Measurement of ventilation and cardiac related impedance changes with electrical impedance tomography
Introduction Electrical impedance tomography (EIT) has been shown to be able to distinguish both ventilation and perfusion. With adequate
filtering the regional distributions of both ventilation and perfusion and their relationships could be analysed. Several
methods of separation have been suggested previously, including breath holding, electrocardiograph (ECG) gating and frequency
filtering. Many of these methods require interventions inappropriate in a clinical setting. This study therefore aims to extend
a previously reported frequency filtering technique to a spontaneously breathing cohort and assess the regional distributions
of ventilation and perfusion and their relationship.
Methods Ten healthy adults were measured during a breath hold and while spontaneously breathing in supine, prone, left and right lateral
positions. EIT data were analysed with and without filtering at the respiratory and heart rate. Profiles of ventilation, perfusion
and ventilation/perfusion related impedance change were generated and regions of ventilation and pulmonary perfusion were
identified and compared.
Results Analysis of the filtration technique demonstrated its ability to separate the ventilation and cardiac related impedance signals
without negative impact. It was, therefore, deemed suitable for use in this spontaneously breathing cohort.
Regional distributions of ventilation, perfusion and the combined ΔZV/ΔZQ were calculated along the gravity axis and anatomically in each position. Along the gravity axis, gravity dependence was
seen only in the lateral positions in ventilation distribution, with the dependent lung being better ventilated regardless
of position. This gravity dependence was not seen in perfusion.
When looking anatomically, differences were only apparent in the lateral positions. The lateral position ventilation distributions
showed a difference in the left lung, with the right lung maintaining a similar distribution in both lateral positions. This
is likely caused by more pronounced anatomical changes in the left lung when changing positions.
Conclusions The modified filtration technique was demonstrated to be effective in separating the ventilation and perfusion signals in
spontaneously breathing subjects. Gravity dependence was seen only in ventilation distribution in the left lung in lateral
positions, suggesting gravity based shifts in anatomical structures. Gravity dependence was not seen in any perfusion distributions.