An Electrocardiogram (ECG) is a graphic representation of the heart’s electrical activity, recording impulses that appear as a distinct waveform on a monitor. This allows medical professionals to diagnose rhythm problems. Cardiopulmonary Resuscitation (CPR) is the mechanical action of chest compressions and artificial ventilation used to manually circulate blood and oxygen when the heart has stopped. When CPR occurs simultaneously with ECG monitoring, the mechanical force directly interferes with the electrical signals. This interaction creates a complex and often confusing visual pattern, making it difficult to discern the heart’s true electrical state.
The Rhythms Requiring CPR
CPR is initiated following a diagnosis of cardiac arrest, defined by the absence of a pulse. Cardiac arrest is associated with four primary electrical states visible on the ECG, categorized as either shockable or non-shockable.
Ventricular Fibrillation (VF) and Pulseless Ventricular Tachycardia (pVT) are the two shockable rhythms. VF appears as chaotic, disorganized electrical activity, while pVT is a rapid, organized rhythm that fails to generate a pulse.
The non-shockable rhythms include Pulseless Electrical Activity (PEA) and Asystole. PEA shows an organized electrical rhythm on the ECG but lacks a mechanical contraction strong enough to produce a pulse. Asystole, often called a “flat line,” represents the complete absence of measurable electrical activity. Electrical monitoring diagnoses the specific rhythm, guiding rescuers to either deliver a shock or focus solely on compressions and medication.
The Visual Chaos: ECG Artifact During Compressions
When chest compressions are performed, the ECG trace transforms into compression artifact. This artifact results from the mechanical movement of the chest wall beneath the monitoring electrodes. The pushing physically moves the electrodes and the heart, creating electrical noise that obscures the true cardiac rhythm.
The artifact typically presents as a rapid, periodic waveform that oscillates with the frequency of the compressions. Since CPR requires a rate between 100 and 120 compressions per minute, the artifact generates a corresponding rhythmic noise on the monitor. This noise can be mistakenly interpreted as an organized electrical pattern by automated external defibrillators (AEDs) and rescuers. The artifact can also cause the ECG baseline to “wander” due to patient movement and changes in chest impedance.
This visual chaos is problematic because it can hide or mimic life-threatening rhythms. For example, the artifact can sometimes look like fine Ventricular Fibrillation (VF), a low-amplitude, disorganized rhythm. Conversely, if the underlying rhythm is truly fine VF, the artifact can mask it, preventing a life-saving electrical shock. The noise also includes high-frequency spikes and erratic patterns, compounding the difficulty of interpreting the trace during active resuscitation.
Interpreting the Underlying Rhythm During Pulse Checks
Since compression artifact makes continuous, accurate rhythm analysis impossible, resuscitation protocols mandate a brief pause in compressions every two minutes. This interruption, known as the rhythm check, is the only time the heart’s true electrical activity can be observed without interference. The goal of this quick pause is to minimize “hands-off” time while allowing for a clear diagnostic look at the ECG.
During this momentary break, rescuers look for organized electrical activity, specifically consistent QRS complexes representing ventricular contraction. If the monitor shows VF or pVT, a shock is immediately delivered before compressions resume. If the rhythm is Asystole or PEA, CPR continues, and medications are administered. If an organized rhythm is present and a pulse is confirmed, the patient has achieved Return of Spontaneous Circulation (ROSC), and CPR is discontinued.