Review the history of the development of impedance cardiography (ICG) from its theoretical base and direct applications for non-invasive measurement of cardiac output, to its most recent applications in assisting CPR, in rate responsive pacemakers and its potential application in automatic electrical defibrillators (AEDs).
Impedance Cardiology often called ICG, is a measure of change across the thoracic region of the body over the cardiac cycle. If there is high fluid volume and blood flow, a low impedance across the region is calculated. Impedance Cardiology is also used to measure blood flow in major vessels of the cardiac region from which stroke volume is obtained. A change of impedance can be useful in calculating stroke volume, cardiac output and systemic vascular resistance due to the fluid volume changing with every heart beat.
Cardiac Output (CO) along with the concentration of haemoglobin and arterial oxygen saturation are the cornerstones in the movement of oxygen. Cardiac output can be used to confirm the usefulness of treatment or if treatment is required as it analyses the functional performance of the cardiovascular system. The measurement of cardiac output is important in cardiothoracic surgery. There are several different methods for calculating the cardiac output. Firstly there is invasive methods that are quite accurate however the use of these methods are usually limited to intensive care units. While using invasive techniques the loss of blood, risk of infection and other complications are usually a matter of concern which leads to a alternative method, that being a non-invasive technique. For example Impedance Cardiology (ICG) is a method that is used quite often as it is easily used, provides a continuous reading of the cardiac output measured and has a better accuracy than that of other non-invasive techniques that are available. Impedance Cardiology involves applying a current field across the thorax using a constant magnitude, high frequency and a low amplitude alternating current. ¹
Bio-impedance is a non-invasive technique where the stroke volume is estimated based on the changing of impedance that occurs in the human arterial system during the cardiac cycle due to the constant change of blood volume. Cardiac output is a lot easier to measure by impedance cardiology compared to thermo dilution with a catheter interested in the pulmonary artery, as it can be applied quickly and easily. It also does not cause risk of blood loss, other complications or infection that would be carried with the arterial catheters. Invasive methods cannot monitor the cardiac output continuously whereas ICG will. Non-invasive techniques are the solution to all these problems. Cardiac output is calculated by multiplying the stroke volume by the heart rate. Stroke volume is the volume of blood that is pumped by the heart during every cardiac cycle. This means that measuring the differences in impedance gives an estimate of the changes in stroke volume.¹
The American Heart Association (AHA) resuscitation guidelines stated the chest compressions are the main source of effective cardiopulmonary resuscitation (CPR). A number of feedback devices have been developed to try to improve the efficiency of chest compressions, all of which improved guideline complaint CPR but did not improve the patients outcome. The ICG provides a non-invasive measure of the hemodynamic status of the body and is being investigated as another method of helping to improve CPR. This led to a study being set up Heartsine Technologies to investigate whether there was any relationship between compression depth, thrust and ICG amplitude during CPR. This also let a correlation between these to be established.
The impedance cardiogram was recorded using 2 electrodes from defibrillation pads. The compression depth (cm), compression thrust (kg), end-tidal CO2 (kPa), systolic blood pressure (mmHg), carotid flow (ml/min) and cardiac output (L/min) were all measured at two minute intervals for each model (13 porcine models in total). ²
The results of the study showed that there is a strong correlation between the correlations achieved with compression depth, compression thrust and between ICG amplitude. The table below shows the results obtained in the study and shows the correlation between the three:
The ICG measurement provides another measurement of CPR efficiency with physiological effects that are compared to chest compression depth and chest compression thrust. The results show that the ICG measurement could be used in the development of CPR feedback algorithms for AEDs (automated electrical defibrillators). ²
Improved impedance cardiogram measurement and recording methods have enabled their use in critical care of patients. Cromie reported that the use of both ICG recordings from two defibrillator pads , which is used to overcome the awkward application of using multiple electrodes and frequency analysis of the calculated derivative of the impedance signal (dZ/dt) which also provides information about circulatory arrest that occurs in the porcine model. ³
He then reported an algorithm that was based on the ICG using the peak magnitude in a frequency range for detecting cardiac arrest. It was brought about that the frequency analysis by Fast Fourier Transform (FFT) in public access defibrillators’ (PAD) and automated electrical defibrillators (AEDs) would compromise its processing capabilities and the use of integer filters to calculate the frequency components was proposed.
An algorithm that was only based on the impedance cardiogram that had been recorded through the use of two defibrillator pads, by using the strongest frequency and amplitude, could lead to a decrease in beginning CPR and could determine the circulatory arrest. Integer filters were used to analyse the frequency of the impedance cardiogram signal. Filters are lighter, simpler and a lot more adaptable when it comes to comparing with Fast Fourier Transform (FFT). This approach is more desirable as it limits the processing abilities of the devices that could compromise usability of the FFT, even though the approach is less accurate. The two techniques were compared with one another using 13 cases of cardiac arrest and 6 of normal controls. The best filters were used on this set and an algorithm that detects cardiac arrest was tested on a much wider set of data. The algorithm was then tested on a validation set and the ICG was recorded. It was tested on 132 cardiac arrest patients and 97 controls. The results indicated that cardiac arrest using the algorithm had a sensitivity average of 81.1% with the samples ranging from 77.6-84.3%. The specificity of the validation set was 97.1% with the samples ranging from 96.7-97.4% at a 95% confidence limit. These results show that automated defibrillators with impedance cardiogram analysis has the potential to improve emergency care by enabling non qualified persons to carry out appropriate CPR and it can also improve the detection of cardiac arrest when the algorithm is combined with ECG analysis.³
He then reported an algorithm that was based on the ICG using the peak magnitude in a frequency range for detecting cardiac arrest. It was brought about that the frequency analysis by Fast Fourier Transform (FFT) in public access defibrillators’ (PAD) and automated electrical defibrillators (AEDs) would compromise its processing capabilities and the use of integer filters to calculate the frequency components was proposed. The results of the frequency spectrum of the first order derivative of the impedance cardiogram (dZ/dt) recorded using the two defibrillator pads can be used as a marker to calculate circulatory collapse. The results obtained provide tools for the development of applications for the use of impedance cardiograms in defibrillators in emergency clinical practice.
Automatic Electrical Defibrillators are available in public areas along with trained rescuers. AEDs that recognise circulatory arrest use a hemodynamic sensor together with algorithms based on ECGs, would aid in the management of collapsed patients where accurate, quick and critical decisions must be made. ³