# The Role of Phase Angle in Non-Invasive Fluid Assessment in Dogs with Patent Ductus Arteriosus: A Novel Method in Veterinary Cardiology

## TL;DR

This study shows that phase angle from bioelectrical impedance analysis can non-invasively track fluid changes in dogs with a heart defect called patent ductus arteriosus.

## Contribution

The study introduces thoracic phase angle at 5 kHz as a novel non-invasive method for assessing fluid status in dogs with PDA.

## Key findings

- Thoracic PhA at 5 kHz was significantly reduced in dogs with PDA and correlated with pulmonary velocity and end-diastolic volume.
- Post-surgery, PhA values increased, indicating recovery of fluid balance.
- Thoracic PhA was more sensitive than trunk or abdominal measurements in detecting fluid changes.

## Abstract

Patent ductus arteriosus (PDA) is a congenital heart defect in dogs that causes left-to-right shunting, increasing pulmonary blood flow and placing excess load on the left heart. This can lead to pulmonary edema and ventricular remodeling. Monitoring fluid imbalance is essential for staging the disease and managing recovery, but conventional tools such as echocardiography or blood markers do not provide real-time, region-specific information. This study evaluated phase angle (PhA), derived from bioelectrical impedance analysis (BIA), as a non-invasive marker of thoracic fluid status in dogs with PDA. Thoracic PhA at 5 kHz was significantly reduced in PDA dogs, strongly correlated with pulmonary velocity, and moderately correlated with end-diastolic volume, reflecting extracellular fluid accumulation. After surgical correction, PhA values progressively increased, indicating recovery of thoracic fluid balance. Regional comparisons showed thoracic PhA was more sensitive than trunk or abdominal values, while associations with structural echocardiographic indices were limited. Overall, low-frequency thoracic PhA appears to be a practical tool for assessing fluid dynamics, monitoring treatment response, and supporting clinical decision-making in veterinary cardiology.

Background: Patent ductus arteriosus (PDA) in dogs causes persistent left-to-right shunting, leading to pulmonary overcirculation, left heart volume overload, and potential congestive heart failure. Accurate assessment of fluid imbalance is essential but challenging with conventional echocardiography or biomarkers. Phase angle (PhA), derived from bioelectrical impedance analysis (BIA), may serve as a non-invasive marker of extracellular fluid distribution and cellular integrity. Objectives: This study aimed to evaluate PhA as an indicator of thoracic fluid imbalance in dogs with PDAby analyzing its correlation with pulmonary velocity (PV) and end-diastolic volume (eV), as well as its responsiveness to surgical correction. In addition, we assessed the relationships between PhA and echocardiographic structural indices (LA/Ao, TDI Sep E/Em, TDI Lat E/Em) and examined the influence of the measurement region. Methods: PhA was measured at 5, 50, and 250 kHz in 30 PDA-affected and 15 healthy dogs, with electrode placement across thorax, trunk, and abdomen. Echocardiography evaluated PV, eV, and PDA-specific structural parameters. Results: Thoracic PhA at 5 kHz was significantly reduced in PDAdogs, strongly correlated with PV and moderately with eV. Postoperative measurements showed progressive PhA recovery. Only TDI Lat E/Em correlated with mid-frequency PhA, while other structural indices showed minimal association. Thoracic PhA was lower than trunk or abdominal values, indicating that thoracic measurements may better capture localized extracellular fluid changes in PDAcompared with other regions. Conclusion: Thoracic PhA at 5 kHz effectively reflects extracellular fluid changes in PDA, complements structural echocardiography, and tracks postoperative fluid normalization. Its non-invasive nature supports clinical utility for monitoring hemodynamic burden and therapeutic response.