The Electrocardiogram
The electrocardiogram (ECG; also EKG) is an essential component in the evaluation of a patient presenting with cardiac symptoms. It should be obtained simultaneously with, or immediately following, the evaluation of a patient’s vital signs. Rapidly obtaining and reviewing the ECG is a vital, time-sensitive step in evaluating all potential cardiac complaints. Professional association guidelines recommend that all patients with suspected coronary heart disease, such as patients complaining of shortness of breath, chest pain, syncope, or palpitations, receive an ECG with physician review within 10 minutes of ED arrival.
The heart is a four-chambered muscle that works as a pump at the center of the body’s circulatory system ( Fig. 8.1 ). The heart receives deoxygenated blood from the body’s tissues (colored blue) and pumps this blood into the lungs where it is oxygenated, receives this blood back from the lungs (colored red), and then pumps the newly oxygenated blood to the body’s vital organs.
The heart muscle contracts and relaxes in an organized manner based upon its own, intrinsic, natural, electrical pacemaker system that sends depolarizing charges to the heart muscle, which mechanically contracts after the heart’s membrane is depolarized. Analysis of the ECG allows a clinician to assess the rhythmic patterns and current flows of the electrical system and cardiac musculature. Detailed analysis permits a comparison of the patient’s current ECG to the known normal or to the patient’s prior ECG, allowing inferences to be made about disease processes affecting the heart. An ECG includes the heart’s rate (the number of contractions per minute) and rhythm (area of the heart where depolarizing impulses arise). By analyzing the ECG, physicians are often able to identify regions of the heart that may be receiving inadequate blood flow (such as during a heart attack).
Cardiac Pacemaker and Conducting System
The heart’s rhythm is determined by specialized tissues located within the heart that regularly emit an electrical pulse called pacemaker tissues. The main pacemaker of the heart called the sinoatrial (SA) node is located within the right atrium and is largely controlled by the body’s nervous system, which can speed it up or slow it down as needed. The cardiac impulse is then conducted throughout the atria until all the atrial cells are depolarized. The impulse then arrives at the atrioventricular (AV) node, where it is delayed for a short period and then released into the His-Purkinje system of the ventricles, which rapidly conducts the impulse throughout the ventricular musculature ( Fig. 8.2 ).
Components of an ECG
The ECG represents current flow between different parts of the heart during the cardiac cycle. To be an effective pump, the heart must contract in an organized manner. The heart’s electrical system normally depolarizes cells in a manner to optimize the heart’s pumping function. As cells are depolarized, current flows between the depolarized cells and those cells that have yet to be depolarized. The ECG measures this flow on the surface of the chest. The multiple leads that are obtained for the ECG provide the clinician with information about the current flows in different areas of the heart. After all the cells in a given region have depolarized, there is a period when there is no current flow measured, and the ECG returns to its baseline. The heart cells then undergo a wave repolarization in the same order as during depolarization, leading to measurable current flow again until all the cells are repolarized, and the ECG strip returns to baseline of no current flow. Flow of electricity on the cardiac surface toward a positive pole on an extremity or chest wall produces an upward deflection in the ECG, whereas electricity flowing away from the positive pole produces a negative deflection. Fig. 8.3 depicts these upward and downward ECG images by lead based upon flow of electricity within the heart.
The Cardiac Cycle
The first step of the cardiac cycle is atrial depolarization. This is represented on the ECG by a deflection called the P wave. This deflection can be upward or downward depending on which lead you examine. After all the atrial cells are depolarized, current flow stops, and the ECG returns to its baseline during the period that the impulse is then being delayed in the AV node. This period, the PR interval, is when the atria contract, filling and priming the “ventricular pump” before its contraction. The next set of deflections is called the QRS complex, which represents ventricular depolarization. As there are more cardiac muscle cells in the ventricles than the atria, the QRS complex is often of greater magnitude than the P wave due to the greater current flow that occurs during ventricular depolarization. When all ventricular cells are depolarized, current flow again ceases, and the ECG again returns to its isoelectric baseline during a period known as the ST segment. Ventricular contraction occurs during the ST segment. The final ECG deflection, the T wave, represents ventricular repolarization, after which current flow ceases and the ECG returns to its baseline. The representation of these elements on a normal ECG can be found in Fig. 8.4 .
The ECG is produced on specially designed graph paper. When viewed directly, it is an assortment of small squares that are 1 mm wide and 1 mm high. These small squares are further categorized on the paper into larger blocks composed of five small squares in height and width. At a standard paper printing speed of 25 mm/sec, the width of each large block is 0.2 seconds long and each small square is 0.04 seconds long. The vertical aspect of the graph paper reflects voltage. A standard ECG machine is calibrated so that each small square represents 0.1 mV. It is important to be aware of the standard settings on the ECG machine and not to deviate from them, as interpretation can be affected by these changes. Settings can be found printed on the side of a 12-lead ECG or at the bottom of a printed rhythm strip.
Obtaining a 12-Lead ECG
A key task for the emergency department technician (EDT) is to obtain ECGs. This is achieved by placement of external electrodes, referred to as leads, on the patient’s skin surface. Some leads are placed on an extremity (limb leads), and some placed adjacent to the chest wall itself (precordial leads). All leads measure current flow in the heart from different positions, resulting in variations among the different leads according to whether the current flow is moving toward the lead (positive deflection) or away (negative deflection). Different leads therefore allow the clinician to study changes in electrical activities for cells located in a specific area of the heart.
To apply the leads, stickers with conductive gel are applied to the skin, and wires transmit the detected electrical impulses to a monitor or ECG machine that can produce 12 simultaneous images measuring the current flows in the different leads. The six limb leads are generated by current flows measured on the extremities, and the six precordial leads are flows that compare the flows measured on the anterior chest wall to those measured on the limbs.
Fig. 8.5 shows the traditional location for appropriate placed limb leads. Usually the leads are applied to the wrists and ankles of the patient, but may also be applied to the anterior shoulder and hips/waist if necessary (this can reduce movement of the wires which can lead to “artifact” or “noise”). The lead wires will often have identification directing the technician to their appropriate placement: right arm, left arm, right leg, left leg.
