Background
Target condition being diagnosed
The insertion of a nasogastric tube (NGT) is the passage of a tube, appropriate for its intended purpose, via the nostril into the stomach (National Institute for Clinical Excellence 2006). NGTs are used within clinical practice for a variety of reasons including: decompression, following gastric surgery, patient assessment, administration of drugs, enteral feeding, and fluid administration. This is an extremely common clinical intervention, with an estimated one million tubes being purchased per annum in England and Wales alone (Hanna 2010). Although the majority of these tubes are inserted and used without incident, there is a recognised risk that the tube can be misplaced into the lungs, or move out of the stomach. Published reports of incidents have included oesophageal, peritoneal and intestinal placement, and NGTs placed within the brain (Burns 2001). Additionally, severe pulmonary complications, indeed deaths, have been reported as a direct result of NGT placement within the respiratory tract (Miller 2011). Between September 2005 and March 2010, 21 deaths and 79 cases of harm relating to feeding through misplaced NGTs were reported in the UK (National Patient Safety Agency 2011a).
Confirmation of NGT placement is required immediately following insertion and subsequently prior to each use, for example, administration of enteral feed or medication. Additionally, the tube should be checked following episodes of vomiting, retching or coughing spasms, after oropharyngeal suction has been required, every four hours during regular feeding or where there is a suggestion of tube displacement (American Association of Critical Care Nurses 2009). Any new or unexplained respiratory symptoms or a drop in oxygen saturation readings is a further indication for seeking repeated confirmation of NGT placement (Durai 2009).
There are various methods used to determine NGT position, including bedside assessment and observing for signs of respiratory distress. Air insufflated (blown) through the NGT in combination with epigastric ausculation (listening to the stomach with a stethoscope) for whooshing sounds has also been used (Fletcher 2011). Although these tests are widely known about, they are not officially recommended for use as standalone measures of NGT placement. Current guidelines from the American Association of Critical Care Nurses 2009 and National Patient Safety Agency 2011b recommend a combination of aspirate testing and radiological confirmation. In a small number of patients for whom the NGT has been placed under direct vision of an anaesthetist or surgeon, it is possible to forego chest x-ray confirmation (National Patient Safety Agency 2011b). Observing the characteristic of fluid aspirate can be used, with gastric secretions differing in colour and consistency to those obtained from tracheal, bronchial or intestinal secretions (Metheny 2001). In addition to this subjective approach, objective measures of aspirate pH can be used to assess NGT placement. A pH reading of between 1 to 5.5 is considered a reliable method for excluding placement in the pulmonary tree (National Patient Safety Agency 2011a). The ability to obtain gastric aspirate may not be achievable in up to 65% of patients (Hanna 2010). Concurrently, radiography or direct visualisation are considered the only reliable methods of confirming NGT placement (Elpern 2007; National Patient Safety Agency 2011b).
Index test(s)
The measurement of carbon dioxide (CO₂) in exhaled air is a widely used clinical observation and is a recognised standard of care during tracheal intubation or laryngeal mask airway (Ahrens 2003; The Intensive Care Society 2009). This can be achieved in one of two ways; capnography and colorimetric capnometry. Capnography is the measurement of inspired and expired CO₂ using the absorption of infrared light by CO₂ molecules to estimate CO₂ concentrations. These measurements are then displayed against time to give a continual graphical trace. Detection of a CO₂ waveform is the test threshold for index test positivity for capnography. Colorimetric capnometry involves the detection of CO₂ using an adapted form of pH filter paper, impregnated with a dye which changes colour from purple to yellow in the presence of CO₂. The colour change is the index test threshold for test positivity for colorimetric capnometry. However, this method does not provide a continual reading and can only be used as a semi-measurement of the amount of CO₂ in the expired gas (Frakes 2001).
The monitoring of CO₂ emanating from an NGT inadvertently passed into the airways would utilise this phenomenon in a reverse manner, confirming tracheobronchial placement rather than the intended alimentary tract (Thomas 1998), provided that there is circulation to deliver CO₂ to the lungs and an absence of complete bronchospasm preventing gas exchange (The Intensive Care Society 2009). CO₂ monitoring for this clinical application has indeed been suggested, and has been a concept acknowledged in the literature for over 20 years (Mercurio 1985).
Alternative bedside methods for detecting NGT placement have been suggested in the literature (e.g. measurement of gastric enzymes by Metheny 1997 or an electromagnetic technique as evaluated by Kearns 2001), however CO₂ monitoring is the only currently available technique identified as a potential viable alternative to the reference standard appearing in clinical guidelines (The Intensive Care Society 2009). We have therefore chosen to focus on the detection of CO₂ only to keep the Review manageable and maximise clinical relevance of the comparison.
Clinical pathway
The measurement of CO₂ in exhaled air is a recognised and mandatory standard of care for confirming and monitoring endotracheal tube or airway placement under general anaesthesia. Additionally, it is also a mandated form of monitoring for patients undergoing moderate and deep sedation (Weaver 2011). The monitoring of CO₂ from an NGT has been suggested as a replacement for the current reference standard of chest radiography.
Rationale
Several studies have examined the accuracy of colorimetric capnometry in predicting gastric placement of NGTs. Very high levels of specificity and sensitivity were reported against a reference standard radiograph control (Araujo-Preza 2002; Thomas 1998) or air insufflation and epigastric auscultation (Elpern 2007; Meyer 2009). Similar results have been reported with capnography when using a radiograph control (Kindopp 2001), and when both capnography and colorimetric capnometry were compared against both radiograph and epigastric auscultation controls (Burns 2006).
Several narrative reviews examining various techniques for verification of NGT placement comment on the use of CO₂ detection. Some authors comment that the use of CO₂ detection shows promise (Ackerman 2006) with suggestions that this approach may result in cost savings (Roberts 2007). Further narrative reviews conclude that radiographic confirmation remains the most reliable method of confirming gastric NGT placement (Bourgault 2009; Metheny 2001; Simons 2012).
Both capnography and capnometry were evaluated in a recent meta–analysis by Chau 2011, which concluded that there was strong evidence available to support their use to confirm NGT position, with a sensitivity ranging from 0.88 to 1.00 and specificity of 0.95 to 1.00. However, this work was limited both by language and publication status and no searching of the grey literature.
A systematic review of CO₂ detection for testing NGT placement in adults is required; to identify and critically evaluate the current evidence base and to establish the diagnostic test accuracy of this new application of an existing clinical technology.