Hemodynamic efficacy of a motor-driven automatic device performing simultaneous sternothoracic cardiopulmonary resuscitation compared to standard cardiopulmonary resuscitation in an animal model of cardiac arrest

2.1 Device description

The automatic SST-CPR consists of two parts: a CPR-performing apparatus and a backboard (Fig. 1). The CPR performing apparatus is composed of a piston, a chest strap, actuating and controlling parts, and a supporting frame. The piston and actuating and controlling parts are mounted on the supporting structure. The piston is a rectangular bar, used to compress the sternum. The chest strap is to constrict the chest circumferentially. It surrounds the thorax when combined with the CPR-performing apparatus and the backboard. The piston pulls the chest straps attached to both sides of the piston as much as the compression depth when compressing the sternum, so the chest constriction occurs twice the depth of chest compression. The up-and-down motion of the piston is driven by a battery-powered motor. The parameters of the piston-driven chest compressions are adjustable. The compression depth of the sternum can be controlled from 0 to 6 cm, and the compression rate can be controlled from 0 to 110 compressions per minute (CPM). The actuating and controlling parts provide the electronic control of the device. The supporting frame is connected to the backboard and serves to support the piston and actuating and controlling parts. When the supporting frame and the backboard are combined, the chest strap on both sides is connected to each other. The backboard includes an apparatus to control the length of the thoracic strap automatically according to the size of the chest.

Fig. 1.Automatic SST-CPR device (X-CPR). The main compression unit is positioned in the top of the device which is operated by a battery-powered motor. The strap is attached to both sides of the central piston.

2.2 Animal experiment

Domestic male Yorkshire pigs, weighing 35–48 kg, aged 8–11 weeks, from a single-source breeder were used in the study. The pigs were housed in a temperature (18–25 °C) and humidity (40–60%) controlled room. The experimental procedures and protocols were carried out in compliance with ARRIVE guidelines [16].

2.3 Preparation of animals

The pigs were fasted overnight but allowed free access to water. Anesthesia was induced through the intramuscular injection of ketamine (30 mg/kg) and xylazine (2 mg/kg), followed by inhaled 3% isoflurane during preparation. Jaw tone was assessed throughout the procedure as an anesthetic depth indicator. Endotracheal intubation was performed with a cuffed endotracheal tube in the prone position. The placement of the endotracheal tube was confirmed by auscultation and measurement of end-tidal carbon dioxide (ETCO₂) levels (CO₂SMO, Philips Respironics, Murrysville, PA, USA). After intubation, the animals were placed in the supine position. The pigs were ventilated with room air, via a volume-controlled ventilator (MDS Matrix 3000, Orchard Park, NY, USA). The tidal volume was set at 10 mL/kg and ventilation rate at 18 breaths per minute. Electrocardiography (ECG lead II) was performed and ETCO₂ levels were monitored continuously.

Under aseptic conditions, an introducer sheath (7.5 Fr, Arrow International Inc., Reading, PA, USA) was inserted into the right femoral artery, as per the Seldinger method, and a micromanometer-tipped catheter (5 Fr, Millar Instruments, Inc., Houston, TX, USA) was introduced into the femoral sheath and advanced to the thoracic aorta to continuously record the arterial pressure [17]. After right cervical dissection, two introducer sheaths were placed in the right external and internal jugular veins. A micromanometer-tipped catheter (6 Fr, Millar Instruments, Inc., Houston, TX, USA) was introduced into the right atrium and a pacing electrode catheter (5 Fr, bipolar lead, Arrow International Inc., Reading, PA, USA) was positioned in the right ventricle. The catheter position was confirmed by characteristic pressure tracing from the cardiac chamber and a postmortem examination. Once the catheters were in place, a heparin bolus (100 units/kg, IV) was administered to prevent thrombosis. The mean right atrial (RA) pressure was maintained at approximately 5 mmHg through the administration of intravenous fluid. Electrode pads for defibrillation placed in the right upper quadrant and the left lower quadrant of the chest. The pigs were stabilized at least 10 minutes before the baseline measurement was recorded.

2.4 Randomization and induction of ventricular fibrillation

After preparation, each animal was randomized into the standard CPR (S-CPR) group or the automatic CPR (A-CPR) group according to the results after the researcher opened the sealed, opaque envelope containing the results of randomization by one of the investigators. After the baseline measurement was done, ventricular fibrillation (VF) was induced by delivering 30–60 mA of electrical current, at 60 Hz for a duration of 5 seconds, to the endocardium, via the electrode in the right ventricle. The occurrence of VF was confirmed by the ECG waveform and the disappearance of arterial pressure. Once VF was induced, the endotracheal tube was disconnected from the ventilator and the pigs were observed for 5 minutes without any procedure or treatment.

2.5 Experimental protocol

After 5 minutes of VF, the animals received either S-CPR or A-CPR, according to the randomization result. During the first 5 minutes, the ratio of compressions to ventilation was maintained at 30:2 as in basic life support (BLS). For S-CPR group, manual chest compressions were performed by investigators according to the CPR guidelines [18]. All compressors were emergency physicians certified with the American Heart Association basic life support training program. Chest compression was performed at the center of the chest with a depth of about 5 cm and at a rate of 100 per minute following audio prompt from metronome. The compressors were switched every 2 minutes. For A-CPR group, the animal was placed on the backboard of the automatic SST-CPR device after induction of VF. Then, the automatic SST-CPR device was applied to the animals with the piston positioned in the center of the chest. During A-CPR, chest compression was set at a rate of 100/min and a depth of 5 cm (a thoracic constriction of 10 cm). Positive pressure ventilations were delivered with a resuscitator bag (silicone resuscitator 870150, Laerdal Medical, Stavanger, Norway). A tidal volume of approximately 300 mL per breath was delivered by a resuscitator bag. Two breaths were given within 6 seconds, after 30 continuous chest compressions. After 5 minutes of CPR, advanced cardiovascular life support (ACLS) was implemented and continued for 10 minutes. Continuous chest compressions were performed without interruption for ventilation, while asynchronous, intermittent, and manual ventilations were administered with the resuscitator bag at a rate of 10 ventilations per minute. Defibrillation was performed every 2 minutes with 2J/kg for the first shock and 4J/kg for subsequent shocks. One mg of epinephrine was administered into the right atrium every 3 minutes. All the resuscitation efforts were stopped if ROSC was not achieved after 10 minutes of ACLS. If ROSC was achieved, isoflurane inhalation was restarted. The animals were stabilized using vasopressors and survival was monitored for 2 hours. If an animal survived for 2 hours, then it was euthanized with an intravenous injection of potassium chloride.

2.6 Data measurements and resuscitation outcomes

Data were digitized with a digital recording system (Powerlab, AD Instruments, CO, USA). All parameters including systolic blood pressure (SBP), diastolic blood pressure (DBP), coronary perfusion pressure (CPP), right atrial pressure (RAP) and, ETCO₂ were continuously recorded during the experiment. CPP was calculated as the difference between the arterial pressure and right atrial pressures during the end-diastolic phase using an electronic subtraction unit. The primary outcome was measured by SBP to compare the hemodynamic effects in the two groups. The secondary outcomes were successful defibrillation, ROSC, and 2-hour survival. Successful defibrillation was defined as the termination of fibrillation with the return to an organized electrical rhythm at 5 seconds after defibrillation. ROSC was defined as the maintenance of a SBP of at least 60 mmHg for at least 10 consecutive minutes [19].

2.7 Statistical analysis

On the basis of a previous study reporting a 44 mmHg difference of SBP with standard deviation (SD) of 35 mmHg between the S-CPR and A-CPR group, it was calculated at least 10 subjects would be needed in both groups to provide a statistical power of 80% with a two-sided alpha value of 0.05 [15]. Twenty animals were chosen, considering that 10% of animals would be excluded in the analysis due to unpredictable experimental failure.

Hemodynamic data were analyzed using the average of each parameter measured during the data-sampling period. The values of each parameter were the averages of the values measured for 30 seconds at 6, 8, 10, 12, 14, 16, 18, and 20 minutes after the induction of VF. Continuous variables were presented as mean ± SD. A student’s t-test was used to compare the continuous variables between the S-CPR and A-CPR group. The nominal variables were reported as counts and percentages, and were compared with a chi-square or Fisher’s exact test, as appropriate. A linear mixed model analysis was used to compare hemodynamic parameters. A value of p < 0.05 was considered significant. Analyses were carried out using SPSS statistics 20.0 for Windows (IBM Corp., Chicago, IL, USA). Any differences were regarded as significant if the p-values were less than 0.05.

3.1 Baseline measurements

Of the twenty pigs used in the study, 10 animals were included in S-CPR group and 10 in the A-CPR group. Among the S-CPR group, one case with dislodged catheter and one case with arterial bleeding were excluded from the analysis. There was no difference in the baseline characteristics and hemodynamic parameters between the two groups (Table 1).

3.2 Comparisons of the hemodynamic effects between S-CPR and A-CPR

SBP was significantly higher in the A-CPR group than the S-CPR group (p = 0.046 by group-time interaction analyses). The hemodynamic parameters including DBP, right atrial diastolic pressure, CPP and ETCO₂ did not differ statistically between two groups (Table 2) (Fig. 2).

Fig. 2.Comparisons of the hemodynamic effects between S-CPR and A-CPR. SBP, systolic blood pressure; DBP, diastolic blood pressure; RADP, right atrial diastolic pressure; CPP, coronary perfusion pressure; ETCO₂, end-tidal carbon dioxide.

3.3 Energy requirement and resuscitation outcome

The total defibrillation energy doses were 775 ± 161 J in the S-CPR group and 901 ± 278 J in the A-CPR group. ROSC was observed in three pigs (38%) in the S-CPR group and one (10%) in the A-CPR group. Two pigs (25%) in the S-CPR group and none in the A-CPR group achieved 2-hour survival (Table 3).

3.4 Complications

Complications, including rib fractures, lung contusion, hemothorax, or hemopericardium, were found on autopsy (Table 4). Rib fracture was noticed more frequently in S-CPR group (p = 0.043) than in A-CPR group. A minimal amount of hemopericardium was seen in a case of both groups.