Functional imaging of exercise-induced pulmonary haemorrhage

At present, the relative severity of the bleeding in exercise-induced pulmonary hemorrhage (EIPH) is better evaluated using scoring methods based on tracheobronchoscopic assessment of bleeding or red blood cells (RBCs) count in broncho-alveolar lavage (BAL). However, misinterpretation might happen with tracheobronchoscopic examination when the hemorrhage is tiny as well as with RBCs count in BAL when the bronchi sampled are not representative of the bleeding site. Quantifying EIPH accurately would be of great interest to define the relationship between bleeding and performance as well as to assess efficacy of treatments or prophylactic measures. Several studies have attempted to determine whether lung imaging could be of value in EIPH detection and quantification.

Radiographic findings when present consist of interstitial opacities limited to the caudodorsal regions of the lungs. Resolution of opacity may last for several days to several months. Interpretation of chest radiographs has found to be of poor value for diagnosing and a fortiori for quantifying EIPH and lung remodeling.

Scintigraphy has been investigated as a mean of studying EIPH. Ventilation/perfusion (V/Q) scans show variable reduction of ventilation and a loss of perfusion in the caudodorsal fields but the relationship between V/Q mismatches and EIPH quantification seems hazardous. The exercise-induced pulmonary perfusion redistribution may be visualized and quantified with labeled radiotracers entrapped in the pulmonary capillaries following their intravenous injection in exercising horses. This technique may not detect EIPH because scanning does not distinguish tracer collected in alveoli (i.e. resulting from bleeding) versus trapped in the capillaries. The same is true with the use of labeled-RBCs: circulating tracer impedes visualization of bleeding. To visualize the bleeding site, radioactivity of the vascular blood pool should be removed. A preliminary study indicated that this background might be removed using a double isotope scintigraphy. In fact, horse would be first injected at exercise with RBCs labeled with 111-Indium (111In-RBCs). In bleeders, a scan taken at the end of exercise would include the hemorrhage hidden by the blood pool. As EIPH is presumed to resolve with exercise cessation due to immediate decrease in pulmonary artery pressure, a second injection of RBCs labeled with another radiotracer (99m-technetium; 99mTc-RBCs) would enable acquisition of the sole blood pool knowing that 99mTc and 111In activities may be recorded on two different channels. Then, the computerized removal of background vascular radioactivity should enable to visualize the bleeding site and possibly to quantify the amount of extravasated blood.