Impact of concomitant vasoactive treatment and mechanical left ventricular unloading in a porcine model of profound cardiogenic shock

Ten female Danish Landrace pigs weighing approximately 70 kg were studied. The study was approved and conducted per guidelines of the Danish Animal Experiments Expectorate (authorization number: 2016-15-00951). Unfractionated heparin (20 IU) was administered every 2 h to avoid blood clotting during the experiment. Amiodarone (300 mg) was injected before instrumentation followed by continuous infusion of 50 mg/h to avoid malignant arrhythmias. Instrumentation was done using the percutaneous Seldinger technique, except for the surgical exposure of the internal jugular vein. Instrumentation included placement of a conductance catheter (Ventri-Cath 512 PV Loop Catheter, Millar Inc.) in the LV for continuous recordings of pressure-volume (PV) relationships, a conductance catheter in the aorta to measure aortic pressure, a central line, and a continuous CO 7.5-Fr Swan-Ganz catheter with SvO₂ recording (Edwards Lifesciences Corp. Irvine, CA, USA) placed in the pulmonary artery. A conventional triple lumen 7-Fr Swan-Ganz catheter (Edwards Lifesciences Corp. Irvine, CA, USA) was placed in the renal vein via femoral venous access, and a 4-Fr double-lumen central venous catheter (Cook Medical, Bloomington, USA) in a retrograde fashion in the internal jugular vein to obtain organ-specific blood gasses for measuring oxygen saturation and lactate levels.

Before the start of the study, a sealed envelope listing the order of the infusions of epinephrine, norepinephrine, and dopamine was handed to an independent individual who prepared and labeled the infusions with a number signifying the order. The infusions were prepared and administered at fixed infusion rates to what was considered equipotent doses and not to target a predefined MAP, equivalent to a dose of norepinephrine 0.10 μg/kg/min, dopamine 10 μg/kg/min, epinephrine 0.10 μg/kg/min, and phenylephrine 10 μg/kg/min. The infusion of phenylephrine alone was not blinded given the long half-life and administered in all pigs in the end. All animals were treated with a fluid regime of 1 L of isotonic saline the first hour and afterwards 900 mL/h, which was shifted between Ringer acetate and isotonic saline.

CS was induced by stepwise injection of polyvinyl alcohol microspheres (Contour™, Boston Scientific, Marlborough, MA, USA) in the left main coronary artery through a JL3.5 guide catheter (Launcher, Medtronic Inc., Minneapolis, MN, USA) [17]. Hemodynamics were allowed to stabilize for 2–3 min after each injection before the administration of the next injection. Stepwise injections of microspheres was continued until CS developed, defined as mixed venous oxygen saturation (SvO₂) reduction to < 30% or ≤ 50% of baseline value and/or sustained cardiac index < 1.5 L/min/m² for ≥10 min. A median of 12 microsphere injections (interquartile range, 9–17) was required to induce CS. Following the onset of CS, Impella CP was advanced from the left femoral artery and placed across the aortic valve with the inlet in the left ventricle and outlet in the ascending aorta. The placement of Impella CP was guided by fluoroscopy, and the maximum pump speed possible was achieved and maintained during the entire study. Vasoactive treatment with norepinephrine, dopamine, or adrenaline was randomized, and the treating team was blinded to the treating order. The first vasoactive infusion started following 30 min of Impella CP support, and each infusion was administered for 30 min. The initial experimental plan included an Impella CP only phase (no vasoactive drugs) for 30 mins and a washout phase (no infusion of vasoactive drugs) between each drug infusion. However, due to severe hypotension (mean arterial blood pressure (MAP) < 50 mmHg) during the Impella alone and washout phase in the pilot pigs, the experimental protocol was changed and Impella support was combined with a minimum dose of norepinephrine to maintain MAP > 50 mmHg. Also, the withdrawal and initiation of subsequent drug infusion overlapped to avoid any drop in arterial pressure. Vasoactive drug infusion was withdrawn following any change in hemodynamics (mean arterial pressure or heart rate). Phenylephrine was administered in all pigs for 20 min after the completion of all three catecholamine infusions, followed by euthanization.

A conductance catheter was inserted through a sheath in the right carotid artery and advanced retrograde into the LV and connected to an MPVS Ultra® Pressure-Volume (PV) loop system (Millar Inc., 6001 Gulf Fwy, Houston, TX, USA). The PV relationships were available in 9 pigs at all time points and not available in 1 pig due to disturbance in the volume signal. The MPVS Ultra® PV loop system was connected to a PowerLab 16/35 (ADInstruments, Dunedin, New Zealand), and PV measurements were continuously recorded in LabChart Pro (ADInstruments, Dunedin, New Zealand). Volumes were calibrated using an alpha correctional value, and parallel wall conductance was determined using the hypertonic saline method. Data recorded from the conductance catheter comprised of the following: pressure-volume area (PVA, mmHg × mL), LV end-diastolic pressure (LVEDP, mmHg), LV end-diastolic volume (LVEDV, mL), LV end-systolic pressure (ESP), LV stroke work (SW, mmHg × mL), LV end-systolic pressure-volume relationship (Ees), and heart rate (HR, bpm). In all the pigs, balloon occlusion of the inferior vena cava was performed in the healthy condition at the start of the study, and the acquired V₀ (theoretical ventricular volume when no pressure is generated) was kept as a constant throughout the study to generate single-beat estimations of Ees and PVA [18, 19]. Ees was derived from Ees = LVESP/(LVESV-V0) [20]. Potential energy (mmHg × mL) was estimated using the formula, PE = LVESP(LVESV-V0)/2 [21]. All other variables were extracted from the software program. The slope of the line from LVEDV to LVESP on the P-V loop was used to calculate arterial elastance (Ea). Ventriculo-arterial coupling was assessed as the ratio between Ea and Ees [22].

Data were collected at seven prespecified time points: baseline before injection of microspheres, the onset of CS before initiation of Impella support, after 30 min of Impella support, at the end of each blinded infusion, and after 20 min of phenylephrine infusion. Collected data included systemic and pulmonary artery blood pressure, central venous blood pressure, blood gasses assessing oxygen saturation and lactate levels from the femoral and pulmonary artery, and renal and internal jugular veins. PV relationships including LVEDP, LVEDV, LVESP, LVEDP, SW, potential energy, PVA, HR, Ees, and Ea were determined for the same time points.

The primary efficacy parameters of the study were PVA and cardiac work (HR × PVA), both parameters closely related to myocardial oxygen consumption [21]. End-organ perfusion was estimated based on organ-specific (cerebral and renal) and overall venous saturations.

Following the induction of CS, defined as a 50% reduction in SvO₂ and cardiac index < 1.5 L/min/m², a significant increase in LVEDV and LVEDP was observed concomitant with a significant reduction in SW and stroke volume (Table 1 and Fig. 1). The systemic blood pressure was compromised with a mean MAP of < 40 mmHg, Table 1.

Initiation of Impella CP resulted in a reduction of LVEDV by 33% and potential energy by > 40% with little change in SW (Table 1). The PV loop shifted leftward and became triangular (Fig. 1). Despite Impella support, the MAP remained < 50 mmHg in 7 pigs. Hence, a low-dose norepinephrine (median 0.02 μg/kg/min [interquartile range 0.02, 0.05]) was administered to increase MAP > 50 mmHg. Impella support reduced the cardiac work (HR × PVA) via a reduction in both potential energy and HR (Table 1 and Fig. 1). Although Impella support increased renal, cerebral, and mixed venous saturations, the levels did not reach the baseline level, while the lactate levels did not change significantly during Impella support (Table 1 and Fig. 2). The SvO₂ increased during Impella support from 37% (95% CI 39, 44) to 55% (95% CI 47, 64). A laminar aortic flow was observed after 30 mins of Impella support in four pigs, suggesting uncoupling between the ventricular and aortic peak pressure.

All vasoactive drugs caused a significant increase in PVA, albeit to different degrees. SW increased significantly with all catecholamines, but remained unchanged with phenylephrine. On the other hand, only phenylephrine caused a significant increase in potential energy. HR increased with all drugs, except for norepinephrine. Thus, cardiac work increased significantly with all vasoactive drugs (Table 2). The average effects on PV loops are summarized in Fig. 1. In general, catecholamines increased SW and reduced LVESV, thus causing a leftward shift of the PV loop. In contrast, phenylephrine caused a rightward shift of the PV loop with an increase in LVEDV and LVEDP (Fig. 1 and Table 2).

Also, phenylephrine significantly increased right atrial and mean pulmonary artery pressure, an effect not observed with any of the other vasoactive drugs (Table 2).

SvO₂ increased with vasoactive drugs, except phenylephrine which caused a slight decrease [(mean difference from Impella alone), − 9% [95% CI (− 19% to 0%)], p = 0.063] (Fig. 2). Renal venous saturations also decreased with phenylephrine while there was no change with any of the other drugs. Cerebral venous saturation increased significantly with dopamine, slightly with norepinephrine, and did not change with phenylephrine (Fig. 2). Signs of end-organ ischemia were observed with phenylephrine with a significant increase in arterial and venous lactate levels (Fig. 2). In contrast, the catecholamines did not cause any change in lactate levels, although there was a trend towards lower lactate concentration with norepinephrine (p = 0.06) (Fig. 2). The correlation between SvO₂ and cardiac work during different stages of the study is shown in Fig. 3.

CS was characterized by a decrease in Ees and an increase in ventriculo-arterial decoupling (Ea/Ees) from 1.3 (1.3–2.0) to 7.2 (3.8–10.6) (Table 1). Dopamine significantly increased Ees, but no significant changes were observed with all the other drugs. Phenylephrine significantly increased Ea (Table 2). Consequently, ventriculo-arterial coupling significantly improved with dopamine (p = 0.03), a trend towards improvement was observed with epinephrine (p = 0.09), but remained unchanged with phenylephrine and norepinephrine.

In this study, vasoactive drugs were administered at what is considered equipotent doses and not titrated against a predefined target MAP, as done in the clinical setting. The doses of dopamine and phenylephrine used in this study might have been too low compared to the new vasoactive inotropic score [34]. In our opinion, a higher dose of phenylephrine would not be beneficial given the adverse effect of the low dose used in this study. Also, increasing the dopamine dose would result in vasoconstriction (alpha-receptor stimulation) with the risk of negative impact on cardiac work and coronary perfusion. The experimental setting based on a standard operating procedure used in this study aids in reproducibility and reducing the variability involved in testing the physiological effects of drug therapies in an acute setting. A high number of animals would have been required to compare the effects inter-individual. Despite being clearly superior, this was not feasible in terms of time required and expenses. Thus, we chose to do the cross-over design and make intra-individual comparisons to allow for a lower number of animals. However, the small sample size of the study is a limitation and may result in a type II error. Given the risk of hemodynamic instability, we did not include washout periods between drug infusions. Nonetheless, we do not expect a carryover effect of a previous drug infusion as the vasoactive agents have a short half-life, and the measurements were taken at the end of each infusion. The diuresis was not recorded systematically as we in design of study considered the individual duration of intervention too short to have confidence that the diuresis during each intervention was not mostly carry over effect of previous. The crossover design and statistical analyses were undertaken considering the potential effect of the timing of the interventions. Phenylephrine was administered in all the pigs at the end of the experiment due to its long half-life. The pigs may have developed more severe CS in the end, which could have affected the results. We attempted to adjust for this effect by using the linear mixed model and believe that the adverse effect of phenylephrine observed in this study reflects the drug’s effect and not a time-dependent artifact. The experimental observation period in this study is probably too short and may not reflect the long-term effects of concomitant vasoactive treatment and LV unloading in AMICS. Given the similar body size and adrenoceptor distribution and function among pigs and humans, the results may apply to the human treatment of CS [24].