Introduction
Medic 22 and Engine 14 respond to a report of an unconscious man in a city office building. When they arrive, they find co-workers performing cardiopulmonary resuscitation on a 48-year-old male. Firefighters take over CPR while the medics place the patient on the monitor/defibrillator.
The patient’s electrocardiogram reveals ventricular fibrillation. The medics deliver a single countershock, and the firefighters resume CPR. One of the medics quickly inserts a supraglottic airway and confirms tube placement.
During the course of the resuscitation attempt, the patient receives 1 milligram of epinephrine, 300 milligrams of amiodarone and two additional countershocks. Just before transport begins, the patient achieves return of spontaneous circulation.
On arrival in the emergency department, the physician confirms the presence of breath sounds but moves quickly to replace the SGA with an endotracheal tube.
The patient’s 12-lead ECG reveals a ST-segment myocardial infarction, and the staff quickly prepares the patient transport to the cardiac catheterization lab.
Before medics leave the ED, the physician tells a nurse that he wishes the medics would stop using SGAs and go back to endotracheal intubation right from the start.
Review
Using an already existing epidemiologic registry of cardiac arrest data, researchers with the Resuscitation Outcomes Consortium examined the outcome differences for patients who received advanced airway management with either ETI or SGA insertion during the resuscitation efforts (Wang et al., 2012).
The ROC is a network of 264 EMS agencies in the United States and Canada who collectively conduct research on cardiac arrest (Morrison et al., 2008) and traumatic injury (Newgard et al., 2008). The data points used in this investigation were first collected as part of the ROC PRIMED study, which investigated two cardiac arrest interventions; 1) delaying defibrillation for a period of three minutes while providing high-quality CPR versus immediate defibrillation and 2) the use of an active impedance threshold device versus a nonfunctional ITD (Aufderheide et al., 2011; Stiell et al., 2011).
Both studies ended early and revealed no statistical differences in outcomes offered by either intervention.
The ROC investigators identified all patients 18 years of age or older who suffered an out-of-hospital cardiac arrest and had successful insertion of an advanced airway. EMS personnel were free to choose between ETI or SGA insertion with an agency-approved device.
The SGAs used in the study were the King LT, the Combitube and the Laryngeal Mask Airway. Patients with successful SGA insertion that was later replaced in the field with an endotracheal tube were placed into the SGA group for analysis. The researchers excluded all patients with an unsuccessful advanced airway attempt and patients receiving only basic airway maneuvers.
The primary outcome for this investigation was survival to hospital discharge with satisfactory disability function, requiring a score of three or less on a Modified Rankin Scale (Rittenberger, Raina, Holm, Kim, & Callaway, 2011). A score of three indicates that a patient has only moderate disability and requires some help but is able to walk without assistance (Van Swieten, Koudstaal, Visser, Schouten, & Van Gijn, 1988).
Although most research projects measure a variety of variables, the primary outcome variable defines the intent of the research project (National Cancer Institute, n.d.). Secondary and other variables can provide insight about the study results or drive future research projects, but readers must exercise caution when ascribing meaning to secondary outcome measures.
Results
The research team identified 10,455 adult patients who suffered an out-of-hospital cardiac arrest and had advanced airway placement by EMS personnel. The overwhelming majority of patients had ETI rather than SGA insertion. Approximately 85 percent of patients had ETI or SGA insertion alone without the need for an alternative advanced airway attempt.
Of the 1,968 SGA insertions, the medics identified in the patient chart the specific device they used in 1,444 cases. There were 909 King-LT airway insertions, 296 Combitube insertions and 239 LMA insertions.
Overall, 4.7 percent of the patients receiving ETI survived long enough to be discharged from the hospital with satisfactory functional status compared to 3.9 percent who received SGA. Although one could see an absolute difference of 0.8 percent between the two groups, the numbers do not provide much detail about how strongly each form of airway control influences the outcome of interest. For that, we need to calculate a statistic known as “effect size.”
One way to measure effect size is through odds-ratio reporting. An odds ratio represents the number of times an event occurred divided by the number of times it did not occur (Riegelman, 2005). Odds ratios greater than 1 suggest an event is more likely to happen, and ratios less than 1 suggest the event is less likely to happen. Table 1 represents the odds ratio for four different outcome variables in this study.
The most important outcome variable, however, is survival to hospital discharge because the researchers designed the entire study around that measurement. The middle column represents the odds ratio of achieving an outcome if a patient received ETI versus SGA insertion.
Based on that number, adult patients who suffered an out-of-hospital cardiac arrest were about 1.5 times likelier to survive to hospital discharge with satisfactory functional status if EMS personnel inserted an endotracheal tube compared to SGA.
The last column in Table 1 represents the confidence interval for each odds ratio. If the experiment were repeated using a similar sample of patients and similarly trained EMS personnel, one could be 95 percent confident that the new odds ratio for that outcome would lie between 1.04 and 1.89. Since the entire interval lies above the value of one, survival to hospital discharge is more likely with endotracheal intubation compared to SGA insertion.
Similarly, with the secondary outcome measures, we could say that adult patients who suffered an out-of-hospital cardiac arrest were about 1.75 times more likely to achieve ROSC or survive for 24 hours if EMS personnel inserted an endotracheal tube instead of an SGA.
The last row requires an explanation. When the 95 percent confidence interval contains the value of one, the event is no more likely to happen than not to happen. In this case, the odds are not better or worse for secondary airway or pulmonary complications if EMS personnel use an endotracheal tube or SGA.
In the initial analysis, the researchers placed patients with successful SGA insertion into the SGA group even when the device was later replaced in the field with an endotracheal tube. In the follow-up analysis, the researchers placed these patients into the ETI group and found similar results.
It is also worth nothing that secondary analyses found a similar association between ETI and each individual type of SGA. However, they could not find one type of SGA that resulted in better survival outcomes compared to another.
What it means for you
The American Heart Association acknowledges the lack of conclusive evidence demonstrating improved survival resulting from advanced airway insertion for adult victims of cardiac arrest (Neumar et al., 2010).
Although researchers in Australia found that adult patients suffering an out-of-hospital cardiac arrest were 3.5 times more likely to survive if paramedics successfully performed ETI compared to airway management without the endotracheal tube (Jennings, Cameron, Walker, Bernard, & Smith, 2006), an investigation of in-hospital cardiac arrest found a lower survival rate for intubated patients (Dumot et al., 2001).
In an evaluation of more than 3,300 patients who suffered an out-of-hospital cardiac arrest, researchers in Japan could not demonstrate statistically significant differences in neurologically intact survival rates between patients managed with an endotracheal tube or a SGA (Kajino et al., 2011).
Researchers in Milwaukee found no difference in ROSC, survival to hospital admission or survival to hospital discharge between adult victims of cardiac arrest whose airways were managed with SGA placed by EMTs or ETI by paramedics (Cady, Weaver, Pirrallo, & Wang, 2009).
Despite the lack of evidence demonstrating superiority over ETI, SGAs do offer logistical and tactical advantages during a resuscitation attempt. Because SGA insertion does not require glottic visualization, medics can often insert the airway devices more quickly than they can perform ETI (Burns, Branson, Barnes, & Tsuei, 2010; Russi, Miller, & Hartley, 2008) and without the need for interruptions in chest compressions (Ruetzler et al., 2011).
However, some speculate that there may be a sinister side to these devices that could offset the benefits. In a swine model of cardiac arrest, researchers have demonstrated that inflating the cuffs on several types of SGAs to the manufacturer’s recommendations results in compression of the carotid arteries with a concomitant 15-50 percent reduction in blood flow to the brain when compared to ETI or no advanced airway (Segal et al., 2012).
A similar finding in humans, if confirmed with follow-up research, could help explain why the patients with SGA insertion had worse outcomes than the patients managed with ETI.
In a secondary analysis of the data, the authors noted that the odds of survival were almost two times greater if EMS used a bag-valve mask (BVM) than if they used any of the advanced airways.
This is similar to data seen in other observational studies. Researchers in Los Angeles County demonstrated that adult patients suffering a non-traumatic cardiac arrest were 4.5 times more likely to survive to hospital discharge if paramedics ventilated with a BVM only compared to a BVM and endotracheal tube (Hanif, Kaji, & Niemann, 2010). And a North Carolina study found the adjusted odds of survival to hospital discharge were 5.5 times greater for adult patients when paramedics made no ETI attempt compared to one successful ETI attempt (Studnek et al., 2010).
Limitations
One major limitation of this study is its retrospective nature. This type of investigation looks at data already present as opposed to collecting data as events unfold. Retrospective investigations do not permit researchers to record and control any variables that could influence survival measures in cardiac arrest.
For example, the authors were not able to determine when EMS personnel gained control of the patient’s airway. A registry study of over 25,000 in-hospital cardiac arrest found increased odds of survival if patients received advanced airway insertion within the first five minutes of the cardiac arrest compared to insertion beyond that time (Wong, Carey, Mader, & Wang, 2010). Researchers in Washington State found increased odds of survival for adult victims of out-of-hospital cardiac arrest when successful ETI occurred within the first 12 minutes compared to later insertion (Shy, Rea, Becker, & Eisenberg, 2004).
Without knowing exactly when the medics placed the advanced airway, it is not possible to determine if one group received advanced airway insertion sooner, which could help to explain the increased odds of survival.
Prospective studies allow researchers to plan for data collection before the study starts. This way, they can be sure to collect all of the information necessary to explore an issue adequately. Simply identifying a successful intubation tells us nothing about the number of intubation attempts, whether there were interruptions in chest compressions and whether there was consistent tidal volume delivery. In fact, the medics in this study failed to mention what type of SGA they used for about one-third of the cases.
The bottom line is that there is still no conclusive evidence that airway insertion improves outcomes for patients who suffer an out-of-hospital cardiac arrest. The endotracheal tubes in this study did offer a slight survival advantage over the SGA, but the BVM offered greater advantages than any of the advanced airways.
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The author has no financial interest, arrangement, or direct affiliation with any corporation that has a direct interest in the subject matter of this presentation, including manufacturer(s) of any products or provider(s) of services mentioned.
Send correspondence concerning this article to Kenneth W. Navarro, The University of Texas Southwestern Medical School at Dallas, 6300 Harry Hines Blvd, MC 9134, Dallas, Texas 75390-9134. E-mail: kenneth.navarro@utsouthwestern.edu
