arsenal along with their own clinical judgement and decision-making skills.
When evaluating benefits of full waveform capnography, consideration must be given to cost of not only price, but also of time and effort in training. The Scottish Ambulance Service (SAS) has already spent £7.5 million on new HeartStart MRx defibrillators with a capnography utility (Scottish Healthcare, 2010), but training prehospital practitioners on interpreting end tidal CO2 waveforms may not be cost effective. In practice, having to decipher a capnogram waveform may be somewhat off-putting to a prehospital practitioner and may lead to depriving patients from an important tool that could help inform their management.
Waveforms which give information on things such as: spontaneous breathing in ventilated patients (curare cleft); cardiogenic oscillations; and severity of airway obstruction, which are useful in definitive in-hospital care where breath-by-breath waveforms are needed for in depth analysis, but are unlikely to alter management in the prehospital setting. The most important part of the waveform is the plateau height, which is the value that a quantitative capnometer without a waveform displays numerically in any case. These devices would require minimal training, cost less and be more likely to be used.
Conclusion
etCO2 monitoring has evolved as an invaluable tool throughout anaesthetic and emergency care providing information on body metabolism, perfusion and ventilation. More recently, this tool has found a role in the often time critical prehospital environment. Ideally, devices suitable for this setting would be stand alone, lightweight, small and easy to use, as locations and situations in prehospital practice are unpredictable.
Recommended standards of practice and equipment do not meet these criteria. Recent technological advancements have allowed for the development of portable, self contained mainstream capnometers which are equally as accurate in detecting etCO2 compared with anaesthetic equipment.
Capnometers do not show an etCO2 waveform unlike the new defibrillators purchased for some ambulance services, however, this is unlikely to have any impact on prehospital patient management. The advantages gained by greater access to etCO2 monitoring, related to both cost and training considerations, have enabled the conclusion that this type of mainstream capnometer, and not capnography, should be considered for the future direction of end tidal CO2 use in prehospital care.