CET NHA Certified EKG Technician - Set 3 - Part 2

Correct Answer: The sinoatrial node near the superior vena cava

The sinoatrial node, located in the upper right atrium near the superior vena cava, is the heart's primary pacemaker and normally fires at sixty to one hundred beats per minute. The SA node sets the pace for the conduction sequence. Option a is incorrect because the atrioventricular node serves as a backup pacemaker at forty to sixty beats per minute. Its role is to delay conduction and take over only when the SA node fails to fire normally. Option b is incorrect because the Purkinje fibers and ventricular myocardium have an intrinsic rate of only twenty to forty beats per minute and serve as the lowest-tier backup. Ventricular escape rhythms are too slow to be the primary pacemaker. Option d is incorrect because the bundle of His conducts the impulse from the AV node into the ventricles but does not normally generate the impulse itself. The bundle is a relay path rather than a pacemaker site.

Correct Answer: Forty to sixty beats per minute

The AV junction's intrinsic rate when it acts as a backup pacemaker is forty to sixty beats per minute. This intermediate rate sits between the SA node's higher rate and the ventricular escape rate from below. Option a is incorrect because twenty to forty beats per minute is the intrinsic rate of ventricular and Purkinje pacemakers, not the AV junction. Ventricular escape sits below the AV junction in the pacemaker hierarchy. Option c is incorrect because sixty to one hundred beats per minute is the intrinsic rate of the SA node, the primary pacemaker. AV junctional escape activity occurs at a slower rate than the SA node generates. Option d is incorrect because one hundred to one twenty beats per minute exceeds the intrinsic rates of all the major pacemaker sites in the conduction system. No normal escape pacemaker fires that fast.

Correct Answer: The atrioventricular node

The atrioventricular node delays the electrical impulse by about one tenth of a second between atrial and ventricular activation. This pause allows the ventricles to fill from atrial contraction before the ventricles themselves contract. Option a is incorrect because the bundle of His conducts the impulse rapidly from the AV node into the ventricular septum without adding the deliberate delay. Conduction through the bundle is fast rather than slow. Option b is incorrect because Purkinje fibers distribute the impulse rapidly throughout the ventricular myocardium. Their conduction is among the fastest in the heart and is the opposite of a delay-producing structure. Option c is incorrect because the right bundle branch carries the impulse rapidly through the right ventricle after the AV node has already added the delay. Bundle branches conduct quickly to enable coordinated ventricular contraction.

Correct Answer: The Purkinje fibers in the ventricles

The Purkinje fibers are the terminal conduction fibers that distribute the electrical impulse throughout the ventricular myocardium for coordinated contraction. Their rapid conduction allows both ventricles to depolarize nearly simultaneously and pump together. Option b is incorrect because Bachmann's bundle is an interatrial conduction pathway that carries the impulse from the right atrium to the left atrium. Bachmann's bundle is an atrial structure rather than a terminal ventricular conductor. Option c is incorrect because the chordae tendineae are mechanical structures that anchor the atrioventricular valve leaflets to the papillary muscles. They have no role in electrical conduction within the heart at all. Option d is incorrect because the coronary sinus is a venous drainage channel that returns blood from the heart muscle to the right atrium. It is a vascular structure rather than part of the electrical conduction system.

Correct Answer: From the right atrium to the left atrium

Bachmann's bundle is an interatrial conduction pathway that rapidly carries the impulse from the right atrium to the left atrium. This rapid transfer keeps the two atria contracting in close sequence rather than independently. Option a is incorrect because the SA-to-AV pathway uses the internodal tracts within the right atrium rather than Bachmann's bundle. Bachmann's bundle goes across to the left atrium, not down toward the AV node. Option b is incorrect because conduction from the AV node to the bundle of His occurs through the AV node itself and into the His bundle in the septum. Bachmann's bundle has no role in that segment of the pathway. Option d is incorrect because conduction from bundle branches to Purkinje fibers occurs as a continuous distribution within the ventricles. Bachmann's bundle is exclusively an atrial structure and is not involved in the ventricular conduction handoff.

Correct Answer: Sodium influx through fast sodium channels

The rapid phase zero upstroke of the ventricular action potential is driven by sodium influx through fast sodium channels. This rapid depolarization shifts the membrane potential from about negative ninety millivolts to a positive value. Option a is incorrect because calcium efflux does not drive phase zero of a ventricular action potential, although calcium influx contributes to the later plateau. Movement direction and timing of calcium do not match the phase described. Option b is incorrect because potassium flow during phase zero is small, and the larger potassium movements drive repolarization in later phases. Potassium tends to flow out, not in, during action potential events. Option c is incorrect because sodium efflux outward through pumps maintains the resting gradient but does not produce the rapid depolarizing upstroke. The pump's slow active transport is the opposite of phase zero's fast inward sodium rush.

Correct Answer: Calcium influx balanced by potassium efflux

The plateau phase of the ventricular action potential is sustained by calcium influx through L-type calcium channels balanced by potassium efflux. This stable depolarization period triggers calcium-induced calcium release that drives myocyte contraction. Option b is incorrect because sodium influx without efflux is the pattern of phase zero, not the plateau phase. The plateau phase is no longer driven by sodium currents after the fast channels close. Option c is incorrect because chloride influx does not predominate during the cardiac plateau phase, and chloride movements are minor in this context. The plateau is dominated by calcium movements paired with potassium currents. Option d is incorrect because sodium efflux through gated channels is not the mechanism that maintains the plateau. The plateau depends on inward calcium and outward potassium balancing each other, not on sodium movement out.

Correct Answer: The QRS and most of the ST segment

The absolute refractory period corresponds roughly to the QRS complex and most of the ST segment on the EKG. During this window, ventricular cells cannot fire a new action potential regardless of stimulus strength. Option a is incorrect because the TP segment occurs between cardiac cycles, when the cells are at rest and can readily respond to a new stimulus. The TP segment is the opposite of a refractory period. Option c is incorrect because the end of the P wave reflects atrial repolarization timing rather than the ventricular absolute refractory window. P-wave events sit before the relevant ventricular phase the question describes. Option d is incorrect because the U wave follows the T wave and occurs well after the absolute refractory period has ended. By the U wave, ventricular cells are largely back to resting potential.

Correct Answer: The T wave, especially its peak

The relative refractory period corresponds roughly to the T wave on the EKG, with the peak being particularly vulnerable. A premature ventricular complex landing on the T wave is the classic R-on-T phenomenon that can trigger ventricular tachycardia. Option a is incorrect because the PR segment falls between atrial and ventricular activation and is not the ventricular relative refractory window. PR-segment timing does not match the T-wave vulnerable period under discussion. Option b is incorrect because the TP segment lies between beats when the heart is at rest, well past any refractory period. R-on-T cannot occur during the resting interval because there is no vulnerable repolarization gradient there. Option d is incorrect because the QRS complex corresponds to the absolute refractory period rather than the relative one. A stimulus during the QRS cannot trigger a new action potential at all, which is the opposite of relative refractoriness.

Correct Answer: Automaticity of pacemaker cells

Automaticity describes the ability of pacemaker cells to spontaneously generate an action potential without external stimulus. SA node, AV junction, and Purkinje cells all possess this property to varying degrees. Option a is incorrect because excitability describes the cell's ability to respond to a stimulus, which is different from spontaneously generating one. An excitable cell needs a trigger to fire, while automatic cells start the action potential without help. Option b is incorrect because conductivity describes the ability to transmit an electrical impulse to adjacent cells rather than to generate one. Conduction moves an existing impulse rather than creating a new one. Option c is incorrect because contractility refers to the mechanical shortening and force generation by the myocyte after depolarization. Contractility describes a mechanical outcome rather than the electrical initiation that automaticity addresses.

Correct Answer: Ten electrodes generating twelve leads

The standard twelve-lead EKG uses ten physical electrodes, four limb and six precordial, to generate twelve different leads or views of the heart. The system combines electrodes mathematically to produce more leads than the number of physical contacts. Option b is incorrect because twelve electrodes would imply one electrode per lead, but the system uses fewer electrodes combined into more views. Twelve physical electrodes is more than the standard requires. Option c is incorrect because six chest electrodes alone would not produce the limb leads that complete the twelve-lead set. Limb electrodes are essential for the standard, augmented, and frontal-plane views the system provides. Option d is incorrect because four limb electrodes alone produce only the limb leads and not the six precordial leads. Without chest electrodes, the system cannot deliver the horizontal-plane views the twelve-lead system requires.

Correct Answer: Lead one plus lead three equals lead two

Einthoven's law states that lead one plus lead three equals lead two, reflecting the geometric relationship of the three bipolar limb leads around the heart. Adding voltages from leads one and three predicts the lead two reading. Option a is incorrect because lead one minus lead two does not equal lead three under Einthoven's law. The correct relationship is additive between leads one and three to equal lead two, not subtractive between leads one and two. Option b is incorrect because lead two minus lead three does not equal lead one in the geometric relationship of the triangle. Rearranging the law as a subtraction in this way does not match the additive form Einthoven described. Option d is incorrect because lead three plus lead two would overcount the contributions when measured against lead one. The standard relationship places lead two as the sum of leads one and three, not as a component in another sum.

Correct Answer: On the left leg viewing from below

The augmented voltage foot lead aVF has its positive electrode on the left leg and views the heart from below, with the letter F standing for foot. This inferior view groups aVF with leads two and three. Option a is incorrect because the augmented lead with a positive electrode on the right arm is aVR rather than aVF. The names use R for right arm, L for left arm, and F for foot to keep placements distinct. Option b is incorrect because the augmented lead with a positive electrode on the left arm is aVL rather than aVF. The L-versus-F suffix carries clinical meaning about which limb provides the positive view. Option c is incorrect because the precordial leads sit on the chest at specific intercostal positions, not as augmented limb leads. Chest electrodes generate the V leads, not aVF or any other augmented limb lead.

Correct Answer: It usually shows the cleanest P waves

Lead two typically shows the cleanest, most upright P waves because its axis aligns well with the direction of atrial depolarization. This makes lead two the preferred lead for rhythm monitoring in most patients. Option a is incorrect because lead two often shows fairly large amplitudes rather than the smallest possible. Small amplitudes are not what makes a lead useful for rhythm monitoring; clear waveform shape matters most. Option c is incorrect because lead two cannot replace the precordial leads, which provide horizontal-plane views the limb leads do not offer. The precordial set is necessary for evaluating ventricular regions across the chest. Option d is incorrect because lead two records electrical activity influenced by the entire heart, not just the right atrium. The lead's vector covers the heart as a whole, with the cleanest P waves being a useful side effect.

Correct Answer: Black for the left arm electrode

Under the AHA system, the left arm electrode is black, paired with the left leg's red electrode using the mnemonic smoke over fire on the left side. The right side pairs white over green as snow over grass. Option b is incorrect because red is the AHA color for the left leg electrode, not the left arm. The mnemonic smoke over fire places black above red on the left side, which is the opposite of this option. Option c is incorrect because green is the AHA color for the right leg electrode, which is essentially a ground reference. The right leg electrode is on the opposite side from the left arm position this option references. Option d is incorrect because brown is generally a chest electrode color in the AHA system rather than a limb electrode color. Brown does not appear on the limb electrodes in the standard color scheme.

Correct Answer: Fourth intercostal space, right of sternum

V one sits at the fourth intercostal space just to the right of the sternal edge. The intercostal space is found by palpating down from the angle of Louis where the second rib attaches. Option a is incorrect because the fourth intercostal space just to the left of the sternum is the position for V two, not V one. V one and V two sit on opposite sides of the sternum at the same level. Option b is incorrect because the fifth intercostal space at the midclavicular line is the position for V four. Moving the electrode down to that position would generate the wrong precordial view at the wrong location. Option d is incorrect because the fifth intercostal space at the midaxillary line is the position for V six, on the lateral chest. Placing V one there would produce a lateral view rather than a septal view of the heart.

Correct Answer: Fifth intercostal space, midclavicular line

V four is placed at the fifth intercostal space on the left midclavicular line. The horizontal level set by V four becomes the reference plane for V five and V six in the anterior axillary and midaxillary lines. Option a is incorrect because the fourth intercostal space to the right of the sternum is the V one location. That position is far from V four's level and on the wrong side of the chest for V four. Option b is incorrect because the fourth intercostal space to the left of the sternum is V two, not V four. V two is one intercostal space higher and considerably more medial than V four's location. Option c is incorrect because the fifth intercostal space at the midaxillary line is the V six position rather than V four. Lateral chest placement at the midaxillary line is reserved for the far lateral precordial lead.

Correct Answer: V one and V two together

V one and V two sit closest to the interventricular septum and provide the most direct septal view among the precordial leads. The septal branches of the left anterior descending artery supply this territory. Option b is incorrect because V five and V six sit on the lateral chest and view the lateral wall of the left ventricle rather than the septum. The far lateral position cannot provide a direct septal view. Option c is incorrect because leads one and aVL view the high lateral wall of the left ventricle rather than the septum. The high lateral grouping is anatomically separate from the septal region between the ventricles. Option d is incorrect because leads two and three view the inferior wall from below rather than the septum from in front. Inferior leads cannot provide the septal view that the anteroseptal precordials offer.

Correct Answer: Red over brown for V one

Under the AHA system, the V one electrode is colored red over brown to distinguish it from the other chest electrodes that share the brown band but pair with different inner colors. Each chest electrode has its own combo. Option a is incorrect because white is the color for the right arm limb electrode under the AHA scheme rather than a chest electrode. White over brown does not appear on any standard chest electrode. Option b is incorrect because yellow over brown is the AHA color for V two, not V one. The yellow-brown pairing identifies the second chest electrode at the fourth intercostal space on the left side. Option d is incorrect because purple over brown is the AHA color for V six, the far lateral precordial electrode. Purple-brown marks the most lateral chest position rather than the right-sided V one.

Correct Answer: V four R at the right midclavicular line

V four R sits at the fifth intercostal space on the right midclavicular line, mirroring V four on the left side. ST elevation in V four R is the most useful single lead for diagnosing right ventricular infarction. Option a is incorrect because V three R sits between V one R and V four R rather than at the right midclavicular line itself. V three R is a different position in the right-sided lead series. Option c is incorrect because V seven sits on the posterior axillary line at the back, used to evaluate the posterior wall rather than the right ventricle. Posterior leads address a separate territory from the right-sided leads. Option d is incorrect because V eight sits at the midscapular line on the back, also part of the posterior lead set. Like V seven, V eight evaluates the posterior wall and not the right ventricle.