Whole-thorax irradiation induces hypoxic respiratory failure, pleural effusions and cardiac remodeling

Abstract

To study the mechanisms of death following a single lethal dose of thoracic radiation, WAG/RijCmcr (Wistar) rats were treated with 15 Gy to the whole thorax and followed until they were morbid or sacrificed for invasive assays at 6 weeks. Lung function was assessed by breathing rate and arterial oxygen saturation. Lung structure was evaluated histologically. Cardiac structure and function were examined by echocardiography. The frequency and characteristics of pleural effusions were determined. Morbidity from 15 Gy radiation occurred in all rats 5 to 8 weeks after exposure, coincident with histological pneumonitis. Increases in breathing frequencies peaked at 6 weeks, when profound arterial hypoxia was also recorded. Echocardiography analysis at 6 weeks showed pulmonary hypertension and severe right ventricular enlargement with impaired left ventricular function and cardiac output. Histologic sections of the heart revealed only rare foci of lymphocytic infiltration. Total lung weight more than doubled. Pleural effusions were present in the majority of the irradiated rats and contained elevated protein, but low lactate dehydrogenase, when compared with serum from the same animal. Pleural effusions had a higher percentage of macrophages and large monocytes than neutrophils and contained mast cells that are rarely present in other pathological states. Lethal irradiation to rat lungs leads to hypoxia with infiltration of immune cells, edema and pleural effusion. These changes may contribute to pulmonary vascular and parenchymal injury that result in secondary changes in heart structure and function. We report that conditions resembling congestive heart failure contribute to death during radiation pneumonitis, which indicates new targets for therapy.

Keywords: mast cells; radiation pneumonitis; right ventricular mass; tachypnea.

Fig. 1.

Radiation-induced decrease in body weight. Graph shows body weights (mean with standard deviation) of rats in grams at time of irradiation and 4 and 6 weeks after irradiation. Irradiated rats (15 Gy) had lower body weight than unirradiated control rats at 6 weeks (*P < 0.05). No difference was observed for results between the irradiation group and control at the other time-points. n = number of rats.

Fig. 2.

Radiation-induced increase in breathing rate. Breaths per minute (Y-axis) are presented as means with standard deviations. Irradiated rats (15 Gy) had higher breathing rate than unirradiated (control) rats at 6 weeks (*P < 0.05). No difference was observed for results between the irradiated group and controls at 4 weeks. n = number of rats.

Fig. 3.

Radiation-induced hypoxia. Percent oxygen (O₂ saturation, y-axis) is presented as means with standard deviations. Rats had lower percent O₂ saturation at 6 weeks after 15 Gy irradiation than unirradiated controls (*P < 0.05 between irradiated and unirradiated, and between irradiated at 5 vs 6 weeks). No difference was observed for results between the irradiated group and controls at 5 weeks. n = number of rats.

Fig. 4.

Radiation-induced changes in representative H&E-stained lung sections (4 μm thick) pointing out four categories scored in Table 1: (A) lung from unirradiated rat; (B) lung from irradiated (15 Gy) rat; (C) magnified image of boxed area in A. (D) magnified image of boxed area in B. Solid and dashed arrows in (A) point to blood vessels and alveolar walls respectively, which appear thicker in the irradiated lung (B). Green arrows in the irradiated lung section (B) point to abundant foamy macrophages. Red arrows point to red blood cells. Note the well perfused alveolar capillaries in unirradiated lungs (C) as compared with the poorly perfused capillaries in irradiated lungs (D).

Fig. 5.

Echocardiographic evidence of pulmonary artery hypertension in irradiated rats. Columns represent control rats (left-side) or irradiated rats (right-side) and rows represents four different imaging windows: (1) parasternal short-axis, (2) apical four-chamber views using B-mode at end-diastole to demonstrate right ventricular remodeling, (3) high parasternal short-axis view with pulsed-wave Doppler is used to detect pulmonary artery flow, and (4) apical four-chamber view with color Doppler is used to detect tricuspid regurgitation (red arrows). The parasternal short axis and apical four-chamber views show right ventricular dilatation with concomitant decrease in left ventricle. The pulmonary artery profiles reflect the presence of pulmonary hypertension. Normal, round-shaped flow profile is found in the control hearts, whereas a blunted flow profile with a sharp peak at early systole and a decreased acceleration time and increased deceleration is seen in the irradiated rat heart. There is no detectable tricuspid regurgitation by color Doppler in control rats, but tricuspid regurgitation is common in the irradiated group. R = right, L = left, A = atrium, V = ventricle.

Fig. 6.

Histological findings in irradiated rat hearts. H&E stains of hearts taken at two magnifications from unirradiated (A and D) and irradiated (B, C, E and F) rats reveal scattered foci of lymphocytic inflammation in a subpopulation of irradiated rats. One irradiated rat showed cardiac myocyte necrosis (C and F, arrows) associated with lymphocytic inflammation. Bars = 80 μm (A, B and C) and 40 μm (D, E and F).

Fig. 7.

Schematic of possible events leading to lethal radiation pneumonitis.