Introduction

An impedance cardiogram (ICG) measures the electrical impedance (resistance) of the blood in the thorax (chest), specifically in the aorta. This impedance depends on the blood levels in the aorta. Every heartbeat shows a change in impedance. When blood levels are high (directly after the opening of the aorta valve in the heart) the impedance is low. The ICG signal (Z(t)), as well as its derivative (dZ/dt) are used to calculate various hemodynamic parameters, such as stroke volume, cardiac output, and the pre-ejection period. ICG is measured in ohms/sec.

Availability, support and advice

For availability of ICG equipment within your unit, contact your lab coordinator. ICG equipment from BIOPAC and VU-AMS can be borrowed from SOLO through the Helpdesk or by contact labsupport@fsw.leidenuniv.nl. 

Protocol

Below, a general protocol of measuring ICG with and 8 lead (BIOPAC) or 4 lead (VU-AMS) configuration is described. Note that other configurations are possible, but not described here. Also note that an adequate training is required before working with physiological data acquisition equipment and collecting such data from participants. If you are uncertain about the procedure, please contact your supervisor or lab coordinator.

Setup

Equipment

ICG can be measured with the NICO-R (wireless) or NICO100D (wired smart amplifier) modules, or with VU-AMS equipment.

Leads and electrodes

NICO-R
Two pairs of BN-EL50-LEAD4 leads for BioNomadix (4x50cm leads) are used to measure ICG. Connect one to each channel (In and Out). Usually, ICG is measured with 8 disposable Ag/AgCl ECG electrodes (see Lab Supplies). The NICO-R transmitter can be attached to a BioNomadix velcro strap with size 137 cm (BN-STRAP-137), which can be tied around the waist of the participant.

NICO100D
The NICO100D smart amplifier is used with the LEAD132, an 8-clip lead. Usually, ICG is measured with 8 disposable Ag/AgCl ECG electrodes (see Lab Supplies).

VU-AMS
The VU-AMS comes with its own electrode lead bundle, one with which you can measure both ICG and ECG. The yellow and blue leads are used to measure ICG. Usually, ICG is measured with disposable Ag/AgCl ECG electrodes (see Lab Supplies).

Materials

For SSH researchers, supplies can be picked up at SOLO 1B11 in PdlC or 2.5.01 in Sylvius, see lab supplies for more info.

• ICG equipment (see Setup)
• Disposable electrodes (Ag/AgCl) (the amount is dependent on the equipment used)
• Scrub gel (NuPrep)
• Non-woven swabs
• Gloves
• Incidin OxyWipe

Skin preparation

Before applying the electrodes it is important to reduce the impedance (resistance) of the skin by removing oils and dead skin cells. It is recommended to use a scrub gel (NuPrep) for this. Do not use alcohol wipes as they are generally too aggressive for the skin.

• Put on disposable gloves.
• Put a little NuPrep scrub gel on a non-woven swab.
• Rub the gel gently onto the skin.
• When the skin remains wet, dry the skin with a clean swab or tissue.
• Apply the electrodes directly after cleaning the skin.

Electrode placement

The number of electrodes used and the placement of the electrodes can differ depending on the equipment and method used. Below, two different methods of placing electrodes for ICG measurement are described (BIOPAC and VU-AMS). Note that in addition to these methods there are other ways to apply electrodes.

Method 1 (BIOPAC)

For this method, eight disposable electrodes are used.

• Four electrodes are placed on the neck, two on the left hand side and two on the right hand side. These pairs are placed over the neck artery with 5 cm between the electrodes (measured from the  center of the electrodes).
• Four electrodes are placed on the ribs, two on each side. Each pair is placed high up on the rib cage (for women just below the bra strap) and in line with the center of the shoulder. The electrodes must be placed 5 cm apart (measured from the center of the electrodes).

Four pairs of electrode leads are used:

• NICO-R with BN-EL50-LEAD4 consists of two pairs of red leads and two pairs of white leads. A tip for remembering the leads is that the “in” leads always connect to the electrodes closest to the heart.
  • The red leads need to be attached to the electrodes in the neck.
    • Attach the red “out” leads to the upper electrodes on the neck.
    • Attach the red “in” leads to the lower electrodes on the neck.
  • The white leads are attached to the electrodes on the chest.
    • Attach the white “in” leads to the upper electrodes on the chest.
    • Attach the white “out” leads to the lower electrodes on the chest.
• NICO100D with LEAD132 consists of a pair of white leads, a pair of white leads, a pair of green leads and a pair of black leads.
  • Attach the white leads to the upper electrodes on the neck. 
  • Attach the red leads to the lower electrodes on the neck.
  • Attach the green leads to the upper electrodes on the chest.
  • Attach the black leads to the lower electrodes on the chest.

When using the wireless RSPEC-R module, you can fix the strap with the transmitter around the participant’s waist and then attach the leads to the electrodes. Ensure the strap is placed between the  transmitter and the skin, so that the transmitter does not come into contact with the skin.

Method 2 (VU-AMS)

VU-AMS - Edited figure from source: Nederend, ten Harkel, Blom, Berntson, & de Geus, 2017 (Resources)

In this method, four electrodes are used.

• Two electrodes are fixed to the chest, one just above the sternum and one just below the sternum.
• Two electrodes are placed on the back, three cm above and three cm below the electrodes on the chest.

The yellow and blue leads of the VU-AMS electrode bundle are used.

• The yellow leads are attached to the chest, the short one to the lower electrode and the long one to the higher electrode.
• The blue leads are attached to the back, the short one to the lower electrode and the long one to the higher electrode.

Cleaning up

• Put on disposable gloves.
• Remove the leads and, if present, the strap with the NICO-R transmitter.
• The participant can then remove the electrodes him/herself. Removing the electrodes can be painful for some people. Depending on the sensitivity of the participant’s skin, there may be red marks visible. These will normally fade within a few hours.
• The electrodes can be discarded in the bin.
• If necessary, the leads can be cleaned carefully with Incidin OxyWipe.
• When a strap has been used for the wireless NICO-R transmitter, the strap can be washed if necessary (use multiple straps to allow for drying), please contact labsupport@fsw.leidenuniv.nl when straps need to be washed.

Tips & Tricks

• The participant must move as little as possible to prevent artefacts in the data.
• The participant must be comfortable and sit in a natural posture with both feet on the floor.
• Check whether the ICG signals before staring the actual recording. The figure below shows typical Z and dZ/dt signals (recorded with BIOPAC AcqKnowledge).
• When the data looks irregular or shows a flat line, check whether the leads are properly attached and whether the electrodes are still properly attached.

Resources

• Nederend, I., ten Harkel, A. D. J., Blom, N. A., Berntson, G. G., & de Geus, E. J. D. (2017). Impedance cardiography in healthy children and children with congenital heart disease: Improving stroke volume assessment. International Journal of Psychophysiology, 120, 136-147