Isolated biventricular working rat heart preparation

Ex vivo evaluation of the biventricular cardiac function for donation after circulatory death model: An experimental study

Few reports on a biventricular working heart model with ex vivo perfusion exist owing to the complexity of establishing a circuit. Hence, we investigated it for donation after circulatory death. The heart in six juvenile pigs (~20 kg) was arrested by asphyxiation. After 30 minutes of global ischemia, the heart was harvested, reperfused with normoxemic blood cardioplegia for 20 minutes, and subsequently perfused with hyperxemic blood. After 70 minutes of controlled reperfusion, the system was switched to the biventricular working mode. Cardiac function was assessed before anoxia and during the biventricular mode. Left and right ventricular functions worsened during the biventricular mode, as compared to those before anoxia (dP/dt_max , 673 ± 120 vs. 283 ± 95 and 251 ± 35 vs. 141 ± 21 mm Hg/s, respectively; P < .001). Systemic (resistance/100 g net heart weight) and pulmonary vascular resistance indexes during the biventricular mode were similar to those before anoxia (829 ± 262 vs. 759 ± 359, P = .707, and 167 ± 57 vs. 158 ± 83 dynes·sec·cm^-5 - l-100-g net heart weight, P = .859, respectively). The biventricular working heart model with ex vivo perfusion was feasible, exhibited stable hemodynamics, and has the potential to be a powerful tool for direct cardiac function assessment.

Rodent Working Heart Model for the Study of Myocardial Performance and Oxygen Consumption

Isolated working heart models have been used to understand the effects of loading conditions, heart rate and medications on myocardial performance in ways that cannot be accomplished in vivo. For example, inotropic medications commonly also affect preload and afterload, precluding load-independent assessments of their myocardial effects in vivo. Additionally, this model allows for sampling of coronary sinus effluent without contamination from systemic venous return, permitting assessment of myocardial oxygen consumption. Further, the advent of miniaturized pressure-volume catheters has allowed for the precise quantification of markers of both systolic and diastolic performance. We describe a model in which the left ventricle can be studied while performing both volume and pressure work under controlled conditions. In this technique, the heart and lungs of a Sprague-Dawley rat (weight 300-500 g) are removed en bloc under general anesthesia. The aorta is dissected free and cannulated for retrograde perfusion with oxygenated Krebs buffer. The pulmonary arteries and veins are ligated and the lungs removed from the preparation. The left atrium is then incised and cannulated using a separate venous cannula, attached to a preload block. Once this is determined to be leak-free, the left heart is loaded and retrograde perfusion stopped, creating the working heart model. The pulmonary artery is incised and cannulated for collection of coronary effluent and determination of myocardial oxygen consumption. A pressure-volume catheter is placed into the left ventricle either retrograde or through apical puncture. If desired, atrial pacing wires can be placed for more precise control of heart rate. This model allows for precise control of preload (using a left atrial pressure block), afterload (using an afterload block), heart rate (using pacing wires) and oxygen tension (using oxygen mixtures within the perfusate).

Oxygen demand of perfused heart preparations: how electromechanical function and inadequate oxygenation affect physiology and optical measurements

NEW FINDINGS

What is the topic of this review? This review discusses how the function and electrophysiology of isolated perfused hearts are affected by oxygenation and energy utilization. The impact of oxygenation on fluorescence measurements in perfused hearts is also discussed. What advances does it highlight? Recent studies have illuminated the inherent differences in electromechanical function, energy utilization rate and oxygen requirements between the primary types of excised heart preparations. A summary and analysis of how these variables affect experimental results are necessary to elevate the physiological relevance of these approaches in order to advance the field of whole-heart research. The ex vivo perfused heart recreates important aspects of in vivo conditions to provide insight into whole-organ function. In this review we discuss multiple types of ex vivo heart preparations, explain how closely each mimic in vivo function, and discuss how changes in electromechanical function and inadequate oxygenation of ex vivo perfused hearts may affect measurements of physiology. Hearts that perform physiological work have high oxygen demand and are likely to experience hypoxia when perfused with a crystalloid perfusate. Adequate myocardial oxygenation is critically important for obtaining physiologically relevant measurements, so when designing experiments the type of ex vivo preparation and the capacity of perfusate to deliver oxygen must be carefully considered. When workload is low, such as during interventions that inhibit contraction, oxygen demand is also low, which could dramatically alter a physiological response to experimental variables. Changes in oxygenation also alter the optical properties of cardiac tissue, an effect that may influence optical signals measured from both endogenous and exogenous fluorophores. Careful consideration of oxygen supply, working condition, and wavelengths used to acquire optical signals is critical for obtaining physiologically relevant measurements during ex vivo perfused heart studies.

NADH changes during hypoxia, ischemia, and increased work differ between isolated heart preparations

Langendorff-perfused hearts and working hearts are established isolated heart preparation techniques that are advantageous for studying cardiac physiology and function, especially when fluorescence imaging is a key component. However, oxygen and energy requirements vary widely between isolated heart preparations. When energy supply and demand are not in harmony, such as when oxygen is not adequately available, the imbalance is reflected in NADH fluctuations. As such, NADH imaging can provide insight into the metabolic state of tissue. Hearts from New Zealand white rabbits were prepared as mechanically silenced Langendorff-perfused hearts, Langendorff-perfused hearts, or biventricular working hearts and subjected to sudden changes in workload, instantaneous global ischemia, and gradual hypoxia while heart rate, aortic pressure, and epicardial NADH fluorescence were monitored. Fast pacing resulted in a dip in NADH upon initiation and a spike in NADH when pacing was terminated in biventricular working hearts only, with the magnitude of the changes greatest at the fastest pacing rate. Working hearts were also most susceptible to changes in oxygen supply; NADH was at half-maximum value when perfusate oxygen was at 67.8 ± 13.7%. Langendorff-perfused and mechanically arrested hearts were the least affected by low oxygen supply, with half-maximum NADH occurring at 42.5 ± 5.0% and 23.7 ± 4.6% perfusate oxygen, respectively. Although the biventricular working heart preparation can provide a useful representation of mechanical in vivo heart function, it is not without limitations. Understanding the limitations of isolated heart preparations is crucial when studying cardiac function in the context of energy supply and demand.

NADH fluorescence imaging of isolated biventricular working rabbit hearts

Since its inception by Langendorff¹, the isolated perfused heart remains a prominent tool for studying cardiac physiology². However, it is not well-suited for studies of cardiac metabolism, which require the heart to perform work within the context of physiologic preload and afterload pressures. Neely introduced modifications to the Langendorff technique to establish appropriate left ventricular (LV) preload and afterload pressures³. The model is known as the isolated LV working heart model and has been used extensively to study LV performance and metabolism^4-6. This model, however, does not provide a properly loaded right ventricle (RV). Demmy et al. first reported a biventricular model as a modification of the LV working heart model^{7, 8}. They found that stroke volume, cardiac output, and pressure development improved in hearts converted from working LV mode to biventricular working mode⁸. A properly loaded RV also diminishes abnormal pressure gradients across the septum to improve septal function. Biventricular working hearts have been shown to maintain aortic output, pulmonary flow, mean aortic pressure, heart rate, and myocardial ATP levels for up to 3 hours⁸.

When studying the metabolic effects of myocardial injury, such as ischemia, it is often necessary to identify the location of the affected tissue. This can be done by imaging the fluorescence of NADH (the reduced form of nicotinamide adenine dinucleotide)^9-11, a coenzyme found in large quantities in the mitochondria. NADH fluorescence (fNADH) displays a near linearly inverse relationship with local oxygen concentration¹² and provides a measure of mitochondrial redox state¹³. fNADH imaging during hypoxic and ischemic conditions has been used as a dye-free method to identify hypoxic regions^{14, 15} and to monitor the progression of hypoxic conditions over time¹⁰.

The objective of the method is to monitor the mitochondrial redox state of biventricular working hearts during protocols that alter the rate of myocyte metabolism or induce hypoxia or create a combination of the two. Hearts from New Zealand white rabbits were connected to a biventricular working heart system (Hugo Sachs Elektronik) and perfused with modified Krebs-Henseleit solution¹⁶ at 37 °C. Aortic, LV, pulmonary artery, and left & right atrial pressures were recorded. Electrical activity was measured using a monophasic action potential electrode. To image fNADH, light from a mercury lamp was filtered (350±25 nm) and used to illuminate the epicardium. Emitted light was filtered (460±20 nm) and imaged using a CCD camera. Changes in the epicardial fNADH of biventricular working hearts during different pacing rates are presented. The combination of the heart model and fNADH imaging provides a new and valuable experimental tool for studying acute cardiac pathologies within the context of realistic physiological conditions.

Norepinephrine-induced expression of cytokines in isolated biventricular working rat hearts

The norepinephrine (NE)-induced hypertrophy of the left ventricle (LV) in the rat is associated with increased interleukin (IL)-6 and IL-1beta expression. In the present study, a newly established model of isolated biventricular working rat heart was used to examine whether NE may directly induce cytokine mRNA expression in a preparation devoid of other circulating hormonal and humoral factors. Representative hemodynamic parameters and the expression of various cytokines of the isolated biventricular working heart (IBWH) were compared with the respective in vivo results. Systolic pressure (SP) of the right ventricle (RVSP) was higher in the IBWH than in the intact anesthetized rat (42.9 +/- 1.89 vs. 32.3 +/- 1.06). However, heart rate (HR), LVSP and the maximal rate of pressure development of LV (LV dP/dt(max)) were lower. After NE infusion (30 nM), SP and dP/dt(max) were increased by 30 and 90%, respectively, in both ventricles. In vivo, the ventricles showed a different response to NE (0.1 mg/kg x h): LVSP increased by 15%, RVSP and RV dP/dt(max) was doubled, LV dP/dt(max) was tripled. The analysis of cytokine mRNA expression with the RNase protection assay revealed that in vivo IL-6 and IL-1beta were increased between 4 and 12 h 80- and 12-fold, respectively, while there was weak expression under control conditions. In the IBWH IL- 1alpha, IL-1beta, IL-6 and tumor necrosis factor (TNF)alpha were increased already during control perfusion. The increase of these stress-activated cytokines indicates that the isolation and perfusion procedure may exert a stress on the heart. NE induced an additional time-dependent increase of IL-6 mRNA after 1 h of infusion. Thus, NE has a direct effect on the cardiac IL-6 expression, which occurred earlier in the in vitro preparation than in the rat heart in vivo.

Laboratory confirmation of clinical heart allograft preservation variability

BACKGROUND

Previously, we reported survival differences from the national heart transplant registry favoring centers that used intracellular organ preservation solutions. To eliminate center selection bias, we tested some of these solutions in a biventricular working rat heart model to determine their relative efficacy.

METHODS

Using 103 Sprague-Dawley rat hearts perfused with modified Krebs-Henseleit buffer, both ventricles functioned with adjustable independent preload and afterload and their pressure-length loops generated load-insensitive measurements of cardiac performance. After 15 minutes of stable function, each heart sustained 180 minutes of cold (4 degrees C) ischemia after a 5-minute perfusion by University of Missouri (UMC), Plegisol, Collins, University of Wisconsin, Custodiol, or Roe solutions. Eighty-two hearts were reperfused and the remainder were used for ATP analyses.

RESULTS

Although the extracellular solution Plegisol showed good recovery of traditional hemodynamic values, including developed pressure and cardiac output, intracellular solutions like Roe had superior preservation of load-insensitive indices such as preload recruitable stroke work: Roe (intracellular) 103%+/-13%; Custodiol (intracellular) 96%+/-9%; UW (intracellular) 69%+/-12%; Collins (intracellular) 68%+/-9%; Plegisol (extracellular) 68%+/-7%; and University of Missouri (extracellular) 56%+/-10% (p = 0.04). Furthermore, recovery with intracellular solutions tended to be gradual but more progressive after ischemia in contrast to an early plateau shown by extracellular (p < 0.001). Right ventricular recovery and ATP measurements were similar between groups.

CONCLUSIONS

These data support the superiority of certain intracellular preservation solutions and provide evidence that optimal heart organ protection may be difficult to judge clinically using hemodynamic values routinely available to the heart transplant surgeon. Care should be taken to verify the performance of some solutions used in heart organ transplantation.

Isolated four-chamber working swine heart model

BACKGROUND

Isolated heart models separate cardiac characteristics from systemic characteristics with subsequent findings used in cardiac research, including responses to pharmacologic, mechanical, and electrical components. The model objective was to develop the ability to represent in situ physiologic cardiac function ex vivo.

METHODS

Swine hearts were chosen over rat or guinea pig models due to their notably greater anatomical and physiologic similarities to humans. An in vitro apparatus was designed to work all four chambers under simulated in situ physiologic conditions. Using standard cardiac surgical techniques, 12 porcine hearts (mean weight 331 +/- 18 g) were explanted into the apparatus. Preload and afterload resistances simulated in situ input and output physiologic conditions. Hemodynamic characterizations, including cardiac output, max +/- dP/dt, and heart rate, were used to determine in situ function leading to explantation (prethoracic operation, postmedial sternotomy, and postperidectomy) and during in vitro function (t = 0, 60, 120, and 240 minutes).

RESULTS

In vitro performance decayed with time, with statistical differences from base line (t = 0) function at t = 240 minutes (p > 0.05).

CONCLUSIONS

An isolation and in vitro explantation protocol has been improved to aid in the study of isolated cardiac responses, and to determine cardiac hemodynamic function during open chest operation, transplantation, and in vitro reanimation with a crystalloid perfusate. The resulting model offers similar working physiologic function, with real-time imaging capabilities. The resulting model is advantageous in representing human cardiac function with regard to anatomic and physiologic functions, and can account for atrial and ventricular interactions.

Load-Insensitive Measurements from an Isolated Perfused Biventricular Working Rat Heart

To determine whether a rat heart model can provide load-insensitive measurements of cardiac function, a recently developed biventricular perfused preparation was tested. Using 29 Sprague-Dawley rat hearts perfused with modified Krebs-Henseleit buffer, ventricles functioned simultaneously with adjustable independent preload (venous reservoirs) and afterload (compliance chambers). Ultrasonic crystal pairs provided continuous left (LV) and right ventricular (RV) short-axis dimensions. LV and RV pressure-length loops (loop area = work) were generated from paired intraventricular pressure and short-axis dimensions. Load-insensitive measurements were obtained from the slopes (elastance) and x-intercepts (L(0)) of regression lines generated from the end-systolic coordinates of these pressure-length loops over ranges of RV and LV preloads. Measurements were made after 15 min of stable function and after 20 min of warm (37 degrees C) ischemia. During perturbations in LV afterload, there were linear changes in dP/dt, but loop work remained relatively unchanged. RV dP/dt and work varied little with physiologic ranges of afterload. Increased RV afterload had little effect on LV function. Ischemia affected LV function more than RV function using these measurements. Elastance, however, increased after ischemia with diastolic 'creep' (increased L(0)) for both ventricles. Load-insensitive and other sophisticated hemodynamic measurements are possible with this new preparation. Copyright 1997 S. Karger AG, Basel