The effect of prone versus supine positioning of Goettingen minipigs on lung density as viewed by computed tomography

Minipigs are frequently used for scientific research as they are easy to handle and the dimensions of their vascular system do not change after 20 months of age. Although surgical interventions under anaesthesia are often performed in the supine position the effects of this positioning on lung functionality in minipigs have not been systematically described. This study aimed to analyse the influence of supine positioning on the macrostructure of the lung and the pulmonary density by the use of computed tomography imaging in pre-adult Göttingen minipigs. Twelve pre-adult female minipigs were used in the study and lung density was investigated in both the prone and the supine positions. The time between the scans in prone and supine positions was less than 5 minutes (296 ± 6 sec). In the prone position lung density did not differ between the dorsal and ventral part of the lung (-641 ± 72 Hounsfield units [HU]). However in the supine position there was a ventrodorsal gradient of decreasing density (ventral part of the lung: -497 ± 106 HU, dorsal part of the lung: -723 ± 51 HU). The changes in lung density were not accompanied by changes in lung volume (829 ± 191 ml). These results suggest an influence of the body position on the ventilation/perfusion (V(A)/Q) matching of the lung which could possibly result in lowered lung oxygenation as well as in an increased heart activity in the supine position. Additionally, due to the steep course of the vena cava caudalis from the caval foramen in the diaphragm across to the heart (in contrast to the more shallow course in the prone position) the activity of the heart necessary to pump the venous blood to the right atrium has to be higher in the supine position than in the prone position. In pigs the capacity of the heart to increase frequency is limited due to a diastolic/systolic (D/S) ratio <1. Supine positioning may possibly increase their risk of cardiovascular complications.