Changes in cardiac pumping efficiency and intra-thoracic organ volume during negative pressure wound therapy of sternotomy wounds, assessment using magnetic resonance imaging

Abstract

Knowledge on the effects of negative pressure wound therapy (NPWT) on the intra-thoracic organs is limited. The present study was performed to investigate the effects of NPWT on the volume of the intra-thoracic organs, using magnetic resonance imaging (MRI), in a porcine sternotomy wound model. Six pigs underwent median sternotomy followed by NPWT at -75, -125 and -175 mmHg. Six pigs were not sternotomised. MR images covering the thorax and heart were acquired. The volumes of the thoracic cavity, lungs, wound fluid and heart were then determined. The volumes of the thoracic cavity and intra-thoracic organs increased after sternotomy and decreased upon NPWT application. The total heart volume variation, which is inversely related to cardiac pumping efficiency, was higher after sternotomy and decreased during NPWT. NPWT did not result in the evacuation of wound fluid from the bottom of the wound. NPWT largely closes and restores the thoracic cavity. Cardiac pumping efficiency returns to pre-sternotomy levels during NPWT. This may contribute to the clinical benefits of NPWT over open-chest care, including the stabilizing effects and the reduced need for mechanical ventilation.

Figure 1

Illustration of manual delineation of organ volumes. Image (A) shows how the intra‐thoracic structures were outlined in a transverse magnetic resonance (MR) image through the thorax of a pig with an open sternotomy wound. The right lung (RL), left lung (LL) and intra‐thoracic fluid (F) were delineated in each image throughout the entire thorax, allowing the volume of each structure to be quantified. Images (B) and (C) show short‐axis MR images of the same animal's heart in end diastole and end systole, respectively. The pericardial border of the heart was manually delineated in all short‐axis slices covering the entire volume within the pericardium. The total heart volume in end diastole and end systole, as well as the total heart volume variation throughout the cardiac cycle, was then quantified.

Figure 2

Representative transverse MR images at the level of the aortic root selected from a stack of 50 contiguous 5‐mm thick sections covering the entire thorax. The images were obtained from: (A) a non‐sternotomised pig and (B) a sternotomised pig before (0 mmHg) and (C) after the application of −125 mmHg. Note the differences in the size, shape and position of the intra‐thoracic organs induced by sternotomy and topical negative pressure. In particular, note how negative pressure wound therapy draws the two sternal edges closer to each other (white bars), thereby decreasing the circumference of the thorax, making it similar to that of the non‐sternotomised pig.

Figure 3

Volumes of: (A) the thoracic cavity, (B) the intra‐thoracic organs, (C) right lung and (D) left lung, measured using MRI, in non‐sternotomised pigs and in sternotomised pigs before (0 mmHg) and after the application of negative pressure wound therapy of −75, −125 and −175 mmHg. The results are presented as mean values ± standard error on the mean. Note how the volumes of the thorax and intra‐thoracic organs decrease upon the application of negative pressure, most prominently upon the change from 0 to −75 mmHg. Further increases in negative pressure only result in minor reduction in volume.

Figure 4

The (A) total heart volume variation and (B) cardiac output in non‐sternotomised pigs and sternotomised pigs before and after the application of negative pressure wound therapy (NPWT) of −75, −125 and −175 mmHg. The results are presented as mean values ± standard error on the mean. Note that after sternotomy, the cardiac pumping effciency appears to decrease (increase in total heart volume variation), and cardiac output increases, and these measures return to control levels upon application of NPWT.

Figure 5

The intra‐thoracic wound fluid volume, measured using MRI, in non‐sternotomised pigs and in sternotomised pigs before and after the application of negative pressure wound therapy (NPWT) at −75, −125 and −175 mmHg. The results are presented as mean values ± standard error on the mean. Note that NPWT does not result in the evacuation of fluid from the bottom of the wound.