'Sweet Spot' of Optimal Compressions, Depth Identified in CPR

Batya Swift Yasgur, MA, LSW

August 21, 2019

New research has identified the optimal combination of chest compression rate (CCR) and chest compression depth (CCD) when performing cardiopulmonary resuscitation (CPR) to improve survival rates.

A team of researchers analyzed data on more than 3600 patients who had experienced an out-of-hospital cardiac arrest and for whom CCR and CCD had been simultaneously recorded.

They found that the optimal CCR-CCD combination was 107 compressions per minute with a depth of 4.7 cm.

When CPR was performed within 20% of this value, survival probability was significantly higher. The findings remained unchanged regardless of age, sex, presenting cardiac rhythm, or CPR adjunct use.

Odds of survival were even higher when a CPR device was used, compared with standard CPR, but the device's effectiveness was dependent on being near the target CCR-CCD combination.

"The findings here not only emphasize the importance of quality CPR performance, but they will likely help paramedics and others on the frontlines now to save many more lives," study coauthor Paul Pepe, MD, MPH, professor of emergency medicine, University of Texas Southwestern Medical School, Dallas, said in a press release.

The study was published online August 14 in JAMA Cardiology.

One Step Further

"A few years ago, we investigated rates of CPR to see if changing the rates of compression affected outcome," Pepe told theheart.org | Medscape Cardiology.

"We found a range of compression rates, from 100 to 120 per minute, which seemed to be right and published that earlier, but we also found a problem — that going faster led to less depth, or going deeper meant the compressions were slower," he said.

"We wanted to do a better job teaching rescuers to develop muscle memory for the right rate and depth, so we looked for an optimal combination," he said.

In addition, Pepe and his team decided to take the investigation "one step further" by investigating differences in patients — for example, whether a large man or a small older woman required different rates or compression, or whether it made a difference how long the cardiac arrest lasted, he noted.

To investigate, the researchers used data collected between June 2007 and November 2009 from the National Institutes of Health (NIH) Resuscitation Outcomes Consortium (ROC), which studied pharmacologic, procedural, and device interventions for out-of-hospital cardiac arrest.

In their previous study, patients were randomly assigned to receive either conventional CPR using a sham impedance threshold device (ITD) or active-ITD device that provided 16 cmH2O resistance.

The study sample included 3643 patients who had experienced out-of-hospital cardiac arrest and for whom CCR and CCD had been simultaneously recorded during an NIH clinical trial of a CPR adjunct.

The researchers conducted subgroup analyses that included evaluations according to age, sex, presenting cardiac rhythm, and application of a CPR adjunct.

American Heart Association recommendations were used to instruct emergency medical service (EMS) first responders:

  • 80 to 100 compressions per minute (cpm)

  • A compression depth of 4.0 to 6.0 cm

  • Use of an advanced airway

  • 10 positive-pressure breaths per minute with approximately 600 mL tidal volume

  • Breaths delivered in a 30:2 ratio of compressions to breaths when using basic airways

ROC sites were required to show that for several months, CPR could be delivered with these predefined metrics ≥50% of the time.

Inclusion criteria required that intact sets of simultaneous CCR-CCD recordings during the first 5 minutes of EMS-performed CPR be available. Those patients with CCR-CCD values outside the prescribed ranges (60 – 160 cpm and 2.0- to 8.0-cm depth) were excluded.

Colormetric Map

The researchers used a regression model with both a linear and a quadratic term for each of the rates and depths (and their interaction), which was fitted to the data overall and then separately fitted for sham-ITD (inactive) and active-ITD groups.

Optimal CCR-CCD combination values were calculated from these models using "numerical optimization techniques," with a proposed optimal combination evaluated further within a range that was within 20% of the identified CCR-CCD target.

Additionally, the researchers performed subgroup analyses to determine whether optimal CCR-CCD targets varied by sex, age, presenting cardiac rhythm, or length of cardiac arrest.

Optimal CCR-CCD combinations for sham-ITD (standard CPR) and active-ITD (adjunct CPR) were estimated within each subgroup as well as across combined subgroups.

To visually display optimal CCD-CCR combinations, contour plots constructed colorimetrically with separate displays for sham-ITD and active-ITD groups showed the relative proportions of survivors across the survivor sample and the weighted survival proportions for the overall cohort within each cell.

"Colder" zones represented lower proportions of survivors or survival probability, while "hotter" zones represented higher proportions of survivors or survival.

"We presented the data in a completely different way," Pepe said. He likened this novel contour plot to a map that uses color to convey the optimal combination of rate and depth.

"Sweet Spot"

The study cohort consisted of 3643 patients (mean [SD] age, 67.5 [15.7] years, 64.4% male). For 41.9% of those patients, bystanders witnessed the OHCA; bystander CPR was performed for 36.3%.

Close to half of patients (47.8%) presented with asystole, while roughly one quarter (24.5%) presented with ventricular fibrillation or ventricular tachycardia.

Across the 130 CCR-CCD combinations, the 100 – 109 cpm and 4.0 cm combination was the most populated for sham-ITD, active-ITD, or the overall cohort.

In the survivor group, the most populated cell was the 90 – 99 cpm and 4.5 cm combination.

The optimal CCR-CCD for all patients was found to be 107 compressions per minute with a depth of 4.7 cm, with a significantly higher survival probability when CPR was performed within 20% of that value (6.0% vs 4.3% outside that range; odds ratio [OR], 1.44; 95% confidence interval (CI), 1.07 – 1.94; P = .02).

These findings remained similar regardless of age, sex, presenting cardiac rhythm, or CPR adjunct use.

"We regard this combination as a 'sweet spot area,' " Pepe commented.

When the 93 sham-ITD (standard CPR) survivors (mRS ≤3) were compared to their 93 active-ITD counterparts using contour plots, optimal CCR-CCD combinations were similar (ie, the cell with the highest proportion of survivors was 100 – 109 cpm and 4.5 – 5.0 cm).

The optimal CCR-CCD was associated with significantly higher probabilities of survival with the CPR device in comparison with standard CPR (OR, 1.90; 95% CI, 1.06 – 3.38; P = .03), with the device's effectiveness dependent on being near the target CCR-CCD combination.

"There did not appear to be conclusive support for a variable favorable combination for any of the predefined subgroups compared with the overall findings," the authors state.

"When we used the device, the optimal rate was the same as for standard CPR, but we got almost double the survival rate," Pepe commented.

"The take-home from that is that ITD improved outcomes significantly, but [the improvement was] highly dependent on being in that optimal range, which makes it more compelling to pay attention to, especially if we use the device," he said.

CPR "Mantra"

Commenting on the study for Medscape Medical News, David Kessler, MD, associate professor of pediatrics in emergency medicine, Department of Emergency Medicine, Columbia University Vagelos College of Physicians and Surgeons, New York City, called it "a great example of secondary analysis that can be done using a large dataset of clinical data on CPR depth and rate."

The study "underscores the need for larger, well-funded registries of actual CPR performance that links to outcomes," stated Kessler, who is also vice chair of innovation.

Kessler, who was not involved with the study, noted several limitations.

"The data on residual lean were not included; therefore, we are unable to assess how this third pillar of high-quality CPR interacts with the other targets for depth and rate," he said.

Moreover, there are "zero data in children, and future studies are really needed to answer similar questions across the age/weight spectrum in children," he commented.

Pepe noted that even well-trained "rescuers" who — to be included in the study — had to demonstrate that they could perform CPR were "quite variable in their performance of CPR, over a large range.

"It is difficult for rescuers to always maintain depth and rate in the real world, so we need to give them technology with real-time feedback, or use mechanical devices that can help with the right rate and depth," he added.

In an accompanying editorial, David Cone, MD, notes that the study findings "are unlikely to lead to a change in international CPR guidelines on their own."

Nevertheless, "they do support the simplest CPR mantra: push hard, push fast, and do not stop," he states.

The data used in the study were provided by the NIH and the Resuscitation Outcomes Consortium. Pepe has disclosed no relevant financial relationships. The other authors' disclosures are listed on the original article. Kessler has disclosed no relevant financial relationships.

JAMA Cardiol. Published online August 14, 2019. Full text, Editorial

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