EKG Archives - BIOPAC Blog

ECG – The Little Wave You Can’t Live Without

For millennia, ancient healers across the globe long recognized the human heart as immeasurably vital to human life. Other organs may take an occasional time out, but not the heart. This sinewy fist-sized pump contracts and relaxes an average 60 to 80 beats per minute and faithfully repeats this cycle some two and a half billion times over an average human lifespan, constantly driving and recirculating lifeblood through 100,000 miles of veins, arteries, and capillaries. It’s a monumental task that we take for granted, but if the process stops for more than a few minutes, so will this mysterious force we call life.

It’s intriguing to consider the wonder with which the ancients must have contemplated this organ that thumps so audibly. They knew it was of critical importance, and over the centuries learned its form and surface physiology, but what did they imagine animated it? What force kept the heart moving so steadily and faithfully, while remaining implacably divorced from the consciousness of its host? (Go ahead. Try to stop your heart. On second thought, don’t.)

The answer, in a word, is electricity. Although the stethoscope, invented in 1816 by French physician René Laennec, augmented audible heart sounds, it wasn’t until 1901 that Dutch physician Willem Einthoven developed a “string galvanometer,” capable of reliably recording the heart’s electrical activity. Einthoven’s work established a standard configuration for recording the electrocardiogram (ECG) and garnered him the Nobel Prize in 1924. Because the ECG reflects electrical activity, it is a useful “picture” of heart normality or abnormality through the observation and analysis of various distinct components of the cardiac cycle. If there are interruptions of the electrical signal generation or transmission, the ECG changes. These changes can be useful in diagnosing changes within the heart.

The methods for recording ECG have become increasingly sophisticated with the advent of digital technology and have put state-of-the art acquisition and analysis of ECG within reach of not only clinicians, but also researchers and educators. Wired in-lab systems such as the BIOPAC MP160 System with AcqKnowledge software are widely used for Research applications, while the MP36 System with Biopac Student Lab software are the industry standard for physiology education around the globe. Wireless systems such as BioNomadix or BioNomadix Smart Center can be used to collect ECG and other body signals in an untethered environment and the BioNomadix Logger is a great solution for acquiring ECG and other body signals over extended periods outside the lab.

We have come a long way since our forebears contemplated the mysteries of the vital mechanical wonder that beats in every chest, and we now have a myriad of accessible options for unlocking its secrets.

For further exploration of ECG, we invite you to enjoy the following comprehensive free ECG webinars:

Recording Great ECG

HRV & RSA – Fundamental ECG Analysis Techniques

BIOPAC offers a wide array of wired and wireless equipment that can be used in your research. To find more information on solutions for recording and analyzing signals such as ECG, heart rate, respiration and more using any platforms mentioned in this blog post, you can visit the individual application pages on the BIOPAC website.

Electrocardiography (ECG)

Electrocardiography (ECG) is a graph of electrical activity of the heart (cardiac) muscle as it changes over time. The basic principle of the ECG is that periodic waves of propagated electrical stimulation cause cardiac muscle to alternately contract and relax.

By Alan Macy, BIOPAC Systems, Inc

Electrocardiography (ECG) results in a graph of electrical activity of the heart (cardiac) muscle as it changes over time. Cardiac muscle contracts in response to electrical depolarization of the heart muscle cells. The ECG is the measured sum, over time, of this electrical activity as it is recorded from electrode connections.

Willem Einthoven’s contribution to ECG recording technology was the development of the string galvanometer that had improved sensitivity over the capillary electrometer. In 1908, Willem Einthoven published a description of a medically useful ECG recording system. This ECG system employed a specific lead connection method presently known as the Einthoven lead system. The Einthoven lead vectors are based on the assumption that the heart is located in a conductive volume. The Einthoven lead vectors are defined as:

Lead I: LA – RA

Lead II: LL – RA

Lead III: LL – LA

Where:

LA is Left Arm

LL is Left Leg

RA is Right Arm

The basic principle of the ECG is that periodic waves of propagated electrical stimulation cause cardiac muscle to alternately contract and relax. These electrical stimulation waves are transmitted from cardiac muscle cell to cell via gap junctions that connect between the cells. The electrical waves spread through the cardiac muscle cells because of cascaded changes in ions between intracellular and extracellular fluid. When the cardiac muscle cells are in a resting (polarized) state, the insides are negatively charged compared to the outsides. Cell membrane pumps maintain this electrically polarized state. Contraction of cardiac muscle is triggered by depolarization. Following depolarization, the cardiac muscle cells return to their resting charge. This process is called repolarization. These waves of depolarization and repolarization result in a changing electrical potential and can be detected by placing electrodes on the surface of the body.

The normal cardiac cycle begins with spontaneous depolarization of the sinoatrial (SA) node, a small volume of specialized tissue located in the right atrium. The SA node sets the rate of the heartbeat. The SA generated wave of electrical depolarization then spreads through the right atrium and across the inter-atrial septum into the left atrium causing the atria to contract. This contraction pushes blood into the ventricles. The atria are separated from the ventricles by a ring of non-conductive tissue. Accordingly, the only conductive path of electrical depolarization, from atria to ventricles, is through the atrioventricular (AV) node. The AV node is a cluster of cells in the center of the heart between the atria and ventricles. The AV node is a pathway that slows the electrical depolarization wave before it enters the ventricles. The wave of depolarization travels down the interventricular septum, through the His-Purkinje fiber network, to the left and right ventricles. The AV node delay gives the atria time to contract, before the ventricles do, to establish robust pumping action. In addition, with normal conduction, the two ventricles contract simultaneously to further maximize cardiac performance. After a volume of cardiac muscle cells depolarizes, the cells repolarize for the next cardiac cycle.

The net (average) path of electrical depolarization, through the heart, is known as the electrical axis. A fundamental ECG measurement principle is that when the wave of electrical depolarization travels toward a recording lead, this results in a positive voltage on that lead. When the wave travels away from a recording lead this results in a negative voltage on that lead. During repolarization, net wave activity towards or from a recording lead results in the opposite polarity as compared to depolarization.

The nature of the repolarization wave is different, from that of the depolarization wave, in that repolarization is not a propagating phenomenon. Cardiac muscle cell repolarization only occurs after the depolarization action potential duration, so cell repolarization does not depend on the repolarization of an adjoining cell. Assuming the action potential of all cells have equal duration, the repolarization wave would follow the same sequence as depolarization wave. However, in considering the entire extent of cardiac muscle, action potential duration is not identical between cells.

Meaning of ECG waves –

P wave: Depolarization of the right and left atria

QRS complex: Depolarization of the ventricles

T wave: Repolarization of ventricles

Meaning of ECG intervals –

PQ interval: Time period between start of atrial depolarization to start of ventricular depolarization

QRS duration: Time period of ventricular depolarization

QT interval: Time period between start of ventricular depolarization and end of ventricular repolarization

RR interval: Time period of complete cardiac cycle (inverse of heart rate).

For more information on BIOPAC’s wide range of tools for recording, displaying, and analyzing ECG measurements from human and animal subjects, visit BIOPAC’s electrocardiograpy, epages and view BIOPAC’s full line of electrodes, amplifiers, and wearable, wireless transmitters and loggers.

BIOPAC Systems, Inc. provides life science researchers and educators with data acquisition and analysis systems that inspire people and enable greater discovery about life. Visit us at www.biopac.com.

Electrocardiogram Waveshape – Pt 1

By Alan Macy, BIOPAC Systems, Inc.

When recording ECG, it’s important to consider the impacts of electrode placement and amplifier filters on the measured biopotential. Electrode placement is important because different orientations of biopotential surface electrodes with respect to the heart’s position will result in different signal shapes. As the heart contracts and expands, during the course of pumping blood, a time-varying, voltage signal is propagated over the volume of the heart. This voltage signal conducts to the surface of the body and is detectable between any two points on the body that straddle the heart. The signal can look quite different depending on whether the signal is collected from left hand to right hand or from right hand to left leg. When the ECG signal is recorded far from the heart, electrode placement is not critical to obtain similar-looking ECG waveforms when recording over time. As electrodes are placed increasingly close to the heart, then electrode placement becomes quite specific to recreate similar-looking ECG signals. This is because the body is a volume conductor and the heart is not a point-source of electrical signal activity.

The ECG waveform has been specified to consist of frequency components in the range of 0.05 Hz to 150 Hz, even though the characteristic ECG waveform shape is largely composed of components from 0.05 Hz to 35 Hz. However, even if the ECG’s primary waveshape is preserved, bandwidth limits affect usability of the data. For a high-resolution ECG recording, the signal bandwidth should be maximized. Although the Association for the Advancement of Medical Instrumentation (AAMI) standard recommends an ECG bandwidth of 0.05-150 Hz, surface electrodes measurements can show ECG components up to 500 Hz. [1]

As with any physiological measurement from a subject, the balance lies in attempting to isolate the signal of interest from residual noise sources. In the case of ECG recording, interfering noise is often generated by skeletal muscle that lies in the superposition path defined by electrode placement. This noise (recorded as the electromyogram) can be especially problematic if the subject is intentionally moving during the ECG measurement. Other noise sources include interference resulting from mains-generated displacement currents (50/60 Hz hum).

Depending on the nature of the ECG measurement requirements, filtering methodology can be a helpful ally to reduce the effects of interfering noise sources. The critical aspects of filtering are a function of the filter’s magnitude and phase characteristics. Choice of filter topologies can have a significant effect on the shape of the ECG. Given that AAMI guidelines are primarily concerned with ECG filter flatness from 0.05-150 Hz, typically, second-order (or higher) Butterworth filters are used to define these limits.

The discussion of ECG waveshape is continued in Electrocardiogram Waveshape Pt. II.

[1] http://circ.ahajournals.org/content/115/10/1306.full

For more information on BIOPAC’s ECG solutions, visit BIOPAC’s ECG-Cardiology page or view BIOPAC’s full line of electrodes, amplifiers, and wearable, wireless transmitters and loggers.

For more information on BIOPAC Systems, visit us at www.biopac.com.