Silas Budd is a paramedic working in the South East. The following blog piece is abstracted from his BSc (Hons) Paramedic Science dissertation: “Exploring the efficacy of respiratory impedance during the management of out-of-hospital cardiac arrest: a review”.
Survival from out-of-hospital cardiac arrest (OHCA) is low in England (survival to discharge: 8% in October 2015) (1), but with appropriate interventions this can be improved. Since the adoption of cardiopulmonary resuscitation (CPR) and closed chest compressions in 1960 (2), many advances in OHCA management have been made and there are a number of interventions now available (3). One such intervention is the impedance threshold device (ITD), known commercially as the ResQPOD® (4). With its small size and ease of application, this device has the potential to be easily adopted in to resuscitation algorithms, theoretically improving outcomes from OHCA. Another intervention that has seen popularity in recent years is automated compression-decompression (ACD) CPR. Examples of these devices include the LUCAS® (Lund University Cardiac Arrest System) device, and the Autopulse®.
The ITD works by selectively impeding respiratory gas exchange in the apnoeic patient. This creates a vacuum in the thorax during chest wall relaxation (when active ventilation is not being performed) which increases blood flow to the heart, ‘priming the pump’ for the subsequent compression (5, 6).
ACD-CPR simply replaces the rescuer providing chest compressions with a mechanical device that provides consistent compressions without fatiguing. Some ACD-CPR devices such as the LUCAS® also enhance the decompression phase of each compression, further maximising negative intrathoracic pressure (7).
The ITD combined with ACD-CPR
The consideration of combining the ITD with ACD-CPR was an early one (5), and one that seemed to be feasible when considering the proposed physiology of both devices, yet there seems to be a disparity in the definition of ACD-CPR. Langhelle et al (8) found improved haemodynamics in an animal trial of ITD+ACD-CPR but both the intervention and control groups received chest compressions from an automated device, thereby negating human factors in both groups.
Plaissance et al (6) used clinician chest compressions (S-CPR), however used a CardioPump® in the ACD-CPR group. While this was found to be beneficial in outcomes, the use of this device resulted in a similar (if opposite) bias to that of Langhelle et al (8): the CardioPump® is a handheld suction cup which enhances the decompression phase of compressions but is also subject to rescuer fatigue and poor technique. As such, it would have been difficult to negate human factors from the results found in the intervention group and this may suggest that the benefit was due to the combination of inspiratory impedance and the enhanced decompression phase, rather than the automated delivery of compressions.
The CardioPump® was also used in the ResQ Trial (9), the first large, pragmatic trial of ITD+ACD- CPR, which found a clear benefit in comparison with S-CPR. Despite these positive results, the ResQ Trial contained some biases, ironically highlighted in its title: the ResQPOD® was co-invented by Lurie (one of the trials researchers) and the CardioPump® was developed under ACS (formely CCx), the company under which Lurie and colleagues developed both devices.
So it seems that the term ACD-CPR is a broad one, and one that requires further definition to ensure results are extrapolated appropriately. Little benefit was found between pneumatically driven S-CPR combined with an ITD, and ACD-CPR as classified by Langhelle et al (8). Furthermore, positive results were found in the ResQ Trial, in which the focus of ‘ACD-CPR’ was the perceived benefit of an enhanced decompression phase. It could therefore be argued that the benefit of ITD+ACD-CPR is derived predominantly from the combination of inspiratory impedance and an enhanced decompression phase in CPR. Therein lies the potential benefit of the LUCAS® device. It ensures both regular systematic compressions, negating human error, and enhances the decompression phase of each compression, meaning it should work well with the ITD.
Early human trials were designed to assess S-CPR against innovative forms of CPR and the resultant ‘bystander phenomenon’ (basic life support given by bystanders until rescuers arrive) meant that a significant number of subjects, regardless of randomisation, would have received S-CPR prior to inclusion (43% in the ResQ Trial). If the theory supporting the combined use of ITD and ACD-CPR is correct, this bystander phenomenon may provide explanation for the shift from positive to neutral data seen between animal and human trials. Such a phenomenon however cannot easily be accounted for and even if it were (perhaps by conducting trials in the in-hospital environment), it would not be pragmatic when extrapolated to out-0f-hospital care, nor ethical to discourage bystander intervention. Thigpen et al (10) for instance found a 75% increase in survival when using the ITD, but theirs was a trial using historical data and based in-hospital, meaning guideline changes and protocols may have yielded significant confounding factors.
Furthermore, despite evident presence of the bystander phenomenon in the ResQ Trial, it must be remembered that a huge benefit was found in the ITD+ACD-CPR group, a benefit supported by the latter re-evaluation of the same participants (11).
It seems clear that there is potential benefit in the use of the ITD+ACD-CPR for OHCA, albeit less distinct than early trials had hoped (5). The disparity in definitions of ACD-CPR cannot be overlooked, and resultant differences in approach between trials makes accurate appraisal difficult. On balance, it seems that the benefit of the ITD is best exploited when combined with the enhanced decompression phase of ACD-CPR provided by the LUCAS®, as inferred by the results from the ResQ Trial (9), and similar trials before it (12).
As such, a large, randomised, double blind, controlled trial, pragmatically conducted in the out-of-hospital environment is recommended. One in which the intervention group is treated with ITD+ACD-CPR and the control group is treated with S-CPR. As discussed, negating confounding factors such as the bystander phenomenon would be unethical, however appropriate statistical analysis to account for this in all groups could be applied.
It is clear that by challenging the management of OHCA, improvements in the number of those that survive can be made (13) and while not all trials will bring the success that is hoped for (7), the Institute for Healthcare Improvement (14) reminds us that “while all changes do not lead to improvement, all improvement requires change”.