Nov 7, 2011
EIT in the ICU: addressing interference
Electrical impedance tomography (EIT) is an emerging clinical tool for tracking regional lung ventilation. The technique, which works by assessing the electrical properties of pulmonary tissue, could prove valuable for monitoring mechanically ventilated patients in intensive care units. In this setting, however, it's imperative to consider the effects of other medical devices on EIT measurements.
Inéz Frerichs and colleagues at the University Medical Centre Schleswig-Holstein in Germany have now published a paper demonstrating how various medical devices can interfere with EIT data. The authors discuss ways to minimize such interference and present methods for processing affected data in order to retrieve useful information (Physiol. Meas. 32 L1).
"I think that the present time is a very important phase in the development of EIT," Frerichs told medicalphysicsweb. "As EIT devices are commercially available now, I wanted to show future users the sources of interference and demonstrate how these affect the EIT scans and waveforms. My hope is that by showing these patient cases, users will be warned. This may eliminate potential and unnecessary discredit for EIT."
To perform EIT, small alternating currents are applied to the body via an array of electrodes and the resulting voltages at the body surface are measured. The acquired data can be transformed into a time series of 2D chest images showing the distribution of pulmonary electrical impedance.
Frerichs and colleagues considered four patients undergoing procedures that could potentially cause mechanical or electromagnetic interference with EIT. Two patients were examined during pulsation therapy, which involves lying on an air suspension mattress containing individual cushions that repeatedly inflate and deflate.
Patient 1 was mechanically ventilated using high-frequency oscillatory ventilation whilst receiving pulsation therapy with an 8 min cycle. EIT measurements were performed using 5 mA excitation currents with a frequency of 73 kHz. The pulsating mattress led to a periodic disturbance of the EIT signal, with fluctuations due to the pulsating mattress more than twice the amplitude of those from the heartbeat and ventilation oscillations. The functional scan was also dominated by pulsation-related artefacts.
This interference was attributed to changes in the external pressure on the electrodes resulting from the mattress motion. To clean up the affected EIT signals, the researchers performed digital band-pass filtering to remove the low-frequency signal modulation arising from the mattress pulsations. The filtered waveforms (and corresponding functional scans) clearly revealed impedance changes due to the heartbeat and ventilation, enabling meaningful interpretation of the data.
Patient 2 was ventilated in conventional pressure-controlled mode and underwent pulsation therapy with a cycle time of 32 min. The EIT waveform (using 4.9 mA, 49 kHz excitation currents) revealed the ventilation-related impedance changes, overlaid with a slow decline of the signal caused by the slower mattress pulsations. The functional scan showed a similar disturbance in the chest wall regions as seen in the first patient. This waveform could also be filtered to eliminate the low-frequency component of the signal.
The third patient was ventilated in pressure-controlled mode and subject to continuous cardiac output (CCO) monitoring, which entails intermittent heating of a thermal filament in the pulmonary artery catheter. This process caused a large broad-band disturbance of the EIT waveform and functional scan. When the monitor was switched off, the disturbance disappeared and the ventilated lung regions became visible in the functional scan.
Here again, frequency filtering to remove the broadband interference improved the appearance of the functional scan, making it similar to the undisturbed measurement. The authors note that while ventilation-related impedance changes were still discernible, if the interference had not been noted, the relatively low ventilation of the dorsal lung regions might have been missed.
The final patient breathed spontaneously and his respiratory rate was monitored using impedance pneumography, in which electrical currents with a frequency of 31.25 kHz were injected through ECG leads. EIT was performed using excitation at 30 kHz (3 mA) and 49 kHz (4.9 mA).
Using an EIT excitation current with a similar frequency to that of the impedance respiration monitor (i.e., 30 kHz) resulted in significant disturbance of the acquired data. This electrical interference was easily addressed by changing to 49 kHz, in which case similar EIT waveforms were seen with and without impedance pneumography.
"It is postulated that EIT may become a clinical tool for monitoring regional lung ventilation in mechanically ventilated patients and providing useful information for optimization of ventilator therapy," said Frerichs. "In our current studies we are trying to develop novel EIT measures that could be used in guiding ventilator therapy based on EIT examinations performed during uninterrupted mechanical ventilation and also during specific ventilatory manoeuvres."
About the author
Tami Freeman is editor of medicalphysicsweb.