Whether US delays resumption of chest compressions and impacts on CPR quality remain a great debate. The results in this study showed US did not impact on CCF. In addition, pause duration of pulse checks could be lessened by using continuous US. If the sonographer insisted or tried to finish US before the resumption of chest compressions, the pause duration was much longer. On the contrary, if he/she allowed resumption of chest compressions and continued US (continuous US), the pause could become much shorter.
In addition to a single interruption of chest compressions, an even more important indicator to monitor quality of CPR is CCF, a fraction of chest compressions duration along the whole CPR process. The American College of Cardiology (ACC)/American Heart Association (AHA) consensus statement recommends a target CCF of more than 60% and at best > 80%[9, 24]. To date, most of the studies regarding resuscitation US reported merely the length of pause rather than CCF[15, 18–20]. Whether lengthening of a single pause would compromise CCF is still unclear. In the current study, the high CCF of 92% was in accordance with the recommendation of the guidelines. The introduction of US during CPR did not influence CCF and survival. The results imply that US during CPR does not prolong overall no-flow time and influence the survival, although US may prolong an individual pause. However, in the CPR with US group, pause became shorter in pulse-checks without US. It possibly compensated for longer US-related pause. Therefore, a similar CCF was preserved, comparing with the CPR without US group.
Such a high CCF in this study may be explained by adequate manpower, structured ALS teamwork, monthly review with continued training, and employment of a timer. In the current study, at least 7 members involved in each resuscitation scenario. The sonographer, all had completed basic US and resuscitative US training. Through continued practice and accumulation of experience, the sonographers exhibited excellent US proficiency[7, 21]. In addition, a pre-set timer is important, which reminds the resuscitation team keeping pause as short as possible. Without the timer, the sonographer would tend to focus on sonographic examination and overlook time elapsing. Moreover, a high-quality portable ultrasound machine ready at the resuscitation room is essential. This helps speed up imaging diagnosis and shortening the interruptions. All of them contribute to whole CPR process in an orchestrated way.
Previous studies reported the pause with US could be prolonged to 17-21s[15, 18, 19]. Clattenburg et al reported shorter pause could be achieved when US was performed by an experienced sonographer or implementation of a structured US protocol[19, 20]. Nevertheless, all these studies focused on US performance during each pulse-check. Evidence regarding CCF and survival were limited. The study for the first time demonstrated that CCF, would not be reduced by employment of US.
This study is characterized by testifying the feasibility of continuous US. The results showed that such breakthrough in concept and intervention would largely broaden the time window of US assessment during CPR. US could be feasible during chest compressions with minimal interruption of CPR.
When performing US during CPR, there are several important organs to target. Subxiphoid view of the heart was the most often scanning target in the hands-off period, to focus on cardiac activity, pericardial effusion, right ventricular size, and left ventricular regional wall motion abnormalities. In the hands-on period, aorta and eFAST were frequently checked. These results to some extent explain how chest compressions might interfere US visualization of certain targets, and how the sonographer would choose in different periods during CPR to search for possible reversible causes at the earlier time point. Subxiphoid view of the heart is more suitable during hands-off period, since it is very close to the location of compressions and the image quality should be better when compression is held. For other targets far from the site of chest compressions, US is feasible during chest compressions. In one case, bloody ascites related to ruptured abdominal aortic aneurysm was detected during compressions. Using such approach of continuous US, not only the pause of chest compressions could be shortened, the reversible cause could potentially be identified and would be managed earlier.
Despite of the contributions, there are several limitations in this study. First, the results were obtained from a single institution. The facilities in the resuscitation room may vary from hospital to hospital. However, the study provides a possible way to improve quality of resuscitation while integration of US into CPR process. Second, selection bias could exist because the results were collected through retrospective video analysis. How to select patients for US during CPR was unclear. However, relatively shorter resuscitation time was noted in the CPR without US group. It was possibly explained that lots of work should be done in the early period of real resuscitation condition, such as endotracheal intubation, defibrillation and application of CPR device. US would be performed after aforementioned chaotic conditions. Third, the quality of US images was not included for analysis. Better quality of focused cardiac US images was suggested to be obtained during rhythm analysis and bag-mask ventilations. In this study, each patient received 1–2 times of US evaluation. Not all US images were stored each time, though the key images with important findings were recorded. Fourth, the correlation between the US findings and the decision making or following management is not analyzed. The impact of the US findings on survival is harder to interpret. For example, cardiac tamponade resulting from aortic dissection was confirmed in 3 patients. The finding of US evidence of cardiac tamponade is almost always followed by pericardiocentesis. However, even early detection, it can hardly reverse the dismal outcomes. The potential value of resuscitative US on survival would be compromised.