Introduction:  Current resuscitation guidelines recommend providing chest compressions at a rate between 100 and 120 min-1 with a depth between 50 and 60 mm. However, two manual chest compressions achieving the same depth and duration (rate) can be very different in terms of impulsivity. Animal studies with mechanical pistons have analyzed different compression/decompression patterns, and concluded that high-impulse compression/decompression improves blood flow. However, there is currently no independent metric to quantify the impulsivity of manual chest compressions. 
Purpose of the study: To characterize four impulsivity metrics and their relationship with rate and depth during manual chest compressions.
Materials and methods: Thirty out-of-hospital cardiac arrest episodes containing the compression depth signal acquired with a CPR feedback system were analyzed. For each chest compression four parameters were annotated: CV: maximum compression velocity (mm/s); RV: maximum decompression velocity (mm/s): Tef: effective compression duration (ms); Tef/Tc: ratio between effective and total compression duration. The linear relationship with compression rate and depth of each parameter was assessed.
Results: A total of 66,971 chest compressions were annotated. Median (5th-95th percentiles) for each parameter was CV: 433 (253, 699); RV: 267 (164, 417); Tef: 368 (276, 446); Tef/Tc: 0.69 (0.54, 0.77). CV and RV showed high linear correlation with depth. For every 10 mm increase in depth, CV and RV increased 97 and 63 mm/s, respectively. For every 10 min-1 increase in rate Tef decreased 21 ms. Tef/Tc showed low dependency with depth and rate. For every 10 mm/10 min-1 increase, Tef/Tc increased 0.015/0.011 ms.
Conclusions: The ratio between effective and total compression duration is a metric capable to assess chest compression impulsivity, independent from depth and rate. Further studies are deserved to analyze the influence of narrowness on outcomes from cardiac arrest.

5 - Metrics to characterize the narrowness of manual chest compressions during out-of-hospital cardiopulmonary resuscitation