Cardiovascular Effects of Increasing Positive End-expiratory Pressure in a Model of Left Ventricular Cardiogenic Shock in Female Pigs

In this prospective experimental study, female Danish Landrace pigs (n = 26) with a mean weight of 60 ± 2 kg were included. The study was conducted in accordance with the Planning Research and Experimental Procedures on Animals: Recommendations for Excellence (PREPARE) guidelines for study planning and Animal Research: Reporting of In Vivo Experiments (ARRIVE) 2.0 guidelines for comprehensive study reporting. A detailed description on animal care and treatment is provided in Supplemental Digital Content 1 (https://links.lww.com/ALN/D666). The current study was conducted in animals included in another study protocol in our laboratory, allowing for maximal power and respect of the 3R framework aimed at reducing the use of animals in research. The original study focused on investigating the hemodynamic effects of ketone body infusion during CS. The protocol for the current study was planned and predefined in the same ethical authorization issued by Danish national Animal Experiment Inspectorate (permit 2023-15-0201-01466, issue date June 19, 2023) and was performed in the animal research facility at Aarhus University Hospital. The number of animals was predetermined for the original study. Several measures were taken to mitigate risk of bias between study protocols. This included the conduction of pilot studies ensuring the reversibility and feasibility of short-term PEEP alterations and the implementation of a proper interstudy washout. A detailed description can be found in Supplemental Digital Content 1 (https://links.lww.com/ALN/D666). The treatment and ethical oversight of the animals strictly adhered to established animal welfare protocols and regulatory standards stipulated by both Danish and European legislation. The methodologies and animal handling conformed to European Union directive 2010/63/EU pertaining to animal experimentation. Upon the end of the study, all animals were humanely euthanized through administration of a lethal dose of pentobarbital (Euthanimal, Scanvet, Denmark).

The study applied a procedure to minimize stress and increase refinement in the care of the pigs. Initially, the pigs received sedation on the farm via intramuscular injection of a commonly used anesthetic mix (Zoletil 50 Vet, Virbac, Denmark) before transportation to the laboratory facility. The pigs were intubated and immediately placed under invasive PPV (GE Datex Ohmeda Avance S5, General Electric, USA). Anesthesia maintenance entailed an intravenous continuous infusion of propofol (3.5 mg · kg–1 · h–1) and fentanyl (15 µg · kg–1 · h–1) in a distal ear vein. Ventilator parameters included a tidal volume of approximately 8 ml/kg with respiratory rate adjustments to maintain end-tidal CO₂ between 4.5 and 5.5 kPa. The ventilator setting was pressure-controlled volume-guaranteed ventilation for all pigs, and the inspiration–expiration ratio was 1:2. PEEP was set to 5 cm H₂O during the instrumentation period until study start. Before baseline measurements, fractional inspired oxygen was adjusted to the lowest level with PaO₂ in a range between 11 and 13 kPa. Body core temperature was monitored using a pulmonary artery (PA) temperature probe, targeting the reference range for domestic pigs (38.5° to 39.5°C). Heart rate (HR) was continuously monitored using electrocardiography. Mean arterial pressure (MAP) was recorded using a fluid-filled intravascular pressure catheter in the right femoral artery. During instrumentation, the pigs received 1 l of isotonic saline (Natriumklorid “B. Braun,” B. Braun Melsungen AG, Germany) to prevent hypovolemia. Afterward, the pigs received a continuous infusion of isotonic saline of 2.9 ml · kg⁻¹ · h⁻¹ to counteract perspiration.

A PA catheter (Swan Ganz, Edwards Lifesciences, USA) was placed under fluoroscopic guidance and used to assess CO by the thermodilution method when inducing LV-CS. A mean value of three repeated measurements with less than 10% variation was used. Mixed venous saturation (SvO₂) was measured during the induction of LV-CS using blood gasses (ABL90 Flex Plus, Radiometer Medical, Denmark). Stroke volume (SV = CO/HR) was calculated. Mean pulmonary arterial pressure (mPAP), pulmonary capillary wedge pressure (PCWP), and right atrial pressure (RAP) were recorded by the PA catheter. After the onset of LV-CS, only right-sided pressures were continuously measured by the PA catheter to assess RV loading conditions. Systemic vascular resistance (SVR = [MAP − CVP]/CO), pulmonary vascular resistance (PVR = [mPAP – PCWP]/CO), and pulmonary artery pulsatility index (PAPi = [PAP_sys − PAP_dia]/RAP) were also calculated. Finally, RV stroke work index (RVSWI = SVI × [mPAP − RAP] × 0.0136) was calculated to assess RV function in relation to right-sided loading conditions.¹⁴ 

A PV admittance catheter (Transsonic, USA) was inserted through the right carotid artery and positioned within the LV using fluoroscopic guidance. The catheter was fixated and left untouched for the whole study period. Before data collection, the admittance catheter was pressure calibrated per the manufacturer’s instructions and volume calibrated using SV acquired from the PA catheter. PV measurements were continuously recorded in LabChart 8 Pro (AD Instruments, Australia) for offline analysis. The data were collected during respiration rather than apnea due to the inability to precisely control the adjustable pressure-limiting valve to achieve the desired PEEP levels during apnea. Mean values of three respiratory cycles were used. The following LV parameters were measured¹⁵ : peak LV pressure (LVP_max), end-systolic pressure (LVESP) and volume (LVEDV), end-diastolic pressure (LVEDP) and volume (LVEDV), ejection fraction (LVEF), maximal first derivative of pressure (dP/dt_max), SV, and CO.

At baseline a balloon occlusion of the inferior vena cava was performed to acquire the theoretical ventricular volume when no pressure is generated (V₀).^16,17  This allowed for estimation of end-systolic elastance (Ees = LVESP/[LVESV − V₀]),¹⁸  which is the slope of the end-systolic PV relationship. Arterial elastance (Ea) was estimated as the slope of the line intersecting at LVEDV and LVESP. Ventriculoarterial coupling was calculated as Ea/Ees. LV wall stress was calculated based on the assumption of a constant LV wall volume of 100 ml LV wall stress (equation 1).¹⁹ 

LV mechanoenergetic parameters were measured by the PV loops at each PEEP increment (Supplemental Digital Content fig. S1, https://links.lww.com/ALN/D669). Stroke work (SW), which is the area inside the PV loop and represents the ventricular energy delivered to the arterial system for maintaining forward blood flow, was calculated by LabChart 8 Pro. Potential energy (PE), which is the area of the PV diagram bounded by the end-systolic pressure–volume relationship, the end-diastolic PV relationship, and end-systolic portion of the PV loop, was estimated as LVESP × (LVESV − V₀)/2,²⁰  assuming that LVEDP is small relative to LVESP. PE represents the ventricular energy that is dissipated as heat during isovolumetric relaxation. PVA represents the total mechanical energy during one cardiac cycle as the sum of SW and PE, which is linearly correlated with MvO_2.^21,22  LV cardiac efficiency (CE = SW/PVA) was calculated. Finally, the total mechanical power expenditure (MPE = PVA × HR) was estimated.²³ 

After animal catheterization and hemodynamic baseline measurements, LV-CS was induced using a well established method of repeated slow injections of polyvinyl alcohol microspheres (Contour, Boston Scientific, USA) in the left main coronary artery.^24–26  Hemodynamic parameters were allowed to stabilize for 3 min after each injection. Inclusion criteria were fulfilled, and CS was considered evident with a 30% reduction in either SvO₂ or CO measured by thermodilution relative to baseline measurements in a healthy state. These prespecified cutoff values were chosen based on pilot studies in our animal laboratory accompanied by previous studies, demonstrating that further hemodynamic deterioration would require treatment escalation, thus remaining out of the scope of the current study.^26,27  Furthermore, this magnitude of hemodynamic deterioration resembles baseline values from clinical CS trials.²⁸  The only exclusion criterium was death of the animal during instrumentation. The hemodynamic effects of increasing PEEP were investigated at three hemodynamic states: (1) healthy state, (2) 5 min (hyperacute LV-CS), and (3) 60 min after onset of LV-CS (fig. 1).

A total of 26 pigs were included in this study. First, 8 pigs were included to assess the hemodynamic effects of elevating PEEP (5 cm H₂O to 10 and 15 cm H₂O) followed by a stepwise decrease (15 cm H₂O to 10 and 5 cm H₂O). This approach assured the reversibility and feasibility of short-term PEEP alterations, providing confidence in its implementation in our multiprotocol design. Next, 18 pigs were included in which PEEP levels were increased in a stepwise manner from 5 cm H₂O to 8, 10, 13, and 15 cm H₂O. All pigs underwent the same instrumentation and preparation period. In both protocols, hemodynamic measurements were performed upon 3 min stabilization on each PEEP level to allow for the acute Anrep reflex to occur. A fixed time of 3 min at each step was chosen to allow for the acute adaptation of hemodynamic parameters, including LV pressure and volume parameters that happen instantaneously when intrathoracic pressure increases. The primary endpoint was PVA. Secondary endpoints comprised SW, CE, MAP, CO, MPE, and parameters reflecting LV preload (LVEDP and LVEDV) and afterload (Ea, LVESP, and LV wall stress).

No sample size calculation was performed because all animals included in this study were to be included in another study protocol in our laboratory. Hence, the number of animals were determined by the calculation performed for the original study. Notably, the sample size in the current study exceeds that of many studies utilizing PV technologies for similar purposes, ensuring sufficient statistical power to assess the predefined study endpoints. Data normality was assessed using qq-plots and histograms. Continuous data, whether normally or non-normally distributed, are presented as means ± SD and medians (interquartile range), respectively, unless otherwise specified. Temporal and relative changes are represented as means ± standard error. We utilized a repeated measurements linear mixed model to analyze the hemodynamic effects of various PEEP levels, with PEEP at 5 cm H₂O as the reference.

Both protocols were combined to assess the effects of increasing PEEP from 5 to 15 cm H₂O. Importantly, only data from the stepwise PEEP increase were included in the combined analysis. Meanwhile, data from the stepwise PEEP decrease part of the reverted protocol (decreasing PEEP levels from 15 through 5 cm H₂O) were only reported as separate stepwise decreasing values; these were not included in the pooled analysis. Furthermore, a sensitivity analysis revealed no significant between-protocol differences in the decremental PEEP steps (Supplemental Digital Content 2, https://links.lww.com/ALN/D667). The main analysis included PEEP as fixed effects and treated each pig at a specific disease state as random effects. The fixed parameters were estimated through a restricted maximum likelihood procedure and compared using Kenward–Roger’s method; whenever a statistically significant effect of PEEP on the hemodynamic outcome was present, the individual effect of each PEEP level was assessed, accounting for multiple comparisons. We examined and addressed mixed model residual normality and homoscedasticity as needed. Statistical significance was defined as two-sided P values of less than 0.05 to accommodate the possibility of both positive and negative effects providing a comprehensive assessment of the evidence. Data analysis was conducted using R software (version 2022.02.3, Build 492, Austria), and figures were generated in Prism (version 8.4.2, GraphPad, USA).

A total of 26 of 27 animals reached LV-CS (96%) according to the prespecified goals. One animal died during induction of LV-CS and was excluded. CO was reduced from 4.6 ± 1.0 l/min at baseline to 2.9 ± 0.5 l/min (37%, P < 0.001) at hyperacute LV-CS and 3.1 ± 0.9 l/min (33%, P < 0.001) after 60 min of LV-CS. SvO₂ was reduced from 55.0 ± 7.5% to 37.7 ± 5.4% at hyperacute LV-CS (31% reduction) and 37.9 ± 6.5% after 60 min of LV-CS (31% reduction; Supplemental Digital Content fig. S2, https://links.lww.com/ALN/D670). PCWP increased from 8 ± 2 to 16 ± 4 mmHg (P < 0.001), whereas MAP declined from 84 ± 14 to 67 ± 8 mmHg after 60 min of LV-CS (P < 0.001). In parallel, SVR was increased in LV-CS (P = 0.05). In the LV-CS states, the PV loop shifted rightwards (fig. 2; Supplemental Digital Content fig. S3, https://links.lww.com/ALN/D671). Ventricular mechanoenergetic parameters declined significantly with lower SW, PE, PVA, MPE and CE (table 1; fig. 3; Supplemental Digital Content fig. S4, https://links.lww.com/ALN/D672). Contractility (Ees) decreased, and afterload (Ea) increased resulting in ventriculoarterial uncoupling (table 2; Supplemental Digital Content fig. S2, https://links.lww.com/ALN/D670). Filling pressure (LVEDP) increased (table 2; Supplemental Digital Content fig. S2, https://links.lww.com/ALN/D670) and ventricular mechanoenergetic parameters declined significantly with lower SW, PE, PVA, MPE, and CE.

In the healthy state, PVA declined inversely with increasing PEEP, reaching statistical significance at 10 cm H₂O (17 ± 0.4%, P < 0.001; table 1; figs. 3A and 4). At 15 cm H₂O, PVA decreased by 36 ± 1% (P < 0.001). SW, PE, and MPE also decreased with higher PEEP levels (figs. 3 and 4; Supplemental Digital Content fig. S4, https://links.lww.com/ALN/D672). CE remained stable, with a minor increase observed at PEEP 15 cm H₂O (fig. 3D). Effect sizes are presented in Supplemental Digital Content table S1 (https://links.lww.com/ALN/D675) and Supplemental Digital Content table S2 (https://links.lww.com/ALN/D676). PV parameters quickly returned to similar levels during stepwise decrease in PEEP (Supplemental Digital Content table S3, https://links.lww.com/ALN/D677; fig. 5). CO and SV decreased at PEEP 15 cm H₂O (table 2; fig. 4), whereas HR remained unaltered. MAP declined in parallel with higher PEEP and was reduced by 20 ± 4 mmHg at PEEP 15 cm H₂O (table 3; fig. 4; Supplemental Digital Content table S2, https://links.lww.com/ALN/D676; Supplemental Digital Content fig. S5, https://links.lww.com/ALN/D673). Meanwhile, SVR remained stable. Ea decreased at PEEP levels above 10 cm H₂O, and Ees increased, leading to a lower Ea/Ees ratio. LV wall stress decreased with higher PEEP, reaching statistical significance at PEEP of 10 cm H₂O (table 2). LVESP, LVESV, and LVEDV decreased progressively (fig. 2A), whereas LVEF increased. RVSWI was significantly decreased (table 3), whereas no effects were seen on PAPi or PVR.

PVA decreased in parallel with increasing PEEP in pigs with LV-CS, reaching a relative reduction of 10 ± 1% at 10 cm H₂O (P < 0.001) and 18 ± 1% at 15 cm H₂O (P < 0.001; figs. 3A and 4; table 1). SW and PE mirrored this reduction with escalating PEEP levels, whereas CE remained unaltered. These effects were highly reversible during stepwise decrease of PEEP from 15 to 5 cm H₂O (Supplemental Digital Content table S3, https://links.lww.com/ALN/D677; fig. 5). Although HR increased slightly, MPE decreased significantly at PEEP 10 cm H₂O and above (table 1; Supplemental Digital Content fig. S4, https://links.lww.com/ALN/D672). CO demonstrated no significant change. MAP decreased by 7 ± 4 and 9 ± 4 mmHg at PEEP of 10 and 15 cm H₂O, respectively (table 3). In parallel, SVR declined with higher PEEP, reaching similar levels as during healthy conditions at PEEP 10 cm H₂O and higher. LV wall stress, P_max, and LVESP were decreased at PEEP levels greater than or equal to 10 cm H₂O. As a minor decline in SV occurred (−5 ± 2 ml, P = 0.04), Ea did not decrease significantly. Ees and Ea/Ees ratio remained unchanged. LVEDV and LVESV decreased (table 2; fig. 2B), and LVEF remained unchanged. In addition, LVEDP decreased at PEEP 15 cm H₂O. RVSWI was significantly decreased (table 3), whereas no effects were seen on PAPi or PVR. The hemodynamic changes to increasing PEEP were largely similar in hyperacute LV-CS (Supplemental Digital Content table S3, https://links.lww.com/ALN/D668; Supplemental Digital Content table S4, https://links.lww.com/ALN/D678; Supplemental Digital Content fig. S6, https://links.lww.com/ALN/D674).

The current study demonstrated beneficial mechanoenergetic effects and cardiac unloading with increasing levels of PEEP within the normal therapeutic range of 5 to 15 cm H₂O in LV-CS. The effects were easily titratable as PEEP was decreased. Despite a decrease in MAP, no detrimental effects on CO or RV load were observed. These salutary effects were consistent in both hyperacute LV-CS and untreated LV-CS lasting for 1 h, mirroring potential unloading benefits at different time points in the clinical scenario of LV-CS.

Key features of LV-CS include myocardial depression, reduced CO, hypotension, increased afterload and SVR, and metabolic deprivation due to limited blood supply to match myocardial demand.¹  Treatment primarily aims at stabilizing hemodynamics and alleviate congestion. Cardiac unloading is attractive to achieve these physiologic treatment goals and has become a major focus of research.^29–31  In essence, LV unloading entails a decrease in PVA, aligning with diminished MvO₂.³²  Although conventional vasoactive substances have historically been preferred for LV-CS treatment, their tendency to increase MvO₂ and lead to adverse events counteracts the unloading objective. PPV with PEEP affects LV loading conditions by reducing preload and afterload and thereby possibly unloads the LV in LV-CS.^5,6,9,10,33  In a canine model of acute LV failure, increased PEEP levels reduced LV wall tension and MvO₂ without causing ischemic myocardial metabolism,³⁴  and a recent study in healthy pigs demonstrated reduced SW with escalating PEEP levels.³⁵ 

The current study assessed the mechanoenergetic and hemodynamic effects of increasing PEEP in LV-CS by utilizing state-of-the-art PV measurements. We demonstrated that PVA, which is linearly correlated with MvO_2,^21,22  decreased with increased levels of PEEP in LV-CS. In addition, because HR remains a major determinant of total myocardial oxygen demand,³⁶  we estimated the total mechanical power expenditure, MPE, by normalizing PVA to HR.²³  MPE also decreased in parallel with increasing PEEP, despite a minor increase in HR. The decrease in PVA was caused by a reduction in both SW and PE, resulting in preserved CE. In fact, CE was even increased in hyperacute LV-CS at high levels of PEEP. The apparent mechanoenergetic effects of increased PEEP were highly reversible when PEEP was reverted to 5 cm H₂O, indicating a direct and immediate effect of the concurrent PEEP setting rather than a cumulative effect over an extended time frame. We observed a slight reduction in MAP at PEEP level of 10 cm H₂O and higher, but importantly, CO (i.e., forward flow) was not compromised during increased PEEP. In addition, SVR returned to similar levels as during healthy conditions, which may benefit organ perfusion despite a drop in blood pressure. Ultimately, the current findings indicate that increasing PEEP provides a feasible and easily accessible treatment strategy to achieve LV unloading in LV-CS.

In line with previous studies,^35,37  preload (i.e., LVEDV and LVEDP) was decreased with increased PEEP, shifting the PV area leftward. This might be a consequence of diminished venous return and elevated right ventricular afterload. PEEP should be applied with caution in settings of RV dysfunction, during which the pathophysiology is different.^35,38,39  Despite this, these decongestive effects of elevating PEEP could offer potential clinical advantages in LV-CS patients with LV impairment. Thus, PEEP application must be customized to the specific condition and the underlying cardiac pathology.

Increased PEEP levels elevate intrathoracic pressure instantaneously. With the systemic circulation operating at atmospheric pressure, the increased intrathoracic pressure during PPV with PEEP leads to a pressure disparity between the LV and thoracic aorta, and the systemic circulation. This reduces the force required for LV ejection by reducing the transmural pressure and lowering LV afterload while simultaneously elevating RV afterload.^5,6,10,33  We demonstrated a decrease in several indices of afterload, including LVESP, P_max, and wall stress during higher PEEP levels. Of importance, increased RV afterload may cause interventricular mechanical dyssynchrony (causing interventricular septal bouncing).⁴⁰  This ventricular interdependency is ultimately reflected by impaired LV diastolic filling and reduced SV. However, we observed no detrimental increases in RV load during incremental PEEP in LV-CS, suggesting that it can be safely used without worsening interventricular mechanical dyssynchrony. Moreover, we observed a decrease in several LV afterload parameters. In contrast to healthy conditions, the failing LV is notably responsive to afterload alterations.^5,6,10  Conversely, we demonstrated that CO decreased in a healthy state with higher PEEP, whereas it remained stable during LV-CS, supporting previous reports.^34,41  Also, elevating PEEP was achievable without affecting LV contractility (Ees) or ventriculoarterial coupling (Ea/Ees). These salutary effects were abruptly present upon every PEEP increment and were highly titratable during stepwise PEEP elevations, as well as decrements.

Utilization of higher PEEP levels at 10 versus 5 cm H₂O has demonstrated improvements in lung compliance and decreased oxygen requirements at discharge after cardiac surgery.⁴²  A PEEP level of 20 cm H₂O as a lung recruitment method results in more stable hemodynamic conditions compared to continuous positive airway pressure in patients after cardiac surgery.⁴³  Furthermore, a PEEP level of 10 cm H₂O is associated with improved weaning off mechanical circulatory support along with higher discharge survival rate compared to mechanical circulatory support alone.¹¹  These results suggest that employing increased PEEP levels provides multiple cardiopulmonary advantages in a variety of situations in cardiac patients. However, it should be emphasized that potential clinical benefits of treatment with PEEP in LV-CS have not been investigated in a randomized trial.

The current study demonstrated potential cardiovascular benefits of increasing PEEP. LV unloading was achieved without compromising CO, LV contractility, LVEF, or CE. When evaluating the advantages of a treatment, it must be balanced against its potential harm. Although PPV is often indicated in LV-CS, it can induce lung injury. Furthermore, in RV failure, PPV with PEEP can deteriorate the hemodynamic status.^5,6,10  Also noteworthy, MAP was reduced by 7 mmHg at PEEP greater than or equal to 10 cm H₂O. However, the impact of reduced MAP on organ perfusion during stable CO is unclear. Recent data suggest that lower MAP might affect renal filtration short-term but not long-term outcomes. Furthermore, cerebral blood flow is more correlated with CO than MAP.^44–46  Therefore, the unloading effects of elevating PEEP might outweigh the slight reduction in blood pressure, especially in the setting of LV-CS, because LV unloading and reduced MvO₂ are associated with myocardial protection.^47–49  Still, a major decrease in MAP is unwarranted in LV-CS, and detrimental hemodynamic effects were found in the highest PEEP settings. Hence, our study underscores the importance of careful PEEP optimization with respect to the underlying CS etiology.

In LV-CS, timing of unloading is imperative, with unloading preceding revascularization having significant benefits.^50,51  Our findings highlight that elevated PEEP may be a useful treatment option to alleviate MvO₂ in LV-CS, even before revascularization. Randomized clinical studies are warranted to investigate whether elevated levels of PEEP have prolonged beneficial effects in patients with LV-CS.

Although right-sided pressures and RVSWI were assessed at different PEEP settings, RV mechanoenergetics were not measured by PV assessment. Elevated PEEP may increase RV afterload and modify pulmonary arterial resistance and RV preload.^5,6,10  However, because we aimed to investigate hemodynamic effects of incremental PEEP in acute LV systolic dysfunction, assessing RV PV loops remained out of the scope of the current study. However, incremental PEEP levels had no significant effects on RAP or mPAP. Furthermore, RVSWI and PAPi remained similar at lower to moderate PEEP levels, indicating preserved RV function. RVSWI was slightly reduced at the highest PEEP level, in accordance with previous studies.³⁵  Conclusively, because CS can arise from various etiologies with different hemodynamic representations, PEEP should be applied with caution during instances of RV dysfunction. Second, because all measurements were performed after 3 min of stabilization at each PEEP adjustment, only acute cardiovascular effects of PEEP were assessed. Although this provides novel insights into immediate hemodynamic and mechanoenergetic responses to PEEP during LV-CS, longer-term impacts of prolonged PEEP remain unexplored. Future studies investigating extended PEEP durations in LV-CS are warranted to comprehensively evaluate its effect on LV function and hemodynamics. Along the same lines, PEEP was not reverted to 5 cm H₂O after each increment, and we cannot definitively rule out the existence of a cumulative impact of PEEP over an extended time frame. However, reverting PEEP returned hemodynamic effects to baseline levels, indicating that PEEP adjustments cause abrupt hemodynamic changes that may be easily titratable. This might even enhance the translatability of the protocol, because slowly adjusting PEEP within therapeutical levels is common clinical practice. Although we conducted an extensive evaluation of LV hemodynamics, continuous measurement of SvO₂ was deemed unfeasible due to the extended time required to reach steady state at each PEEP level.⁵² 

Finally, preclinical research must be translated into clinical practice with caution. In addition, this study only used pigs of female sex to minimize variability, due to the well-being of the animals in the stable by not mixing sexes and due to the relative ease of placing the urinary catheter. Furthermore, pigs resemble close anatomical similarities with human thoracic anatomy, and their cardiovascular and respiratory physiology closely resemble those of humans. Hence, intersex differences in hemodynamic and cardiopulmonary interactions to different PEEP settings in LV-CS seems unlikely.⁵³ 

In the current study, we demonstrated that elevated PEEP within the therapeutic range caused LV unloading in LV-CS by inversely reducing PVA, mediated by reduced preload and afterload parameters. Controllable PEEP adjustments were possible to revert these changes. In LV-CS, high PEEP levels were safely applied with no detrimental effects on cardiac output or efficiency and without compromising contractility and ventriculoarterial coupling, highlighting PEEP as a promising therapeutic strategy in LV-CS if titrated optimally.

The authors thank Alexander Møller Larsen, B.M.Sc. (Department of Cardiology, Aarhus University Hospital, Aarhus, Denmark); Lasse Juul Christensen, B.M.Sc. (Department of Cardiology, Aarhus University Hospital, Aarhus, Denmark); Markus Hedin Fedder, B.M.Sc. (Aarhus University, Aarhus, Denmark); and Selma Elisabeth Fogh Rubenius, B.M.Sc. (Aarhus University, Aarhus, Denmark) for their voluntary contributions to data acquisition.

Supported by the Novo Nordisk Foundation (Hellerup, Denmark), the Novo Nordisk Steno Collaborative Grant (Hellerup, Denmark), Aase and Ejnar Danielsens Foundation (Kongens Lyngby, Denmark), the Graduate School of Health (Aarhus University, Aarhus, Denmark), the Independent Research Fund Denmark (Odense, Denmark), and the Director Emil C. Hertz and Wife Inger Hertz Foundation (Copenhagen, Denmark).

Dr. Wiggers has been the principal or a subinvestigator in studies involving the following pharmaceutical companies: Merck & Co. (Rahway, New Jersey), Bayer (Leverkusen, Germany), Daiichi-Sankyo (Tokyo, Japan), Novartis (Copenhagen, Denmark), Novo Nordisk (Hellerup, Denmark), Sanofi-Aventis (Copenhagen, Denmark), and Pfizer (New York, New York). The other authors declare no competing interests.

Supplemental Content 1. Animal Ethics and Risk of Bias Management, https://links.lww.com/ALN/D666

Supplemental Content 2. Sensitivity Analysis, https://links.lww.com/ALN/D667

Supplemental Content 3. Results of PEEP in Hyperacute LV-CS, https://links.lww.com/ALN/D668

Fig. S1. Parameters Calculated from the Pressure–Volume Loop, https://links.lww.com/ALN/D669

Fig. S2. Endpoint Parameters during Induction of LV-CS, https://links.lww.com/ALN/D670

Fig. S3. Healthy and LV-CS Pressure–Volume Loops, https://links.lww.com/ALN/D671

Fig. S4. Evolution of Total Mechanical Energy Expenditure, https://links.lww.com/ALN/D672

Fig. S5. Systemic and Pulmonary Pressures, https://links.lww.com/ALN/D673

Fig. S6. Results of PEEP in Hyperacute LV-CS, https://links.lww.com/ALN/D674

Table S1. Effect Sizes of PEEP Settings on Cardiac Parameters, https://links.lww.com/ALN/D675

Table S2. Effect Sizes of PEEP Settings on Systemic and Pulmonary Pressures, https://links.lww.com/ALN/D676

Table S3. Reversible Effects of PEEP Adjustments, https://links.lww.com/ALN/D677

Table S4. Results of PEEP in Hyperacute LV-CS, https://links.lww.com/ALN/D678