Leads that are bipolar measure the electrical difference between two recording sites. The limb leads I, II, and III do just that by comparing different pairs of limbs: I compares the right arm and left arm, II compares the right arm and left leg, and III compares the left arm and left leg. Because each lead records the potential difference between two points, they are considered bipolar. The other leads are unipolar. Precordial chest leads V1 through V6 have a single positive electrode on the chest with a reference point derived from the Wilson central terminal, so they measure the heart’s activity relative to a central reference rather than between two limbs. The augmented limb leads aVR, aVL, and aVF also use a single limb electrode as the positive input and an augmented central reference, making them unipolar as well, though with signal amplification. So the correct set of bipolar leads is the limb leads I, II, and III.

The concept is how we name recurring PVC patterns based on which beat in the sequence is the ectopic beat. If a PVC occurs on every fourth beat, the cycle repeats every four heartbeats with the fourth being a PVC. This pattern is called quadrigeminy, since quadri means four. Other patterns include trigeminy (every third beat), bigeminy (every other beat), and pentigeminy (every fifth beat). So the term for every fourth beat being a PVC is quadrigeminy.

Pulmonary veins deliver oxygen-rich blood from the lungs to the heart, and the chamber that receives this blood is the left atrium. From the left atrium, blood passes through the mitral valve into the left ventricle to be pumped to the body. (Note the exception that pulmonary veins carry oxygenated blood, unlike most veins.)

Phase 0 is the rapid upstroke of the cardiac action potential. In fast-response cardiac cells like atrial and ventricular myocytes, the resting membrane potential sits around -90 mV. When these cells are excited, fast voltage-gated sodium channels open and a large inward Na+ current drives the membrane potential quickly from about -90 mV toward +20 mV in a very short time. This swift depolarization is what defines Phase 0. After this sharp rise, the action potential proceeds to Phase 1 (brief repolarization), Phase 2 (plateau due to calcium influx balanced by potassium efflux), Phase 3 (repolarization as potassium exits), and Phase 4 (resting potential ready for the next impulse). In nodal tissue, the upstroke can be calcium-driven, but for the typical fast fibers in exams, the initial rapid depolarization is due to sodium entry during Phase 0.

When the frontal-plane axis doesn’t match what you’d expect from the patient’s presentation, the first thing to check is the limb-lead setup. The limb leads largely determine the heart’s axis in the frontal plane, so a misplacement or mix-up (such as swapping arms, swapping legs, or a loose connection) can produce an axis that looks off even though the heart’s electrical activity is normal. Re-attaching the limb leads correctly and then recalculating the frontal-plane axis helps confirm whether the apparent deviation is technical or real. Chest leads are essential for the overall view of the ECG, especially the horizontal plane, but they don’t verify limb-lead orientation. If you rely only on chest leads, you might miss a misplacement that is distorting the axis. Re-checking limb leads and recalculating the axis after correcting any issues provides a reliable basis for interpretation. If, after ensuring proper limb-lead placement, the axis still conflicts with the clinical picture, further evaluation would be warranted, but the immediate, best step is to verify limb-lead orientation.