A Novel Porcine Model of Septic Shock Induced by Acute Respiratory Distress Syndrome due to Methicillin-resistant Staphylococcus aureus

Abstract

Background: Sepsis is one of the main causes of mortality in critically ill patients following progression to septic shock. To investigate the pathophysiologic changes of sepsis, we developed a novel porcine model of septic shock induced by acute respiratory distress syndrome (ARDS) due to methicillin-resistant Staphylococcus aureus(MRSA) pneumonia.

Methods: Twenty-six male Landraces (Lvyuanweiye, Beijing, China) weighing 30 ± 2 kg were divided into four groups: sham group (SH; n = 5); cotton smoke inhalation group (SM; n = 6); MRSA pneumonia group (MR; n = 6); and septic shock group with cotton smoke inhalation + MRSA pneumonia (SS; n = 9). Extensive hemodynamics, oxygen dynamics, and lung function were monitored for 24 h following the injury or until death. Tissues were collected, and histopathology evaluations were carried out.

Results: Blood cultures from 6 of 9 animals in the SS group were positive for MRSA. Two hours following the injury, decreased mean arterial blood pressure (60-70 mmHg) and cardiac index (<2 L.min-1.m-2) were observed in the animals in the SS group, while systemic vascular resistance index was increased. The hemodynamic characteristics of septic shock were only observed in the SS group but not significant in the other groups. The PO2/FiO2in the SM and SS groups decreased to 300 and 100, respectively. In the SS group, extravascular lung water index increased to 20 ml/kg, whereas thoracopulmonary compliance decreased to 10 ml/H2O after injury. Deterioration of pulmonary function in the SS group was more serious than the SM and MR groups. Severe lung injury in the SS group was confirmed by the histopathology evaluations. The lung injury confirmed by high-resolution thin-section computed tomography and histopathology in the SS group was more serious than those of other groups.

Conclusions: In the present study, we developed a novel porcine model of septic shock induced by ARDS due to severe MRSA pneumonia with characteristic hyperdynamic and hypodynamic phases in 24 h, which mimicked the hemodynamic changing of septic shock in human.

Figure 1

Changes in blood temperature and WBC. (a) Blood temperature in the SM and SS groups increased progressively from 2 h postinjury. It was higher in the SS group than in the SM and MR group; significant differences were found between the SS group and other two lung injury groups (SM and MR) at each time point after 2 h. Blood temperature in the SH group stayed normal at all time points. (b) Similar changes with blood temperature were found in the WBC count among the four groups. The increasing of blood temperature and WBC in the MR group was slower than those in the SM and SS group. WBC: White blood cell; SH: Sham group; SM: Cotton smoke inhalation group; MR: Methicillin-resistant Staphylococcus aureus pneumonia group; SS: Septic shock group. *P < 0.05 versus the SM group; ^†P < 0.01 versus the SM group; ^§P < 0.01 versus the SS group; ^||P < 0.05 versus the MR group; ^¶P < 0.01 versus the MR group.

Figure 2

Hemodynamic changes. (a) MAP decreased to 60–70 mmHg exactly 2 h after injury in the SS group; significant changes were found between the SS and other three groups. (b) HR increased significantly in the lung injury groups (SM, MR, and SS), especially in the SS group. Very significant differences were found between the SS and anyone of other three groups at each time point after 2 h postinjury. (c and d) In the SS group, the changes in the CI and SVRI were opposing: The CI increased at 2 h and then gradually decreased, whereas the SVRI decreased at 2 h and then gradually increased. (e) The change in dP_max was identical to that in CI. (f) The CVP of individuals in the SS group increased progressively, but remained normal in the other three groups. CI: Cardiac index; CVP: Central venous pressure; dP_max: Index of left ventricular contractility; HR: Heart rate; MAP: Mean artery pressure; SH: Sham group; SM: Cotton smoke inhalation group; MR: Methicillin-resistant Staphylococcus aureus pneumonia group; SS: Septic shock group; SVRI: Systemic vascular resistance index. *P < 0.05 versus the SM group; ^†P < 0.01 versus the SM group; ^‡P < 0.05 versus the SS group; ^§P < 0.01 versus the SS group; ^||P < 0.05 versus the MR group; ^¶P < 0.01 versus the MR group.

Figure 3

Changes in respiratory parameters. All parameters in the SS group deteriorated gradually and the changes were more obvious than those in the SM and MR groups. Significant differences were found between the SS and anyone of other two lung injury groups (SM and MR) at each time point after 2 h injury. (a) Ctp of the SM group was improved before 8 h by positive pressure ventilation but deteriorated after that. (b-d) Vd, R, and ELWI increased after injury in the SM, MR, and SS groups, whereas they were stable in the SH group. Ctp: Thoracopulmonary compliance; ELWI: Extravascular lung water index; R: Airway resistance; P-V: Pressure-volume; SH: Sham group; SM: Cotton smoke inhalation group; MR: Methicillin-resistant Staphylococcus aureus pneumonia group; SS: Septic shock group; Vd: Dead space. *P < 0.05 versus the SM group; ^†P < 0.01 versus the SM group; ^‡P < 0.05 versus the SS group; ^§P < 0.01 versus the SS group; ^||P < 0.05 versus the MR group; ^¶P < 0.01 versus the MR group.

Figure 4

Changes in oxygen dynamics. (a and b) In the SS group, pH decreased progressively, whereas Lac increased. Significant differences were found between the SS group and anyone of other three groups after 2 h. (c) The PO₂/FiO₂ of the SM and SS group decreased gradually after smoke inhalation. It was lower in the SS group than in the SM group. Very significant differences were found between anyone of them and the SH group. (d-f) DO₂ and VO₂ in the SS group increased to peak at 2 h and then decreased to the nadir, but ERO2 stayed at a high level until the end of the protocol. DO₂, VO₂, and ERO₂ in the SH and SM group stayed normal throughout the entire protocol. VO₂ and ERO₂ in the MR group increased significantly after 2 h of injury. DO₂: Oxygen supply; ERO₂: Oxygen extraction rate; Lac: Lactic acid; SH: Sham group; SM: Cotton smoke inhalation group; MR: Methicillin-resistant Staphylococcus aureus pneumonia group; SS: Septic shock group; VO₂: Oxygen consumption. *P < 0.05 versus the SM group; ^†P < 0.01 versus the SM group; ^‡P < 0.05 versus the SS group; ^§P < 0.01 versus the SS group; ^||P < 0.05 versus the MR group; ^¶P < 0.01 versus the MR group.

Figure 5

HRCT scans of the SS group. (a and b) The transverse section of the lung window and the longitudinal diaphragm window of HRCT, respectively, showing the consolidation of double lower lung. The black arrow shows the pleural thickening, and the white arrow shows the mucous plugging. (c and d) The vertical plane and the coronal section of lung, respectively, showing the serious effusion with the ground glass-like appearance. All sections were from the SS group. HRCT: High-resolution computed tomography; SS: Septic shock group.

Figure 6

Histopathologic figures. (a-d) Histopathologic slides under LM (H and E, ×200) from the SH group, SM group, MR group, and SS group, respectively. The slide was normal in the SH group, but very significant lung injuries were found in the SM, MR, and SS groups. ①Neutrophils, lymphocytes, and macrophages infiltrated in the alveoli; ②microthrombus formatted in the capillaries; ③interalveolar septum thickened by the infiltration of inflammatory cells. (e-g) Histopathologic figures from the SS group under EM (bars: 2 μm in e and f, 1 μm in g). (e) ④MRSA was found in the parenchymal area. (f) The bandings of Type I alveoli and capillary epithelium were loose and disordered. (g) ⑤The chromatin and nucleus vacuolar of Type II alveoli epithelium degenerated; ⑥The mitochondria swelled and the cristae disappeared. LM: Light microscope; EM: Transmission electron microscope; SH: Sham group; SM: Cotton smoke inhalation group; MR: Methicillin-resistant Staphylococcus aureus pneumonia group; SS: Septic shock group; MRSA: Methicillin-resistant Staphylococcus aureus.