One option may be electrical impedance tomography (EIT), an emerging clinical tool that uses electrical currents to probe impedance changes within the body. EIT is currently employed to track changes that occur as a patient breathes, providing real-time information regarding regional lung ventilation. Researchers at the University of Sydney in Australia have now investigated the use of EIT with a saline-based contrast to examine perfusion redistribution after an artificially induced PE in adult sheep (Physiol. Meas. 36 1297).

"Ventilation and perfusion are two sides of the same coin," said lead author Trang Nguyen. "We and many other researchers believe that in order to understand the physiological state of the patient's lungs, both ventilation and pulmonary perfusion information are required. We are particularly motivated by the possibility of using EIT for diagnosis of PE."

Proof of principle

Nguyen and colleagues performed EIT on eight sheep. The animals were anaesthetized and saline contrast injected into the right atrium during a 40–45 s period of apnoea, in order to image conductivity changes due to the contrast without interference from the ventilation signal. "We are currently trying to use post-processing to eliminate this period of apnea altogether," Nguyen noted.

The researchers acquired EIT images with two contrast dilutions: 0.13 ml/kg of NaCl 20% (C1) and 0.77 ml/kg of NaCl 3% (C2), inducing an artificial PE by inserting a balloon catheter into the animal's pulmonary artery trunk. They then used these images to generate averaged contrast dilution curves in regions-of-interest within each lung, for baseline and PE states.

Statistical analysis of four parameters in each of the curves – peak value, maximum uptake, maximum washout and perfusion (estimated from the area under the curve) – revealed three pairs of parameters with significantly differing means between PE and baseline: maximum washout in the left lung with C1; perfusion in the left lung with C2; and peak value in the left lung with C2.

To study the redistribution of perfusion due to a one-sided PE-like event, the researchers computed right lung-to-left lung (R2L) ratios for the four parameters. Of these, the R2L perfusion ratio showed the most obvious distinction between PE and baseline states. The R2L peak value ratio also showed similar distinction between the two states. In both cases, the ratio was lower for the PE state than for the baseline.

One significant finding of this study was that that the two contrasts exhibited similar performance in distinguishing PE from baseline, implying that EIT can be performed using the less concentrated saline solution. Hypertonic 3% saline is available clinically and less likely to cause cell injury than a 20% solution (as used in previous studies). The total salt delivered is also reduced in the 3% solution, which is important for patients with compromised sodium control.

The team concluded that EIT can reliably detect the difference between normal and embolized lungs with a one-sided perfusion defect, pointing out that a larger trial is required before the method can be used clinically for diagnosis of PE.

"While PE can lead to severe and possible long-term damage of the cardiovascular system, its symptoms are highly unspecific," senior author Alistair McEwan told medicalphysicsweb. "As EIT is less invasive and does not use radiation, we are hopeful that it can be used as a first-stop imaging diagnosis for PE before a CT pulmonary angiogram is prescribed, to reduce radiation and contrast exposure in patients."

• Westmead Hospital Cardiology provided the researchers with the animal facilities and clinical support. Trang was also supported by an Australian Postgraduate Award from the Australian Research Council and McEwan was supported by a Microsoft Faculty Fellowship.

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