Electrical Impedance Tomography

Revised and reviewed 16 July 2014

OVERVIEW

  • Electrical impedance tomography (EIT) is a noninvasive, radiation-free monitoring tool that allows real-time imaging of ventilation
  • EIT has medium spatial resolution and a very high temporal resolution
  • EIT may be performed as:
    1. relative or differential EIT (most commonly used) (aka functional EIT or f-EIT), or
    2. absolute impedance EIT (aka a-EIT)

USES

  • describe the distribution of alveolar ventilation
  • detect lung collapse
  • assess lung hyper-expansion
  • monitor lung recruitment
  • diagnose pneumothorax
  • detect endobronchial intubation
  • estimate ventilation/perfusion distributions (using hypertonic saline as ‘contrast’)
  • determination of optimal PEEP (can detect simultaneous presence of overdistended and collapsed lung compartments)

DESCRIPTION

  • tomograph (a transportable device)
  • a computer for control and data sampling
  • a cable tree with 16 cables (+1 neutral) and 16 or 32 electrodes (+1 electrode = neutral) attached to the thorax

PRINCIPLES

Impedance

  • Impedance is an abstract physical variable that describes the resistivity characteristics (or the reciprocal value: conductivity) of an electric circuit in the presence of an alternating current (AC)
  • Different biological tissues have distinct impedance characteristics (bio-impedance)
  • increasing air in lungs causes increased impedance
  • Perfusion causes a change in thoracic bio-impedance from diastole to systole in a range of 3%, due to:
    1. lung displacement during systole
    2. RBC alignment during systole, and random positioning during diastole
    3. localised changes in the cardiac fossa due to cardiac contraction

Impedance tomography

  • cross-sectional images of the lungs are created by the administration of  high frequency (50-80 Hz), low amplitude (5 mA peak-to-peak) alternating electrical currents, typically through 16 or 32 electrodes
  • the pathways followed by the administered currents vary according to:
    1. chest wall shape
    2. differences in thoracic impedance (e.g. due to variations in lung volume during ventilation)
  • the resulting electric potentials on the surface of the chest wall are measured
  • an electric impedance distribution within the thorax is created using a reconstruction algorithm
  • IV injection of hypertonic saline can be used as a contrast agent for EIT images due to its extremely low impeditivity. When combined  with a breath hold maneuver this allows measurement of lung perfusion
  • extent of lung recruitment can be measured by:
    • the center of gravity of ventilation images moves dorsally during lung recruitment and ventrally during lung collapse
    • variation in the time delay between start of inspiration and start of regional inflation (ventilation delay index)

OTHER INFORMATION

Pathological processes that affect lung impedance include:

  • extravascular lung water, e.g. pulmonary edema
  • intrathoracic blood volume
  • fluid in cavities (e.g. pleural effusion, pericardial effusion, bronchial and alveolar fluid)
  • foreign bodies (pleural drain)
  • lung fibrosis

Determinants of spatial resolution

  • accuracy and noise of the measurements
  • number of electrodes
  • the ‘regularisation’ used

PROS AND CONS

Advantages

  • cheap
  • non-invasive
  • allows ventilation to be continuously monitored at the bedside
  • no radiation
  • relative or differential EIT cancels out errors caused by assumptions made about thoracic shape
  • resolution is better with increased number of electrodes (e.g. 6-10% of thoracic diameter (1.5 to 3 cm) with 32 electrodes versus 12-20% for 16 electrodes)
  • high temporal resolution (e.g. 50 images/second)
  • use of brief periods of apnea or by filtering out ventilation allows monitoring of perfusion

Disadvantages

  • poor availability
  • not widely accepted
  • small errors in voltage measurements can lead to the creation of markedly different impedance distributions, requiring the use of ‘regularisations’ (assumptions) in the estimation algorithms, which sacrifice spatial resolution and attenuate extreme perturbations
  • determination of absolute impedance distributions requires characterization of the shape of the thorax
  • differential EIT does not provide information about baseline impedance or absolute measurements (e.g. a change in lung impedance from 5 to 10 ohms produces the same relative image as a change from 10 to 20 ohms) (this does not seem to have adverse clinical ramifications)
  • differential EIT images only show regions of the thorax that change their impedance over time, so preexisting areas of lung consolidation, pleural effusions or large bullae are not represented
  • absolute EIT tends to have lower quality images
  • spatial resolution varies between EIT devices, and within a single device depending on the settings used
  • spatial resolution in the craniocaudal direction is lower (~ 7–10 cm slices)
  • less spatial resolution than CT or MRI
  • baseline measurement is needed for pneumothorax detection

References and Links

Journal articles

  • Bodenstein M, David M, Markstaller K. Principles of electrical impedance tomography and its clinical application. Crit Care Med. 2009 Feb;37(2):713-24. doi: 10.1097/CCM.0b013e3181958d2f. Review. PubMed PMID: 19114889.
  • Costa EL, Lima RG, Amato MB. Electrical impedance tomography. Curr Opin Crit Care. 2009 Feb;15(1):18-24. Review. PubMed PMID: 19186406.
  • Frerichs I, Becher T, Weiler N. Electrical impedance tomography imaging of the cardiopulmonary system. Curr Opin Crit Care. 2014 Jun;20(3):323-32. doi: 10.1097/MCC.0000000000000088. PubMed PMID: 24739268.
  • Lundin S, Stenqvist O. Electrical impedance tomography: potentials and pitfalls. Curr Opin Crit Care. 2012 Feb;18(1):35-41. doi: 10.1097/MCC.0b013e32834eb462. Review. PubMed PMID: 22201705.

CCC 700 6

Critical Care

Compendium

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

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