On each of the quality measures of CPR, the best average CPR performance occurred with visual feedback (on duty cycle visual feedback was better than auditory but equivalent to no feedback). A significantly larger percent of subjects achieved the CPR performance standards when doing CPR with visual feedback (no worse or better than no feedback on full release of pressure and duty cycle). Among these subjects, visual feedback also reduced performance variability; an effect observed in clinical resuscitation with audiovisual feedback [7
]. Quality of CPR declined over the course of the 10 minutes but not significantly so when subjects received visual feedback.
Both auditory and visual feedback improved the percent of correct compressions (recommended rate, depth, and full release of pressure at the end of compression) and percent of compressions of adequate depth (38 mm or more) among females certified in basic life support performing 10 minutes of 30:2 CPR. Similar positive benefits of feedback in simulated resuscitation are reported by others using auditory feedback (VAM) [8
] and audiovisual feedback (CPREzy™, Health Affairs, London, UK) [12
Our results indicated that auditory feedback provided by the voice-assisted manikin (VAM) decreased the average rate of compressions, increased the percent of compressions without full release of pressure, and decreased duty cycle. Auditory feedback yielded performance that was significantly poorer than that observed with no feedback or with visual feedback. A potential hypothesis for this result is the nature of the feedback; it was error corrective occurring only when performance decayed to a preset threshold. Among the subjects in this study, average rate tended to drift downward with duration of CPR. When this occurred in the auditory feedback condition, the VAM advised the subject to 'press faster' and the subject responded by increasing the rate of compressions which gradually drifted back down to the threshold thereby increasing variability in compressions (displayed in Figures and ). One explanation for the observed performance variation is that the auditory advice disrupted the subject's CPR performance rhythm. Partial support for this hypothesis is provided in the results of Kern et al.[14
] in simulated resuscitation and Chiang et al.[15
] in clinical resuscitation. In each study, a metronome was used to provide continuous feedback during CPR and in each study compression rate improved significantly. Neither of these studies examined compressions without full release of pressure or duty cycle.
Incomplete release of force during decompression results in higher intra-thoracic pressures during the decompression phase of CPR which reduces venous blood flow to the heart and increases intracranial pressure which in turn decreases vital organ blood flow and the likelihood of survival [16
]. Both Aufderheide et al. [19
] and Niles et al. [20
] report that feedback on release of pressure significantly decreases the number and percent of compressions with incomplete release of pressure during decompression. Aufderheide et al. [19
] used an impedance threshold device (ResQPOD) to provide continuous feedback on intra-thoracic pressure during CPR while Niles et al.[20
] used a commercial monitor/defibrillator system (Heartstart MRx Phillips, Andover, MA) to provide audiovisual directive and corrective feedback during clinical resuscitations. With corrective feedback Niles et al. report significant declines in the percent of compressions with incomplete release; but, that even with feedback, 28 percent of compressions had a residual force exceeding the threshold of 2.5 Kg estimated for use in pediatric CPR.
These results along with that reported by others [19
] using error corrective feedback support Kramer-Johansen et al.'s [4
] conclusions that complexity of feedback and its effects on performance need further examination. The results of this study and the residual error in CPR performance reported by others (e.g., Niles et al.[20
]) suggest that there may be two different but equally important underlying constructs that need examination: sensory modality and periodicity of the feedback. These two aspects of feedback were completely confounded in this study as the visual feedback was continuous and the auditory feedback was periodic (error corrective). Kramer-Johansen et al.'s [4
] results of audiovisual feedback during EMS clinical resuscitations in which personnel elected to turn off auditory feedback when given the option lends support to the notion that auditory feedback may be less useful than is visual feedback. On the other hand, the significantly better performance among subjects in this study when receiving visual feedback that was continuous as well as studies using metronomes [14
] which provide continuous auditory feedback point to the superiority of continuous feedback over periodic feedback. Further study of single channel (visual or auditory) continuous and periodic feedback is needed to understand the unitary effects of both sensory modality and periodicity of feedback.
The results of this study argue against the hypothesis offered by Sugarman et al. [3
] that decay in CPR quality even with feedback is due to rescuer fatigue. In this study, subject's ratings of perceived exertion were less with feedback than without it (Figure ). Even though blood lactate levels (Figure ) were significantly higher when these subjects performed 30:2 CPR with visual feedback their ratings of perceived exertion were not higher. In each feedback condition the blood lactate levels obtained before CPR were less than those at post and post 5 minutes CPR which indicated that the effort continued to be anaerobic. The elevated blood lactate levels with visual feedback coincide with the observation that subjects consistently had greater depth and rate when receiving visual feedback relative to receiving auditory feedback. This finding suggests that during visual feedback subjects were paying attention to their performance on the computer screen and possibly physiologically loading their anaerobic system to a greater level, but they did not experience a higher perceived fatigue.
Generalization of the results of this study to clinical resuscitation or to resuscitation training environments must proceed with caution as the study had several important limitations. The study was conducted in the laboratory where resuscitation was performed on a training manikin; one that did not simulate the changing force/pressure dynamics seen in human chests during resuscitation [21
]. The study did not evaluate the kind of feedback most commonly seen in clinical resuscitation - a combination of auditory and visual feedback. However, the results of the study are sufficiently robust to warrant further study in simulated practice. Such studies should evaluate the comparative benefits of feedback when feedback (a) is continuous or real time versus error correcting or delayed and (b) uses single or multiple sensory modalities.