Ultrasonography is used to subjectively assess fluid volumes; to our knowledge no methods of ultrasonographic pleural fluid volume estimation in canines are reported.

Aim

To assess the viability of Ultrasonography in estimation of pleural fluid volumes in dogs.

Method

Nine canine cadavers of mixed breed were selected. The dogs' weight and greatest thoracic height, width and circumference were recorded. The dogs were placed in sternal-recumbency on an echocardiography table with their sternum over the cut-out. Dogs were excluded if pathology was detected on initial ultrasound exam. A 2.5- 5.5 MHz curvilinear probe was placed suprasternally, perpendicular to the skin surface in a transverse plane. An acoustic window using the caudal aspect of the sternebra at the level of the centre of the heart was used. Symmetry of the image was assessed using the shape of the sternebra and adjacent thoracic wall. Linear measurements from the sagittal plane of the sternebra, at the level of the pleural surface of the thoracic wall, to the furthest ventro-lateral point of both right and left lung edges were recorded. All measurements were determined by one operator and repeated three times with the average measurement used for statistical analysis. 25ml increments of isotonic saline were injected using ultrasound guidance into both right and left pleural spaces and the measurements were repeated each time up to 500mls total volume, after which 50ml increments were injected until 1000mls total volume was reached.

Results

No relationship was identified between mean distances and injected volumes up to 100ml. Thereafter the mean distance increased in an approximately linear relationship with the cube root of fluid volume.

Individual gradients for different dogs varied between 0.99 and 1.72. The situation was modelled using Stata 10 (Statacorp, College Station, Texas, USA), using a mixed model linear regression with dog identity as a random effect and assuming an independent covariance structure. The overall equation was:

Volume = [-2.001+1.378(mean)]³

(P < 0.001), with a 95% confidence interval for the gradient of 1.19 to 1.46. However there were differences between dogs (P < 0.001) for both the initial constant and the gradient of the regression line.

Conclusion

Good correlation was found between the ultrasonographic measurement and fluid volume within individual dogs. Further analysis with reference to thoracic measurements may increase accuracy.