electrical impedance tomography

Dr. med. Michael Czaplik

Electrical Impedance TomographyA path-breaking monitoring technique?


My affiliations

Chair for Medical Information TechnologiesHelmholtz Institute for Biomedical Engineering

Department ofAnaesthesiologyUniversity Hospital Aachen


Outline

1. Do we need another monitoring technique?

2. How does Electrical Impedance Tomography work?

3. Is there any evidence that EIT generates more than colourful images?

4. Is EIT clinically established yet?

5. What will be in future?


Do we need another monitoring technique?


Manoeuvring in respiratory treatment


Mechanical ventilation in 2012


Acute respiratory distress syndrome (ARDS)

Acute onset

Bilateral infiltrates

Horowitz ratio (paO2/FiO2)

<300 / <200 / <100 mmHg

Exclusion of a cardiac origin


“Therapy” of ARDS


Birth of protective ventilation

ARDSnetwork. (2000) New Engl J Med 342:1301-8

Mor

talit

y

Tidal volume12ml 6ml


Current ICU “concept”


Individual variation

Non-responder

responder

Protective ventilation needs an individual tuning of ventilation pressures, especially of PEEP

Protective ventilation needs an individual tuning of ventilation pressures, especially of PEEP


Role of CT imaging

Over-distended areas

Collapsed areas

Pro

precise anatomical information

Radiological density measure 1000 to -900 HU overdistended

-900 to -500 HU normal

-400 to -200 HU reduced ventilation

-100 to +100 HU atelectatic

Con

High level of radiation

No functional image

Not at the bedside


How does Electrical Impedance Tomography

work?


Bioimpedance vs. biopotentials

• Internal electrical power sources• Monitoring of internal ion currents flowing

through bio membranes• Examples: ECG, EMG, EEG, …

• Internal electrical power sources• Monitoring of internal ion currents flowing

through bio membranes• Examples: ECG, EMG, EEG, …

• External electrical power sources• Inject a small electrical AC current• Evaluation of passive electrical properties

of all body tissues• Examples: BIS, EIT

• External electrical power sources• Inject a small electrical AC current• Evaluation of passive electrical properties

of all body tissues• Examples: BIS, EIT


Electrical impedance of biological tissue


Principle of impedance tomography

Circular attachment of N electrodes (e.g. N=16)

Pairwise current injection (e.g. 5 mA at 50-200 kHz)

Voltage measurement between all remaining pairs

Recording of N x (N-3) voltages per frame (e.g. 208)


Presentation of impedance measuresStatic or absolute EIT frames

Relative or dynamic sequences of EIT frames

Functional EIT images (fEIT)

1111

22

33

Time course of impedances44


Is there any evidence that EIT generates more than colourful images?


Assessment of tidal volume


Analysis of regional ventilation

Correlation of air content changes determined by CT and impedance changes in 10 adult patients during slow inflation.

Victorino et al. Am J Respir Crit Care Med 2004; 169: 791-800


Analysis of regional ventilation II

Hinz J et al. Chest 2003; 123: 314-22.

Animal model, 12 pigs

ARDS induction with oleic acid

EIT compared to scintigraphy

Correlation coefficient (20 segments): 0,92 [0,86-0,97]


Estimation of regional compliance

9 patients suffering from ARDS

Aim: Evaluation of regional ventilation regarding compliance

Successful recognition of breathes (96,0%; 97,6%)

Calculation of compliance: correlation coefficient of 0.8

Examination of regional ventilation under stepwise pressure increase

Gómez-Laberge C et al. IEEE Trans Med Imaging. 2012; 31: 834-42


Monitoring of recruitment effects


Clinical application: endotracheal suction

Tingay DG et al. Intensive Care Med. 2010; 36: 888-96.

Porcine animal model, ARDS induced by surfactant depletion

Comparison of two suction techniques

Effect depends on catheter diameter >> technique


Clinical application: one-lung ventilation

Steinmann D et al. BJA 2008 Sep; 101: 411-8.

Clinical trial, 40 patients

Thorax surgery requiring one-lung ventilation

Correct placement of double-lumen tube was clearly visible


Clinical application: one-lung ventilation

Pulletz S et al. Acta Anaesthesiol Scand. 2008; 52:1131-9.


Thorax surgery requiring one-lung ventilation

Reliable detection necessitates ROI definition


Clinical application: emergency medicine

Conclusion: EIT is an imaging modality for monitoring mechanical ventilation during the relocation of patients. The application is non-invasive and without any exposure to radiation

Karmrodt J et al. Notfall & Rettungsmedizin 2004; 7:572–576


Is EIT clinically established yet?

Is there any evidence that EIT improves clinical outcome?


Clinical trial examining outcome

Title: Strategies to Optimize Positive End-expiratory Pressure (PEEP) in Patients With Acute Lung Injury (EIT-PEEP) (University Hospital Bonn)

Primary Outcome Measures: homogeneity of regional ventilation delay

Estimated enrollment: 40 patients

Estimated study completion: June 2013


What will be in future?


Clinical application: Monitoring of cerebral oedema

Clinical trial, 200 healthy patients vs. 107 patients with cerebral infarction

0,1mA @ 50kHz

Usage of contralateral side as reference

Aim: Monitoring of volume of cerebral oedema with EIT compared to MRT or CT

Accordance of about 75-85% (dependent on absolute volume)

He LY et al. Application of non-invasive cerebral electrical impedance measurement on brain edema in patients with cerebral infarction. Neurol Res. 2010;32:770-4


Clinical application: Cerebral haemorrhage

Porcine animal model

Blood injection to the cerebral frontal lobe

Usage of contralateral side as reference

16 electrodes attached to the head

Calculation of mean impedance and relative changes in defined ROI

High correlation with injected blood volume was achieved

Xu CH et al. Real-time imaging and detection of intracranial haemorrhage by electrical impedance tomography in a piglet model. J. Int. Med. Res 2010; 38: 1596-1604.


Cardiac related impedance changes


Measurement of ventilation, perfusion and ventilations-perfusions-ratio

Signal processing by band-pass filtering

Grant CA et al. Crit Care. 2011;15:R37.


Separation of ventilation and perfusion I


Separation of ventilation and perfusion II

Overlap of frequency bands

Pikkemaat, Leonhardt et al. Unpublished data


Separation of ventilation and perfusion III

Approach: using constrasting agents (10% NaCl)

Reference: SPECT / CT (99mTC labelled albumine)

Collaboration with Uppsala University (G. Hedenstierna)

4 healthy pigs (28-33 kg)

Volume-controlled ventilation

EIT device EEK2


Quelle: Henning Lüpschen, "Automatisierte protektive Beatmung durch Bestimmung von Ventilation und Perfusion der Lunge mittels Elektrischer Impedanztomographie“, Shaker Verlag, 2012, to appear.

• NaCl-Bolus: 10 ml of 10 %

• First, NaCl-Bolus reaches the right heart

• Then, high NaCl-concentration is flowing to the lungs

• Finally, bolus reaches the left heart The impedance curves show curves

similiar to thermodilution

• Regional separation of the signal according to the physiology seems possible

Separation of ventilation and perfusion IV

Pikkemaat, Leonhardt et al. Unpublished data


Application of internal electrodesImage reconstruction

Perfusion/ventilation voltage ratio

conv tracheal oesoph

conv tracheal oesoph

Czaplik, Hoog-Antik et al. Unpublished data


The future


Thanks for your attention!

The future II

electrical impedance tomography

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