The Electrocardiogram

Because the respiratory care practitioner frequently works with critically ill patients who are on cardiac monitors, a basic understanding of normal and common abnormal ECG patterns is important. An ECG monitors, both visually and on recording paper, the electrical activity of the heart. Figure 6-1 illustrates the ECG pattern of a normal cardiac cycle. The P wave reflects depolarization of the atria. The QRS complex represents the depolarization of the ventricles, and the T wave represents ventricular repolarization.

In normal adults the heart rate is between 60 and 100 beats per minute (bpm). In normal infants the heart rate is 130 to 150 bpm. A number of methods can be used to calculate the heart rate. For example, when the rhythm is regular, the heart rate can be determined at a glance by counting the number of large boxes (on the ECG strip) between two QRS complexes and then dividing this number into 300. Therefore if an ECG strip consistently shows four large boxes between each pair of QRS complexes, the heart rate is 75 bpm (300 ÷ 4 = 75). When the rhythm is irregular, the heart rate can be determined by counting the QRS complexes on a 6-second strip and multiplying by 10. The following heart arrhythmias are commonly seen and should be recognized by the respiratory care practitioner.

Common Heart Arrhythmias^†

Sinus Bradycardia

In sinus bradycardia the heart rate is less than 60 bpm. Bradycardia means “slow heart.” Sinus bradycardia has a normal P-QRS-T pattern, and the rhythm is regular (Figure 6-2). Athletes often normally demonstrate this finding because of increased cardiac stroke volume and other poorly understood mechanisms. Common pathologic causes of sinus bradycardia include a weakened or damaged sinoatrial (SA) node, severe or chronic hypoxemia, increased intracranial pressure, obstructive sleep apnea, and certain drugs (most notably the beta-blockers). Sinus bradycardia may lead to decreased cardiac output and blood pressure. In severe cases, sinus bradycardia may lead to a decreased vascular perfusion state and tissue hypoxia. The patient may demonstrate a weak or absent pulse, poor capillary refill, cold and clammy skin, and a depressed sensorium.

Sinus Tachycardia

In sinus tachycardia the heart rate is greater than 100 bpm. Tachycardia means “fast heart.” Sinus tachycardia has a normal P-QRS-T pattern, and the rhythm is regular (Figure 6-3). Sinus tachycardia is the normal physiologic response to stress and exercise. Common causes of sinus tachycardia include hypoxemia, severe anemia, hyperthermia, massive hemorrhage, pain, fear, anxiety, hyperthyroidism, and sympathomimetic or parasympatholytic drug administration.

Sinus Arrhythmia

In sinus arrhythmia the heart rate varies by more than 10% from beat to beat. The P-QRS-T pattern is normal (Figure 6-4), but the interval between groups of complexes (i.e., the R-R interval) varies. Sinus arrhythmia is a normal rhythm in children and young adults. The patient’s pulse will often increase during inspiration and decrease during expiration. No treatment is required unless significant alteration occurs in the patient’s arterial blood pressure.

Atrial Flutter

In atrial flutter the normal P wave is absent and replaced by two or more regular sawtooth waves. The QRS complex is normal and the ventricular rate may be regular or irregular, depending on the relationship of the atrial to the ventricular beats. Figure 6-5 shows an atrial flutter with a regular rhythm with a 4 : 1 conduction ratio (i.e., four atrial beats for every ventricular beat). The atrial rate is usually constant, between 250 and 350 bpm, whereas the ventricular rate is in the normal range. Causes of atrial flutter include hypoxemia, a damaged SA node, and congestive heart failure.

Atrial Fibrillation

In atrial fibrillation the atrial contractions are disorganized and ineffective, and the normal P wave is absent (Figure 6-6). The atrial rate ranges from 350 to 700 bpm. The QRS complex is normal, and the ventricular rate ranges from 100 to 200 bpm. Causes of atrial fibrillation include hypoxemia and a damaged SA node. Atrial fibrillation may reduce the cardiac output by 20% because of a loss of atrial filling (the so-called “atrial kick”).

Premature Ventricular Contractions

The premature ventricular contraction (PVC) is not preceded by a P wave. The QRS complex is wide, bizarre, and unlike the normal QRS complex (Figure 6-7). The regular heart rate is altered by the PVC. The heart rhythm may be quite irregular when there are many PVCs. PVCs can occur at any rate. They often occur in pairs, after every normal heartbeat (bigeminal PVCs), and after every two normal heartbeats (trigeminal PVCs). Common causes of PVCs include intrinsic myocardial disease, hypoxemia, acidemia, hypokalemia, and congestive heart failure. PVCs also may be a sign of theophylline or alpha- or beta-agonist toxicity.

Ventricular Tachycardia

In ventricular tachycardia the P wave is generally indiscernible, and the QRS complex is wide and bizarre in appearance (Figure 6-8). The T wave may not be separated from the QRS complex. The ventricular rate ranges from 150 to 250 bpm, and the rate is regular or slightly irregular. The patient’s blood pressure is often decreased during ventricular tachycardia.

Buy Membership for Pulmolory and Respiratory Category to continue reading. Learn more here