An electrocardiogram (ECG) records the electrical impulses generated by the heart muscle, providing a visual representation of its rhythm and rate. Cardiopulmonary resuscitation (CPR) is a mechanical intervention designed to manually pump blood when the heart has failed. When compressions are applied, the intense physical movement creates significant electrical interference, or “artifact,” on the ECG tracing. This interference obscures the actual rhythm, making it nearly impossible to determine the heart’s true state while CPR is actively underway.
The Rhythms of Cardiac Arrest
Before CPR begins, the ECG reveals one of four primary cardiac arrest rhythms, each dictating a different treatment path. Ventricular Fibrillation (VF) is characterized by chaotic, disorganized electrical activity, appearing as rapid, irregular waveforms with no discernible pattern. The uncoordinated electrical impulses prevent the ventricles from contracting effectively to pump blood.
Pulseless Ventricular Tachycardia (pVT) shows organized electrical complexes but beats so rapidly that the heart cannot effectively pump blood, resulting in no pulse. These two rhythms (VF and pVT) are considered “shockable” because an electrical shock can potentially reset the heart’s electrical system. In contrast, Asystole is the complete absence of any measurable electrical activity, represented by a flat line on the monitor. Pulseless Electrical Activity (PEA) presents as an organized electrical rhythm that lacks the mechanical contraction required to generate a pulse. Asystole and PEA are classified as “non-shockable” rhythms, requiring continuous chest compressions and medication instead of defibrillation.
The Appearance of CPR Artifact
The act of performing high-quality chest compressions creates a profound distortion on the ECG monitor known as compression artifact. This artifact occurs because the physical force of pushing on the chest wall, ribcage, and heart causes movement at the electrode sites, generating electrical noise. Visually, the artifact appears as repetitive, high-frequency, chaotic oscillations superimposed over the underlying cardiac rhythm.
These rapid, irregular fluctuations are often described as a jagged, “sawtooth” or “zig-zag” pattern that aligns exactly with the rate of compressions, typically between 100 to 120 compressions per minute. The mechanical movement generates a voltage that the electrodes interpret as electrical activity, even if the heart is completely silent. The sheer magnitude of the compression artifact can completely hide a subtle underlying rhythm, making accurate real-time diagnosis during active compressions nearly impossible.
This interference can sometimes be mistaken for a true, weak rhythm, such as fine Ventricular Fibrillation. The mechanical signal makes it impossible for automated external defibrillators (AEDs) to accurately analyze the underlying rhythm, requiring a mandatory pause in compressions. Although some advanced monitoring systems attempt to filter out this noise using specialized algorithms, the artifacts are often so strong that they still corrupt the true waveform morphology. These filtering techniques aim to subtract the predictable noise signature of the compression from the actual electrical signal to offer a clearer view.
Checking for Return of Spontaneous Circulation (ROSC)
Clinicians must periodically perform a brief “rhythm check” by temporarily halting chest compressions. This pause is typically limited to five to ten seconds to minimize interruptions in blood flow to the brain and heart. This short cessation clears the monitor of the compression artifact and reveals the heart’s actual electrical state.
If the patient achieves Return of Spontaneous Circulation (ROSC), the ECG displays a genuine, organized electrical rhythm. A successful ROSC rhythm shows clear QRS complexes or a stable pattern independent of the compression rate. This organized electrical activity indicates the heart’s natural pacemaker has regained electrical control, though the pulse must be confirmed clinically.
If the tracing remains flat (Asystole) or returns to a shockable rhythm (VF/pVT), compressions are immediately resumed. Even after ROSC is achieved, the heart’s electrical activity may still be unstable, often displaying abnormal rhythms like slow idioventricular or rapid junctional rhythms. Initial ECGs recorded immediately after ROSC may show nonspecific changes due to the heart’s recent trauma, requiring serial ECGs to assess for underlying causes like a heart attack.