Development of Minimally Invasive Technique for Assessment of Fluid Status in Animal Models of Heart Failure and Pulmonary Hypertension

Background:
Heart failure (HF) is characterised by reduced myocardial performance resulting in fluid retention, the management of which is critical to clinical care. Due to altered vascular Starling forces in HF, the interstitial compartment and lymphatic system act as large reservoirs of fluid accumulation, increasing up to 3-4 times that of the intravascular compartment. Interstitial fluid pressure (IFP) is increased in interstitial congestion; however, its continuous temporal relationship with indicators of impaired cardiac function, including reduced cardiac output (CO) and increased cardiac filling pressures, remains unknown. IFP may be measured from a preserved fluid pocket following the maturation of subcutaneously implanted bioresin made perforated capsules in animals. However, the structural understanding of tissue neovascularisation within a perforated capsule and the temporal relationship between cardiopulmonary haemodynamics, including left ventricle end-diastolic pressure (LVEDP), central venous pressure (CVP), and mean pulmonary artery pressure (mPAP) and IFP are unknown.

Methods:
Ten Yorkshire White swine (45-50kg) were implanted with subcutaneous bio-resin made perforated capsules (Home Office Project Licence PP1785781). After 3 weeks, animals were anaesthetised, and continuous monitoring of IFP and cardiopulmonary haemodynamics was established. Esmolol (10mg/ml, 50ml/hr) was used to blunt the sympathetic response and HF induced by infusion of 0.9% NaCl (10% body weight/3 hours, i.v.) until a CVP of >15mmHg was maintained and fluid was then removed to euvolemia via ultrafiltration (~3 hours). The simultaneous measurements of IFP and cardiopulmonary pressures were established, and cardiopulmonary haemodynamics were altered by 1) positional changes and 2) preload and afterload were simultaneously modulated by a semi-obstructive balloon inflation in the inferior vena cava (IVC) and the descending aorta. To characterise the perforated capsule: 1) serial measurements of venous, interstitial, and subcutaneous glucose were made following a venous glucose bolus (0.5g/kg); 2) a silicone-based liquid perfusion-fixation agent (MicroFil) was infused into the arterial system to cast the vasculature for MicroCT and microscopy at 3 and 6 weeks. The 3 and 6 week time points for studying perforated capsules were chosen to capture the initial and established biological responses post-implantation. The 3 week mark allows for the observation of early inflammatory and healing processes, essential for assessing initial capsule integration, whereas the 6 week mark evaluates long-term stability and biological integration, including fibrotic or vascular changes. Statistical analysis was performed on GraphPad Prism (Version 9.3.1 Macintosh).

Results:
Micro-CT and microscopy demonstrated neovascularisation of the perforated capsule with a central fluid pocket at 3 weeks and collapse of pocket at 6 weeks due to continued tissue growth. A venous bolus of glucose demonstrated diffusion of the micro-molecule from the vasculature into the fluid pocket (p <0.0001); however, macro-molecules such as MicroFil did not. Orthostatic manoeuvres and balloon inflation significantly modulated preload, afterload, cardiopulmonary haemodynamics, and IFP (p <0.0001) and demonstrated a strong correlation between these parameters. IFP closely tracked the metrics of congestion during the development of HF and subsequent offloading.

Conclusion:
A mature perforated capsule forms a neovascular structure with a central fluid pocket. IFP is closely related to invasive haemodynamic indicators of congestion at rest and during a haemodynamic challenge. The assessment of IFP may provide a direct measure of fluid status and an indirect measure of haemodynamics, facilitating early identification of clinical worsening events (CWEs) and optimisation of fluid balance.