L,Varying Chest Compression Rates During Neonatal Cardiopulmonary Resuscitation: A Randomized Controlled Animal Trial

Background To compare chest compression (CC) rates of 90/min with 180/min and their effect on the time to return of spontaneous circulation (ROSC), survival, hemodynamic, and respiratory parameters. We hypothesized that asphyxiated newborn piglets that received CC at 180/min vs. 90/min during cardiopulmonary resuscitation would have a shorter time to ROSC. Methods Newborn piglets (n=7/group) were anesthetized, intubated, instrumented and exposed to 45 min normocapnic hypoxia followed by asphyxia and cardiac arrest. Piglets were randomly allocated to a CC rate of 180/min or 90/min. CC was performed using an automated chest compression machine. Hemodynamic and respiratory parameters and applied compression force were continuously measured. Results The mean (SD) time to ROSC was 91 (34) and 256 (97) sec for CC rates of 180/min and 90/min, respectively (p=0.08). The number of piglets that achieved ROSC was 7 (100%) and 5 (71%) with 180/min and 90/min CC rates, respectively (p=0.46). Hemodynamic parameters (i.e., diastolic and mean blood pressure, carotid blood ow, stroke volume, end-diastolic volume, left ventricular contractile function) and respiratory parameters (i.e., minute ventilation, peak ination and peak expiration ow) were all improved with a CC rate of 180/min. Conclusion Time to ROSC and hemodynamic and respiratory parameters were all improved, with a CC rate of 180/min vs. 90/min. Higher CC rates during neonatal resuscitation warrant further investigation. ROSC. Our previous showed a (standard ROSC of 220 (25) sec with a CC rate of 90/min. that a CC of 180/min the time to ROSC. size


Introduction
Current neonatal resuscitation guidelines recommend a 3:1 compression to ventilation (C:V) ratio with 90 chest compressions (CCs) and 30 in ations to achieve approximately 120 events per minute 1,2 . However, the optimal CC rate to optimize coronary and cerebral perfusion while providing adequate ventilation of an asphyxiated newborn remains unknown.
A mathematical study suggests that the most effective CC rate to optimize systemic perfusion depends upon body size and weight 3 . This would translate to CC rates of 180/min in term infants and higher rates for preterm infants. However, CC rates of 180 CC/min or above are impossible to achieve using a 3:1 C:V ratio. If a clear effect of body size and weight on the optimal compression frequencies for CPR exists, then optimizing compression frequencies for neonatal CPR has the potential to improve short-and longterm outcomes in newborn infants.
We aimed to examine different CC rates and their effect on the time to return of spontaneous circulation (ROSC), survival, and hemodynamic and respiratory parameters. We hypothesized that a CC rate of 180/min compared to 90/min during cardiopulmonary resuscitation would result in a shorter time to ROSC in asphyxiated newborn piglets.

Results
Fourteen newborn mixed breed piglets (0-3 days, weight 1.9-2.4 kg) were obtained on the day of the experiment and randomly assigned to a CC rate of 180/min (n=7) or 90/min (n=7). There were no differences in the baseline parameters or the start of CPR (end of asphyxia) between groups ( Table 1).

Hemodynamic parameters
Hemodynamic parameters at baseline and at commencement of resuscitation were not different (Table   1). Diastolic and mean blood pressure and carotid blood ow were signi cantly higher with a CC rate of 180/min, while systolic blood pressure was not different between groups ( Figure 2). Stroke volume and end-diastolic volume were improved, but this did not reach statistical signi cance (Figure 3), while dp/dt max and dp/dt min signi cantly improved with a CC rate of 180/min ( Figure 3).

Respiratory parameters
The tidal volume was not different between groups, while the minute ventilation was signi cantly increased with CC rate 180/min with mean (SD) 945 (249) compared to 522 (79) mL/kg/with CC rate 90/min group (p=0.003) (Table 2). Similarly, peak in ation ow and peak expiration ow were signi cantly higher with a CC rate of 180/min (Table 2).

Discussion
Current neonatal resuscitation guidelines recommend providing 90 CC and 30 in ations (=120 events/minute) using a 3:1 C:V ratio to optimize cardiac output and oxygen delivery 1,2 . However, the optimal CC rate during neonatal CPR remains unclear. A mathematical study suggests that the most effective CC rate depends upon body size and weight and that higher CC rates, as currently recommended, might improve survival in newborn infants 3 . In the current study, we compared CC rates of 180/min and 90/min using CC+SI. The results of the study can be summarized as follows: using a CC rate of 180/min resulted in i) 30% higher survival; ii) 46% shorter time to ROSC; iii) less epinephrine administration; iv) higher blood pressure (diastolic and mean) and carotid blood ow ( Figure 2); v) similar stroke volume and end-diastolic volume, and signi cantly improved left ventricular function ( Figure  3); and vi) signi cantly increased minute ventilation and thereby oxygen delivery (Table 2).
Mathematical models described that the maximal cardiac output during CPR depends on the fraction of cycle time available for pump lling and pump emptying, with pump lling being dominant 43 . Fitzgerald et al compared CC rates between 20-140/min in 6-16 kg mongrel dogs and reported that the maximum cardiac output would be achieved with a CC rate of 126/min 4 . Similarly, a mathematical model by Babbs et al calculated the optimal CC rate to achieve maximum cardiac output with a CC rate of 124/min for a 10 kg child and 184/min for a 3 kg newborn infant, indicating a clear effect of body size and weight 3 . In the current study, stroke volume was higher at a CC rate of 180/min (Figure 3), and as cardiac output is simple, the product of compression rate × stroke volume, cardiac output was also higher (results not shown).
The physiological heart rate in neonates ranges between 120-160/min 5 , and an increase in the CC rate might have the potential to boost arti cial cardiac output compared to recommended CC rates, which are based largely on experimental work in animal models larger than neonates. Patel et al randomized asphyxiated piglets to continuous CC with asynchronized ventilation with CC rates of 90/min, 100/min, and 120/min and reported similar mean times to ROSC of 90 sec, 90 sec, and 120 sec, respectively 6 . Furthermore, the hemodynamic recovery (indicated by carotid blood ow and cerebral and renal perfusion) was similar between all three intervention groups 6 . Notably, the piglets in the 90/min and 100/min groups had higher cerebral in ammation and brain injury than those in the 120/min group 6 . In comparison, Li et al reported a shortened median time to ROSC with a CC rate of 90/min compared with a CC rate of 120/min of 34 sec versus 99 sec 7 . However, CC with a higher rate had higher cardiac output and left ventricular function expressed by dp/dt max and dp/dt min 7. Similarly, our own observations indicate that stroke volume, cardiac output, and left ventricular function increase with higher CC rates.
Indeed, stroke volume and cardiac output were highest with a CC rate of 180/min, while end-diastolic volume, dp/dt max or dp/dt min was highest at a CC rate of 150/min. A further increase to a CC rate of 180/min did not further increase the end-diastolic volume, dp/dt max or dp/dt min . These data suggest that CC with a rate of 150-180/min might have optimal cardiovascular performance.
In the current study, we used a custom-designed chest compression machine 8,9 , which allowed consistent delivery of randomized CC rates. Furthermore, it reduced any potential bias (e.g., fatigue during CC or inability to constantly achieve allocated rate). Indeed, manikin studies compared CC rates of 90/min vs. 120/min and observed higher fatigue and up to 50% decay in CC depth after 90 or 120 sec CC rate, respectively 10,11 . While we recognize that CPR using a CC rate of 180/min using a 3:1 C:V ratio is nearly impossible, it might be feasible using continuous compressions.
In the current study, we used continuous CC during sustained in ation to examine different CC rates. Although continuous CC during sustained in ation is mentioned in the knowledge gap section of the neonatal resuscitation guidelines, it is currently not recommended 1,2 . Using the 3:1 C:V ratio might have yielded other results. However, our automated CC machine can only provide continuous CC 8,9 .

Limitations
Our use of a piglet asphyxia model is a great strength of this translational study, as this model closely simulates delivery room events, with the gradual onset of severe asphyxia leading to bradycardia. Our asphyxia model uses piglets that have already undergone the fetal-to-neonatal transition, and piglets were sedated/anesthetized. Our model requires piglets to be intubated with a tightly sealed endotracheal tube to prevent endotracheal tube leak; this may not occur in the delivery room, as uncuffed endotracheal tubes are routinely used. A strength of this study is the use of our automated chest compression machine, which can apply high rates of CC 6-9,11,13,14,17,19-28 .

Conclusion
Time to ROSC and survival were improved with a CC rate of 180/min compared to 90/min. Respiratory and hemodynamic parameters were also improved with a CC rate of 180/min compared to 90/min. A higher CC rate might improve organ perfusion and oxygen delivery compared to lower CC rates and warrants further investigation.

Materials And Methods
All experiments were conducted between January and November 2020 in accordance with the guidelines and approval of the Animal Care and Use Committee (Health Sciences), University of Alberta (AUP00001764), presented according to the ARRIVE guidelines 12 , and registered at preclincialtrials.eu (PTCE0000148). A graphical display of the study protocol is presented in Figure 1. The authors declare that all supporting data are available within the article.

Randomization
Piglets were randomly allocated to two groups ("CC rate 180/min or "90/min"). Randomization was 1:1 with variable block sizes using a computer-generated randomization program (http://www.randomizer.org). Sequentially numbered, sealed, brown envelopes containing the group allocation were opened during the experiment (Figure 1).

Sample Size and Power Estimates
The primary outcome measure was the time of CPR to achieve ROSC. Our previous studies showed a mean (standard deviation) ROSC of 220 (25) sec with a CC rate of 90/min. We hypothesized that a CC rate of 180/min would reduce the time to achieve ROSC. A sample size of 7/group would be su cient to detect a clinically important (20%) reduction in time to ROSC (i.e., 176 sec vs. 220 sec) with 90% power and a 2-tailed alpha error of 0.05. Blinding TFL opened the randomization envelope and set the rate on the automated CC machine. GMS assessed cardiac arrest and was blinded to group allocation prior to the start of CC. However, due to the varying acceleration speed of the plunger, blinding was not feasible during CC. The statistical analysis was blinded to group allocation and unblinded after completion.

Inclusion and exclusion criteria
Newborn mixed-breed piglets (0-3 days of age) obtained on the day of experimentation from the University Swine Research Technology Center were included. There were no exclusion criteria.

Animal Preparation
Piglets were instrumented as previously described with some modi cations 13 . Following the induction of anesthesia using iso urane, piglets were intubated via tracheostomy, and mechanical ventilation (Sechrist infant ventilator model IV-100; Sechrist Industries, Anaheim, CA) was commenced at a 20/min rate, peak inspiratory pressure of 25 cmH2O and positive end-expiratory pressure of 5 cmH2O. Oxygen saturation was kept within 90-100%, glucose level and hydration were maintained with an intravenous infusion of 5% dextrose at 10 mL/kg/h. During the experiment, anesthesia was maintained with intravenous propofol 5-10 mg/kg/hr and morphine 0.1 mg/kg/hr. Additional doses of propofol (1-2 mg/kg) and morphine (0.05-0.1 mg/kg) were also given as needed, and their body temperature was maintained at 38.5-39.5°C by using an overhead warmer and a heating pad.

Hemodynamic Parameters
A 5-French Argyle ® (Klein-Baker Medical Inc. San Antonio, TX) double-lumen catheter was inserted into the femoral vein for uid administration and medications. A 5-French Argyle ® single-lumen catheter was inserted above the right renal artery via the femoral artery for continuous arterial blood pressure monitoring and arterial blood gas measurements. The right common carotid artery was exposed and encircled with a real-time ultrasonic ow probe (2 mm; Transonic Systems Inc., Ithica, NY) to measure carotid blood ow. A Millar catheter (MPVS Ultra, ADInstruments, Houston, TX) was inserted into the left ventricle (LV) via the left common carotid artery for continuous measurement of stroke volume, enddiastolic volumes, dp/dt max (maximal rate of rise of left ventricular pressure), and dp/dt min (minimum rate of change of ventricular pressure), which served as a surrogate for cardiac output. Because of the size difference between the Millar catheter and LV longitudinal axis, which poses a limitation for the accuracy of in vivo volume measurement, an alpha factor = 0.46, based on comparison between Millar's recording and direct echocardiographic measurements in three piglets, was used to correct the conductance volume 14 .
Piglets were placed in the supine position and allowed to recover from surgical instrumentation until baseline hemodynamic measures were stable (minimum of one hour). The ventilator rate was adjusted to keep the partial arterial CO 2 between 35-45 mmHg as determined by periodic arterial blood gas analysis.
Arterial blood pressure, heart rate, and percutaneous oxygen saturation were continuously measured and recorded throughout the experiment with a Hewlett Packard 78833B monitor (Hewlett Packard Co., Palo Alto, CA).

Respiratory Parameters
A respiratory function monitor (NM3, Respironics, Philips, Andover, MA) continuously measured tidal volume, airway pressures, gas ow, and end-tidal CO 2 . The sensor was placed between the endotracheal tube and the ventilation device. Tidal volume was calculated by integrating the ow signal, and end-tidal CO 2 was measured using a nondispersive infrared absorption technique 15,16 . The accuracy for gas ow was ±0.125 L/min, and the end-tidal CO 2 was ±2 mmHg.

Automated Chest Compression Machine
The settings for the automated CC machine were anterior-poster depth 33%, acceleration of compression 500 cm/s2 , speed of recoil 50 cm/s, a simulated two-thumb technique, and a CC rate of 90/min or 180/min according to group allocation 8,9 .

Experimental Protocol
Piglets were randomized into two groups: "CC rate 180/min" or "CC rate 90/min". Following surgical instrumentation and stabilization, the piglets were placed onto the automated CC machine, which was placed on the surgical bed. The piglets' chest diameter was measured from the sternum to the vertebrae touching the bed (anterior to posterior) with a measuring tape, and then the anterior-posterior depth of 33% was calculated 8,9 . Piglets were then exposed to 45 minutes of normocapnic hypoxia, which was followed by asphyxia. Asphyxia was achieved by disconnecting the ventilator and clamping the endotracheal tube until asystole. Asystole was de ned as zero carotid blood ow and no audible heartbeat during auscultation. Fifteen seconds after asystole, positive pressure ventilation was provided for 30 sec with a Neopuff T-Piece (Fisher & Paykel, Auckland, New Zealand) with 21% oxygen, peak inspiratory pressure of 30 cmH2O, positive end-expiratory pressure of 5 cmH2O, and gas ow of 8 L/min. After 30 sec of positive pressure ventilation, mechanical CC was initiated 8,9 using 21% oxygen 17,18 , with an antero-posterior chest diameter depth of 33% 8,9 , and continuous CC was initiated during sustained in ation (CC+SI) with a peak inspiratory pressure of 30 cmH2O for 30 sec [19][20][21] . Sustained in ation was interrupted for 1 sec before a further 30 sec of sustained in ation was provided, and this was continued until ROSC 13 . Epinephrine (0.02 mg/kg per dose) was administered intravenously 2 minutes after the start of positive pressure ventilation and every 3 minutes until ROSC with a maximum of three doses 1,2 , as the maximum resuscitation time was 10 minutes. The administration of epinephrine was immediately followed by a saline ush of 3 ml. ROSC was de ned as an unassisted heart rate >100/min for at least 15 sec. After ROSC, piglets recovered for one hour before being euthanized with an intravenous overdose of sodium pentobarbital (120 mg/kg). If there was no ROSC, piglets were euthanized immediately with an intravenous overdose of sodium pentobarbital (120 mg/kg).

Data collection and statistical analysis
Demographics of study piglets were recorded. Transonic ow probe, heart rate and pressure transducer outputs were digitized and recorded with LabChart ® programming software (AD Instruments, Houston, TX). To analyze the hemodynamic data until time to ROSC (i.e., arterial blood pressure, central venous pressure, and carotid blood ow), the duration of CC time was divided into 10 epochs. To analyze stroke volume, end-diastolic volume, dp/dt max , and dp/dt min , the pressure-volume loops were compared between groups. Airway pressures, gas ow, tidal volume, and end-tidal CO 2 were measured and analyzed using Flow Tool Physiologic Waveform Viewer (Philips Healthcare, Wallingford, CT, USA). For all respiratory parameters, the median values for each piglet during CPR were calculated rst, and then the mean of the median was calculated for comparison.
The data are presented as the mean (standard deviation -SD) for normally distributed continuous variables and median (interquartile range -IQR) when the distribution was skewed. The data were tested for normality (Shapiro-Wilk and Kolmogorov-Smirnov test) and compared using t tests or rank sum for normally or skewed distributed data. P values are 2-sided, and p<0.05 was considered statistically signi cant. Statistical analyses were performed with SigmaPlot (Systat Software Inc, San Jose, USA). We would like to thank the public for donating money to our funding agencies: The project was supported by a project grant from the Laerdal Foundation. GMS was a recipient of the Heart and Stroke Foundation/University of Alberta Professorship of Neonatal Resuscitation, a National New Investigator of the Heart and Stroke Foundation Canada and an Alberta New Investigator of the Heart and Stroke Foundation Alberta.   Changes in systolic blood pressure, diastolic blood pressure, mean blood pressure, and carotid blood ow during chest compression were divided into 10 epochs Figure 3 Stroke volume, end-diastolic volume, dp/dt max (maximal rate of rise of left ventricular pressure), and dp/dt min (minimum rate of change of ventricular pressure)