Presentation of preload and afterload to isolated heart preparations

Figures

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  Figure 1. Langendorff perfusion: constant‐pressure mode versus constant‐flow mode Two common methods of Langendorff‐perfusing are used for an isolated heart: (1) constant‐pressure mode, achieved by maintaining the perfusate reservoir at a fixed height and (2) constant‐flow mode, achieved by setting the peristaltic pump to a constant speed. The three‐way stopcock allows switching between these two modes of Langendorff perfusion. Perfusion pressure can be measured with a catheter inserted into the cannulated aorta, whereas flow rate can be measured with a flow sensor placed in the aortic line. In addition to the cannulated ascending aorta all other vessels can be ligated except for the pulmonary artery, which is kept open to allow coronary effluent to exit and be collected.

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  Figure 2. A balloon inserted into the left ventricle to prescribe different ranges of preload to a Langendorff‐perfused heart The heart can only undergo isovolumic contractions in this system because the left ventricle contracts without ejecting fluid. A portion of the left atrium is dissected from the isolated heart to expose the ventricular inlet for balloon insertion. A pressure–volume (PV) catheter is inserted into the balloon to measure the pressure–volume changes at each preload level. A syringe is used to increase the left‐ventricular volume by inflating the balloon.

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  Figure 3. Isolated working‐heart system set‐up The preload reservoir is bubbled with oxygen (not shown). The preload is kept constant by keeping the preload reservoir at a fixed height. The afterload driving against the heart comprises a compliance chamber, a resistance element (stopcock) and a hydrostatic pressure head. The system uses a stopcock to model the peripheral resistance and a compliance chamber to model the compliance of the aorta. The arrows indicate the direction of flow of the perfusate entering the left ventricle and exiting through the aorta. The figure is drawn based on the concept introduced by Otto Frank (Frank, 1959).

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  Figure 4. The improved heart–lung preparation set‐up by Starling The system implements a variable resistor that models the peripheral resistance of the systemic circulation, thereby setting the afterload. Venous pressure, and thereby the preload, is adjusted by the height of the venous reservoir. The lungs provide artificial ventilation. The major vessels, including the superior vena cava, pulmonary artery, pulmonary vein and aorta, are all cannulated. The arrows indicate the direction of flow of the perfusate. The illustration has been adapted from a figure by Starling (1930).

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  Figure 5. Isolating working heart developed by Neely and Morgan in 1967 Ventilation was achieved through the use of oxygenation chambers instead of isolated lungs. The compliance chamber was used to model the elasticity and distensibility of the aorta. The afterload was made up of a hydrostatic pressure head, which allowed it to be varied by adjusting the height of the pressure head with respect to the heart. This schematic is drawn based on a figure by Neely et al. (1967).

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  Figure 6. Electrical equivalence circuit diagrams for isolated working‐heart systems that represent afterload as a pressure (A) or as an impedance (B) A, the left ventricle is represented as a voltage source. The diode represents the aortic valve, which allows one‐directional flow of perfusate. The aorta provides resistance to the flow of perfusate, which is represented by a resistor. The afterload is represented as voltage source 2. B, the ejected perfusate is stored in the ‘aorta’ (tubing connected to the aorta) during systole, with a compliance represented by a capacitor and dissipated through the rest of the system (body) that has a peripheral resistance represented by the peripheral resistor. Afterload is thus characterised by the three variables (characteristic impedance, capacitor and peripheral resistance) which make up a three‐element Windkessel system.

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  Figure 7. Afterload loading system A, Isolated working‐heart model that implements a variable pressure‐controlled afterload. The centrifugal pump rotates in the direction that opposes the flow out of the heart. The spinning centrifugal pump effectively modulates pressure by pushing blood back against the heart, thus creating a controlled and desired afterload. The pump functions as a controlled flow/pressure source rather than a representation of the arterial tree and, therefore, does not model vascular resistance, compliance or wave reflections. This illustration has been redrawn based on a figure originally published by Schechter et al. (Schechter et al., 2014). B, an impedance‐controlled afterload system employing a motor‐driven aortic cuff. The cuff acts as a variable resistor, modulating aortic outflow to achieve targeted left‐ventricular pressures. The left‐ventricular pressure is controlled by adjusting the resistance in the aortic column. The adjustable cuff represents peripheral vascular resistance, modelling the steady component of afterload without compliance. The drawing is adapted from a figure originally published by Pigot et al. (2022) (Pigot et al., 2022). C, physical…

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  Figure 8. Impedance‐controlled afterload system based on a three‐element Windkessel model The physical Windkessel elements are modelled in software and adjusted programmatically. As indicated by the arrows, the Windkessel models take an input of measured left‐ventricular pressure and produce an output of a volume command signal that allows the volume servo pump to control the filling and ejection of perfusate in and out of the balloon via a piston connected to a linear motor. Schematic adapted from figure originally published by Sunagawa et al. (Sunagawa et al., 1982).