Cardiopulmonary resuscitation and defibrillation are the only effective therapeutic options during cardiac arrest caused by ventricular fibrillation. This model presents a standardized regimen to induce, assess, and treat this physiological state in a porcine model, thus providing a clinical approach with various opportunities for data collection and analysis.
Cardiopulmonary resuscitation after cardiac arrest, independent of its origin, is a regularly encountered medical emergency in hospitals as well as preclinical settings. Prospective randomized trials in human subjects are difficult to design and ethically ambiguous, which results in a lack of evidence-based therapies. The model presented in this report represents one of the most common causes of cardiac arrests, ventricular fibrillation, in a standardized setting in a large animal model. This allows for reproducible observations and various therapeutic interventions under clinically accurate conditions, hence facilitating the generation of better evidence and eventually the potential for improved medical treatment.
Cardiac arrest and cardiopulmonary resuscitation (CPR) are regularly encountered medical emergencies in hospital wards as well as preclinical emergency provider scenarios1,2. While there have been extensive efforts to characterize the optimal treatment for this situation3,4,5,6, international guidelines and expert recommendations (e.g., ERC and ILCOR) usually rely on low-grade evidence due to the lack of prospective randomized trials3,4,5,7,8,9. This is in part due to obvious ethical reservations regarding randomized resuscitation protocols in human trials10. However, this may also point towards a lack of strict protocol adherence when confronted with a life-threatening and stressful situation11,12. The protocol presented in this report aims to provide a standardized resuscitation model in a realistic clinical setting, which generates valuable, prospective data while being as valid and accurate as possible without the need for human subjects. It adheres to common resuscitation guidelines, can be easily applied, and enables researches to examine and characterize various aspects and interventions in a critical but controlled setting. This will lead to 1) a better understanding of the pathological mechanisms underlying cardiac arrest and ventricular fibrillation and 2) higher quality evidence in order to optimize treatment options and increase survival rates.
The experiments in this protocol were approved by the State and Institutional Animal Care Committee (Landesuntersuchungsamt Rheinland-Pfalz, Koblenz, Germany; Chairperson: Dr. Silvia Eisch-Wolf; approval no. G16-1-042). The experiments were conducted in accordance with the ARRIVE guidelines. Seven anesthetized male pigs (sus scrofa domestica) with a mean weight of 30 ± 2 kg and 12-16 weeks in age were included in the protocol.
1. Anesthesia, intubation, and mechanical ventilation13,14
2. Instrumentation
3. Pulse contour cardiac output
4. Ventricular fibrillation and mechanical resuscitation
5. End of experiment and euthanasia (in the case of ROSC)
Cardiac arrest was induced in seven pigs. Return of spontaneous circulation following CPR was achieved in four Pigs (57%) with a mean of 3 ± 1 biphasic defibrillations. Healthy and adequately anesthetized pigs should stay in supine position without shivering and signs of agitation throughout the entire experiment. Mean arterial blood pressures should not drop below 50 mmHg before initiation of fibrillation18. For optimal results, blood gas analyses can be performed and all values including temperature should be normalized.
If placed in the right position, the pacing catheter should start to influence heart rhythm. This can result in extrasystoles, tachycardia and all forms of ventricular and supraventricular arrhythmias. Cardiac arrest can be assumed if 1) the ECG reading shows ventricular fibrillation and 2) no cardiac output or pressure variations are measured by the arterial line (Figure 1). If this state persists with the generator turned off, fibrillation is likely not to spontaneously subside anymore17.
Once chest compressions are started, sufficient cardiac output generation is indicated by a mean arterial pressure of 30-50 mmHg. (Figure 1) If adhering to resuscitation guidelines, the administration of adrenaline (1 mg) should result in a substantial rise in blood pressure within 1 min.
ROSC is confirmed by a dramatic increase in expiratory carbon dioxide measurements (usually increasing from 10-20 mmHg during arrest to 45 mmHg and above), organized heart rhythm in the ECG, and respective cardiac output as shown by arterial measurement. Hypercapnia and a decreased Horovitz index (PaO2/FiO2) are commonly observed after ROSC. Reestablishment of controlled mechanical ventilation leads to recompensation and stable respiratory conditions (Figure 2). A ROSC rate of 50%-70% can be expected depending on the time between cardiac arrest and the start of chest compressions.
Figure 1: Typical hemodynamic values. (A) Heart rate monitoring during trial (depicted as mean values with standard deviation [SD] error bars). Heart rate drops to zero at cardiac arrest (CA) and is standardized during CPR according to the specifications of the chest compression device (here, 100 bpm). Tachycardia is regularly seen after achieving ROSC, initially as a result of adrenaline administration and metabolic acidosis compensation. Values usually normalize over a period of 1-2 h. (B) Mean intra-arterial blood pressure values. At cardiac arrest (CA), pressure does not drop below 10-20 mmHg but loses all signs of effective output. During CPR, especially before vasopressor effects are registered, adequate chest compressions are indicated by pressure values between 30-50 mmHg. Post-ROSC, norepinephrine might be necessary to cover low blood pressure intervals during metabolic recompensation. Please click here to view a larger version of this figure.
Figure 2: Oxygenation and decarboxylation parameters during and after resuscitation. (A) Arterial partial pressure values of carbon dioxide (PaCO2) during and after CPR (depicted as mean values with standard deviation error bars). Under guideline-based ventilation, no significant differences should be detected. An increase in CO2 levels directly after ROSC is to be expected but should normalize within 1 h. (B) Typical values of Horovitz index (arterial partial pressure of oxygen [PaO2]/inspiratory oxygen fraction [FiO2]; depicted as mean values with SD error bars). During CPR, oxygenation is often highly impaired but usually fully recovers post-ROSC during the first 2 h. Please click here to view a larger version of this figure.
Some major technical issues regarding anesthesia in a porcine model have previously been described by our group13,14. These include the strict avoidance of stress and unnecessary pain for the animals, possible anatomical problems during airway management, and specific personnel requirements19.
Additionally, the benefits of ultrasound-guided catheterization was highlighted previously and remains the preferable approach to prevent vascular damages during instrumentation. However, only professionally trained users should work with this technique to yield its advantages20. For this experimental model, it must be stressed that handling electrical frequency generators as well as defibrillators should only be handled by specifically trained personnel or under their direct supervision. Failure to provide adequate expertise while conducting such trials may result in serious injury and can be life-threatening.
Correct positioning of the pacing catheter and initiation of ventricular fibrillation may prove difficult and can require reinsertion of the catheter or frequency variation. When repositioning or removing the catheter, the balloon should be deflated first to prevent internal injuries as well as damage to the catheter itself. If frequency variations are used, the catheter should be placed near the myocardium in order to detect ECG changes, then frequency should slowly be changed according to the manufacturer's instructions. Importantly, the chest compression device has to be positioned correctly and the pig has to be properly immobilized (as shown in the video). Repositioning during CPR can be necessary but often leads to insufficient resuscitation. Even though thoracic anatomy and bone structure differs compared to humans, our studies showed sufficient perfusion generation and ROSC rates with a compression device placed on the lower third of the sternum in median position.
Porcine models have been successfully used in critical care studies for decades17,21,22,23. Similar anatomic and physiologic properties comparable to humans allow for reasonably accurate deductions regarding patient reactions to certain stimuli or clinical situations. The presented resuscitation model has been used and modified in various trials18,24,25,26. It provides an experimental setting that enables the evaluation of guideline effectiveness, since (in contrast to resuscitation models in rodents) equal chest compression intervals, blood pressure thresholds, blood gas values, and defibrillation energies can be used for human comparisons as recommended by ILCOR and ERC, respectively. This facilitates internationally comparable and comprehensible study designs, thus generating a higher quality of evidence overall. The model additionally allows for adequate assessment of drug effects not only qualitatively, but also in a dose-dependent fashion.
Assuming guideline-based resuscitation with intervals of 2 min between defibrillations, pigs usually achieve ROSC within the first four shocks or within 8-10 min27. A ROSC rate of 50%-70% can be expected depending on the time between cardiac arrest and the start of chest compressions. If acceptable ROSC rates or adequate blood pressure values cannot be achieved, it is possible to add vasopressine (0.5 IU/kgBW) to the therapy regimen during CPR. During and directly after CPR, pulmonary gas exchange is heavily impaired. This is largely dependent on the ventilation mode used during chest compressions and can have long-term effects on end organ damage and inflammation18,25,28. Additionally, metabolic acidosis and stunned myocardium can lead to persistent hypotension, especially in the first 1 h following ROSC. This can be treated by fluid administration (20-30 mL/kgBW) and continuous norepinephrine infusion. Excessive acidosis can also be treated with 8.4% sodium bicarbonate solution with a maximum of 4 mL/kgBW.
This experimental protocol provides a standardized setting for resuscitation research in which the aspects of hemodynamic effects of specific drug treatments, influence of ventilation modes on ROSC rates, end-organ damage, and post-resuscitation reactions can be analyzed and evaluated under various circumstances. This will help further scientific insight into the pathophysiologic mechanisms underlying ventricular fibrillation and may lead to more effective treatment options.
The authors have nothing to disclose.
The authors want to thank Dagmar Dirvonskis for excellent technical support.
1 M- Kaliumchlorid-Lösung 7,46% 20ml | Fresenius, Kabi Deutschland GmbH | potassium chloride | |
Arterenol 1mg/ml 25 ml | Sanofi- Aventis, Seutschland GmbH | norepinephrine | |
Atracurium Hikma 50mg/5ml | Hikma Pharma GmbH, Martinsried | atracurium | |
BD Discardit II Spritze 2,5,10,20 ml | Becton Dickinson S.A. Carretera Mequinenza Fraga, Spain | syringe | |
BD Luer Connecta | Becton Dickinson Infusion Therapy AB Helsingborg, Schweden | 3-way-stopcock | |
BD Microlance 3 20 G | Becton Dickinson S.A. Carretera Mequinenza Fraga, Spain | canula | |
CorPatch Easy Electrodes | CorPuls, Kaufering, Germany | defibrillator electrodes | |
Corpuls 3 | Corpuls, Kaufering, Germany | defibrillator | |
Datex Ohmeda S5 | GE Healthcare Finland Oy, Helsinki, Finland | hemodynamic monitor | |
Engström Carestation | GE Heathcare, Madison USA | ventilator | |
Fentanyl-Janssen 0,05mg/ml | Janssen-Cilag GmbH, Neuss | fentanyl | |
Führungsstab, Durchmesser 4.3 | Rüsch | endotracheal tube introducer | |
Incetomat-line 150 cm | Fresenius, Kabi Deutschland GmbH | perfusorline | |
Ketamin-Hameln 50mg/ml | Hameln Pharmaceuticals GmbH | ketamine | |
laryngoscope | Rüsch | laryngoscope | |
logicath 7 Fr 3-lumen 30cm lang | Smith- Medical Deutschland GmbH | central venous catheter | |
LUCAS-2 | Physio-Control/Stryker, Redmond, WA, USA | chest compression device | |
Masimo Radical 7 | Masimo Corporation Irvine, Ca 92618 USA | periphereal oxygen saturation | |
Neofox Oxygen sensor 300 micron fiber | Ocean optics Largo, FL USA | ultrafast pO2-measurements | |
Ölsäure reinst Ph. Eur NF C18H34O2 M0282,47g/mol Dichte 0,9 | Applichem GmbH Darmstadt, Deutschland | oleic acid | |
Original Perfusor syringe 50ml Luer Lock | B.Braun Melsungen AG, Germany | perfusorsyringe | |
Osypka pace, 110 cm | Osypka Medical GmbH, Rheinfelden-Herten, Germany | Pacing/fibrillation catheter | |
PA-Katheter Swan Ganz 7,5 Fr 110cm | Edwards Lifesciences LLC, Irvine CA, USA | PAC | |
Percutaneous sheath introducer set 8,5 und 9 Fr, 10 cm with integral haemostasis valve/sideport | Arrow international inc. Reading, PA, USA | introducer sheath | |
Perfusor FM Braun | B.Braun Melsungen AG, Germany | syringe pump | |
Propofol 2% 20mg/ml (50ml flasks) | Fresenius, Kabi Deutschland GmbH | propofol | |
Radifocus Introducer II, 5-8 Fr | Terumo Corporation Tokio, Japan | introducer sheath | |
Rüschelit Super Safety Clear >ID 6/ 6,5 /7,0 mm | Teleflex Medical Sdn. Bhd, Malaysia | endotracheal tube | |
Seldinger Nadel mit Fixierflügel | Smith- Medical Deutschland GmbH | seldinger canula | |
Sonosite Micromaxx Ultrasoundsystem | Sonosite Bothell, WA, USA | ultrasound | |
Stainless Macintosh Größe 4 | Welsch Allyn69604 | blade for laryngoscope | |
Stresnil 40mg/ml | Lilly Deutschland GmbH, Abteilung Elanco Animal Health | azaperone | |
Vasofix Safety 22G-16G | B.Braun Melsungen AG, Germany | venous catheter | |
Voltcraft Model 8202 | Voltcraft, Hirschau, Germany | oscilloscope/function generator |