Reading one cardiac cycle
Every beat is the same four events in the same order, and every wave, segment and interval on this figure is a way of naming or measuring one of them. Follow the sequence once and the rest of the page is detail.
The cycle, in order
- - The atria depolarize. That is the whole of the P wave.
- - The impulse is held at the AV node, long enough for the atria to finish emptying before the ventricles fire. The PR interval is measured from the start of the P wave to the start of the QRS, so it covers both the P wave and the flat pause after it.
- - The ventricles depolarize, and quickly, because the His-Purkinje system delivers the impulse almost everywhere at once. That speed is exactly why a normal QRS is narrow.
- - The ventricles repolarize and reset, drawing the T wave. The flat stretch just before it is the ST segment.
- - Then it repeats. How long until the next beat is the RR interval, and the heart rate comes out of it.
What gets measured
Two different things get measured here, and the figure marks them differently. Durations run along the trace: the brackets underneath show what each interval covers, and the dashed calipers mark the two points it is measured between. Amplitudes run vertically, from the isoelectric baseline up to a peak or down to a trough. Every entry says which of the two it is and where to read it from.
Entries describe what each part represents and how to measure it. They deliberately carry no normal ranges, because those depend on species, breed and body size, and this is a prototype preview rather than a reference.
Click any labelled part of the tracing, or pick one from the list under the figure.
Isoelectric baseline
The flat reference line the trace returns to between deflections.
Key takeawayEvery amplitude and every ST shift is judged against this line, so finding it first is what makes the rest of the measurements mean anything. A drifting baseline imitates real findings convincingly.
What it representsThe baseline is drawn whenever no net electrical current is moving through the heart - either because the myocardium is uniformly at rest or because it is uniformly depolarized
The baseline is drawn whenever no net electrical current is moving through the heart - either because the myocardium is uniformly at rest or because it is uniformly depolarized. It is not a period in which nothing is happening; it is a period in which the electrodes see no net difference between them. Every amplitude you read and every segment shift you judge is measured against this line, which makes finding it correctly the first step of any measurement.
How to measure itTake the baseline from the flat stretch between the end of the T wave and the start of the next P wave - the TP segment
Take the baseline from the flat stretch between the end of the T wave and the start of the next P wave - the TP segment. That is the most reliably isoelectric part of the cycle, and it is the reference the rest of the complex is judged against.
What changes itA wandering baseline is the single most common reason a tracing is hard to read
A wandering baseline is the single most common reason a tracing is hard to read. Panting, purring, shivering, muscle tremor and a loose or dry electrode all make the line drift, and drift mimics ST-segment shift convincingly. Settle the patient and check electrode contact before you call a baseline finding real.
P wave
Atrial depolarization - the first deflection of the cycle.
Key takeawayWhat the P wave is doing tells you where the rhythm is coming from. Whether one precedes every QRS, and whether it always looks the same, answers most rhythm questions before you measure anything.
What it representsThe P wave is atrial depolarization
The P wave is atrial depolarization. The impulse leaves the sinoatrial node and spreads across the right atrium and then the left, and the wave you see is the sum of that spread. Its size reflects atrial muscle mass and its shape reflects the sequence of activation. Atrial repolarization also happens, but it is buried inside the much larger QRS and is not normally visible.
How to measure itDuration runs from the point where the trace first leaves the baseline to the point where it returns
Duration runs from the point where the trace first leaves the baseline to the point where it returns. Amplitude runs from the baseline to the peak. Choose the lead where the P wave is largest and its start and end are crispest, and read the same landmark on several beats rather than trusting one.
What changes itWhat the P wave is doing tells you where the rhythm is coming from
What the P wave is doing tells you where the rhythm is coming from. No P waves at all points toward atrial fibrillation or atrial standstill. A P that arrives early and looks different from its neighbors is an atrial premature complex. A P that is not followed by a QRS means the impulse was blocked at the AV node. P waves marching through the tracing with no fixed relationship to the QRS complexes mean the atria and ventricles are beating independently.
Common abnormalitiesAtrial fibrillation · Atrial standstill · Atrial premature complexes · Third-degree (complete) AV block
No P waves anywhere on the strip
The baseline between QRS complexes never lifts into a P wave. Either the atria are firing chaotically rather than as a unit, or they are not depolarizing at all - the RR intervals tell the two apart, since chaotic atrial activity makes them irregular.
📄 Read the guide🫀 See it move
Early, and shaped unlike its neighbors
One P wave arrives sooner than the rhythm predicts and has a different contour, because it started somewhere other than the sinus node. The QRS that follows it usually looks entirely normal, since the impulse still reaches the ventricles the usual way.
📄 Read the guide🫀 See it move
Marching on with no fixed relationship to the QRS complexes
The P waves keep their own steady rhythm and the QRS complexes keep a slower one, and the spacing between them changes constantly. Nothing is getting through the AV node, so the ventricles have fallen back on an escape rhythm of their own.
📄 Read the guide🫀 See it move
See it moveNormal sinus rhythm
PR segment
The flat stretch from the end of the P wave to the start of the QRS.
Key takeawayThis is where AV conduction happens invisibly. You never watch it on the trace, you infer it from how long the line stays flat, which is why the PR interval is the number that gets reported.
What it representsDuring the PR segment the impulse is travelling through the AV node, the bundle of His and the bundle branches
During the PR segment the impulse is travelling through the AV node, the bundle of His and the bundle branches. That tissue carries far too little muscle mass to produce a deflection the surface electrodes can see, so the trace sits flat. This is why AV nodal conduction is effectively invisible on an ECG: you never watch it happen, you only infer it from how long the trace stays flat before the ventricles fire.
How to measure itRuns from the end of the P wave to the onset of the QRS
Runs from the end of the P wave to the onset of the QRS. In practice it is described rather than measured - the clinically useful number is the PR interval, which includes the P wave itself.
What changes itKeep the PR segment and the PR interval separate in your head: the segment is the flat part alone, the interval starts at the beginning of the P wave
Keep the PR segment and the PR interval separate in your head: the segment is the flat part alone, the interval starts at the beginning of the P wave. Mixing them up is one of the most common measurement errors.
Learning guidesAtrioventricular block (AVB) types explained
In the ECG AtlasFirst-degree AV block
See it moveFirst-degree AV block
PR interval
Onset of atrial depolarization to onset of ventricular depolarization.
Key takeawayThe PR interval tells you how long the impulse takes to travel from atria to ventricles. A changing PR relationship is often more important than the absolute value.
What it representsThe PR interval is the whole journey from the atria to the ventricles: atrial depolarization, the deliberate delay at the AV node, and rapid conduction down the His–Purkinje system
The PR interval is the whole journey from the atria to the ventricles: atrial depolarization, the deliberate delay at the AV node, and rapid conduction down the His–Purkinje system. Most of it is AV nodal delay, which exists so the atria can finish emptying before the ventricles contract. Because the AV node is richly innervated, the PR interval is sensitive to autonomic tone - vagal tone lengthens it, sympathetic tone shortens it.
How to measure itMeasure from the first deflection of the P wave off the baseline to the first deflection of the QRS, whether that first deflection is a Q or an R
Measure from the first deflection of the P wave off the baseline to the first deflection of the QRS, whether that first deflection is a Q or an R. Use a lead where both the start of the P and the start of the QRS are sharply defined, and check several consecutive beats - whether the PR is constant matters as much as what it measures.
What changes itThe behavior of the PR interval across consecutive beats is what separates the AV blocks from one another
The behavior of the PR interval across consecutive beats is what separates the AV blocks from one another. Constant but long is first-degree block. Lengthening beat by beat until a QRS drops out is Mobitz I. Constant and then a sudden dropped beat is Mobitz II. A PR that varies with no consistent relationship between P waves and QRS complexes is not a PR interval at all - it is AV dissociation.
Common abnormalitiesFirst-degree AV block · Second-degree AV block (Mobitz I) · Second-degree AV block (Mobitz II) · Third-degree (complete) AV block
Constant from beat to beat, but prolonged
Every P wave is still followed by a QRS, so nothing is actually dropped - the impulse is simply taking longer than usual to cross the AV node. The rhythm itself is unchanged, which is why this is easy to miss without measuring.
📄 Read the guide🫀 See it move
Lengthening beat by beat until a QRS drops out
Each successive PR interval is a little longer than the last until one P wave finally fails to conduct, and then the cycle restarts. The progressive lengthening is the signature - look at the PR before and after the gap and they are visibly different.
📄 Read the guide🫀 See it move
Constant, then a QRS drops out without warning
The conducted beats all share the same PR interval, and then a P wave simply arrives with no QRS behind it. Nothing builds up to the dropped beat, which is what separates this from the lengthening pattern.
📄 Read the guide🫀 See it move
No consistent relationship between P waves and QRS complexes at all
There is no PR interval to measure here, only the appearance of one: the P waves and the QRS complexes are each regular but independent, so their spacing drifts. Measuring a PR from any single beat would be reading a coincidence.
📄 Read the guide🫀 See it move
Q wave
The first negative deflection of the QRS, when there is one.
Key takeawayA small q wave is normal in plenty of animals, so its presence alone means nothing. What raises a question is a Q that is unusually deep or wide, and even then lead placement and body position come first.
What it representsA Q wave is depolarization of the interventricular septum, which spreads from left to right - away from most left-sided leads, which is why it reads as a small downward deflection
A Q wave is depolarization of the interventricular septum, which spreads from left to right - away from most left-sided leads, which is why it reads as a small downward deflection. A Q wave is defined by its position, not its size: a deflection is a Q only if it is negative and only if it comes before any positive deflection.
How to measure itRead from where the trace first drops below the baseline at the onset of the QRS to where it crosses back up on its way to the R wave
Read from where the trace first drops below the baseline at the onset of the QRS to where it crosses back up on its way to the R wave.
What changes itPlenty of normal animals show a small septal q wave, and plenty show none at all, so its mere presence means nothing
Plenty of normal animals show a small septal q wave, and plenty show none at all, so its mere presence means nothing. Q waves that are unusually deep or wide are the ones that raise the question of myocardial loss - and even then, lead placement and body conformation have to be ruled out first.
Common abnormalitiesPathological Q wave
Deep and wide, rather than a small initial dip
The downward deflection before the R wave is far more pronounced than the small septal q many normal animals show. Lead placement and body conformation have to be excluded first, because both can exaggerate it.
R wave
The first positive deflection of the QRS - usually its tallest part.
Key takeawayR-wave height is often treated as a measure of ventricular mass, but the chest wall sits between the heart and the electrode. Treat amplitude as one piece of evidence rather than a verdict.
What it representsThe R wave is the bulk of ventricular free-wall depolarization, dominated by the left ventricle simply because it carries far more muscle than the right
The R wave is the bulk of ventricular free-wall depolarization, dominated by the left ventricle simply because it carries far more muscle than the right. Its height is often treated as a proxy for ventricular mass, but the electrode is separated from the heart by the chest wall, so amplitude also depends on body conformation, obesity, and anything that puts fluid or air between the heart and the electrodes.
How to measure itMeasure amplitude from the baseline up to the peak
Measure amplitude from the baseline up to the peak. If a second positive deflection appears after an S wave it is called R prime, written R′ - notched and R′ patterns are a clue that the ventricles are being activated in an abnormal sequence.
What changes itBecause so much besides muscle mass affects R-wave height, treat amplitude as one piece of evidence rather than a verdict
Because so much besides muscle mass affects R-wave height, treat amplitude as one piece of evidence rather than a verdict. Pericardial effusion, pleural effusion and obesity all damp it; thin-chested patients exaggerate it. Beat-to-beat variation in R-wave height on a single strip is its own finding.
Common abnormalitiesBundle branch block
A second positive deflection (R prime), or a notched, slurred complex
Instead of a single clean upstroke the complex carries a second peak or a visible notch, and it is broader than usual. The two ventricles are no longer depolarizing together, so their separate contributions show up as separate deflections.
📄 Read the guide
S wave
A negative deflection that follows an R wave.
Key takeawayThe S wave is the last part of the ventricles to be reached. Its depth feeds into the enlargement patterns, but conformation and lead placement move it around too much for it to stand alone.
What it representsThe S wave is the tail end of ventricular depolarization, usually the basal regions of the ventricles - the last muscle to be reached
The S wave is the tail end of ventricular depolarization, usually the basal regions of the ventricles - the last muscle to be reached. Like the Q wave it is named by position: a negative deflection is an S only if a positive deflection came before it.
How to measure itRead from the baseline down to the trough
Read from the baseline down to the trough. The S wave ends where the trace returns to the baseline, at the J point.
What changes itDeep S waves in particular leads form part of the classic ventricular enlargement patterns, but the same caveat as the R wave applies - conformation and lead placement move S-wave depth around enough that it is never diagnostic on its own
Deep S waves in particular leads form part of the classic ventricular enlargement patterns, but the same caveat as the R wave applies - conformation and lead placement move S-wave depth around enough that it is never diagnostic on its own.
QRS complex
The whole of ventricular depolarization, taken together.
Key takeawayWidth is the most useful thing about the QRS, because it tells you how the impulse arrived. Narrow means it came from above through the normal conduction system; wide means it did not.
What it representsThe QRS complex is the ventricles depolarizing
The QRS complex is the ventricles depolarizing. Its duration is the single most informative thing about it, because duration tells you how the impulse got there. A narrow QRS means the impulse came from above and used the His–Purkinje system, which distributes it almost simultaneously. A wide QRS means it spread muscle cell to muscle cell instead, which is far slower - that happens when the beat originates in the ventricles, when a bundle branch is blocked, or when a pacemaker is driving the beat.
How to measure itMeasure from the first deflection off the baseline at the onset of the complex - Q, R or S, whichever comes first - to the J point where it returns
Measure from the first deflection off the baseline at the onset of the complex - Q, R or S, whichever comes first - to the J point where it returns. Not every complex has all three waves, and the naming follows whichever deflections are actually present.
What changes itWidth tells you origin; a wide QRS is your cue to ask whether the beat is ventricular, conducted abnormally, or paced
Width tells you origin; a wide QRS is your cue to ask whether the beat is ventricular, conducted abnormally, or paced. Comparing the shape of an abnormal complex to the patient's own normal complexes on the same strip is usually more informative than comparing it to a textbook.
Common abnormalitiesVentricular premature complexes · Ventricular tachycardia · Accelerated idioventricular rhythm · Bundle branch block
Wide and early, with no P wave in front of it
A single complex arrives ahead of schedule, broader than the others and with no P wave preceding it, and its T wave typically points the opposite way. It started in ventricular muscle, so it had to spread the slow way rather than through the conduction system.
📄 Read the guide🫀 See it move
Wide, fast and sustained
A run of consecutive wide complexes at a fast rate, with no P waves in front of them. Every beat originates in the ventricles, so the whole run is broad and the atria play no part in it.
📄 Read the guide🫀 See it move
Wide and sustained, but only modestly faster than the underlying rhythm
The same broad, P-less complexes as a ventricular run, but the rate is far closer to the patient's own. Rate is the only thing separating the two on the tracing, which is why it has to be counted rather than eyeballed.
🫀 See it move
Wide, yet every complex still follows its own P wave
The rhythm and the P waves are entirely normal and each beat is conducted, but the complexes are broad. One bundle branch is not conducting, so one ventricle depolarizes late through muscle instead of through its branch.
📄 Read the guide
J point
Where the QRS ends and the ST segment begins.
Key takeawayThe J point is the anchor for every ST judgement, so it is worth pinning down before calling a segment elevated or depressed. It is also one of the hardest landmarks to find at fast rates.
What it representsThe J point marks the moment ventricular depolarization is complete and repolarization takes over
The J point marks the moment ventricular depolarization is complete and repolarization takes over. It has no width of its own - it is a junction, not a phase - but it matters because it is the landmark every ST-segment judgement is anchored to.
How to measure itFind where the steep slope of the QRS abruptly flattens into the ST segment
Find where the steep slope of the QRS abruptly flattens into the ST segment. That change of slope is the J point.
What changes itThis is often the hardest landmark on the tracing to pin down, especially at fast rates where the QRS runs straight into the T wave
This is often the hardest landmark on the tracing to pin down, especially at fast rates where the QRS runs straight into the T wave. Check the same point across several beats and, where you have them, across several leads before committing to it.
ST segment
From the J point to the start of the T wave.
Key takeawayA real ST shift says the myocardium is under stress, but this is also the finding most often faked by a drifting baseline. Confirm it across several beats before believing it.
What it representsDuring the ST segment the ventricular myocardium is uniformly depolarized
During the ST segment the ventricular myocardium is uniformly depolarized. With no gradient between regions there is little net current, so the trace sits close to the baseline. It is the electrical plateau between the ventricles depolarizing and the ventricles visibly repolarizing.
How to measure itJudge the level of the ST segment just after the J point against the TP baseline
Judge the level of the ST segment just after the J point against the TP baseline. You are reading a displacement from a reference line, not a duration, so establishing the baseline first is essential.
What changes itST elevation or depression is associated with myocardial ischemia and hypoxia, pericardial disease, and some drug and electrolyte effects
ST elevation or depression is associated with myocardial ischemia and hypoxia, pericardial disease, and some drug and electrolyte effects. It is also the finding most often faked by artifact - a drifting baseline will produce convincing ST shift on a handful of beats. Confirm it persists across beats and leads before you believe it.
Common abnormalitiesST segment deviation
Displaced above or below the baseline
The stretch just after the J point sits off the level of the TP baseline instead of on it. Confirm it across several beats and, where you have them, several leads first - a drifting baseline mimics this convincingly.
T wave
Ventricular repolarization - the last deflection of the cycle.
Key takeawayT-wave polarity is not fixed in dogs and cats, so a single odd-looking T means little. A T that has changed from that patient's own earlier tracing means a great deal.
What it representsThe T wave is the ventricles repolarizing and resetting for the next beat
The T wave is the ventricles repolarizing and resetting for the next beat. Unlike the QRS, its polarity is not fixed in dogs and cats: a normal patient may have a T wave that is positive, negative, or biphasic. That makes the shape of any single T wave weak evidence on its own - what carries information is a change in T-wave morphology on serial tracings from the same patient.
How to measure itRead from where the trace leaves the ST segment to where it returns to the baseline, and take amplitude from the baseline to the peak or trough
Read from where the trace leaves the ST segment to where it returns to the baseline, and take amplitude from the baseline to the peak or trough. The end of the T wave is notoriously vague; a tangent drawn along its steepest downslope, extended to the baseline, gives a repeatable endpoint.
What changes itTall or peaked T waves are classically associated with hyperkalemia, and T-wave changes also follow heart rate, myocardial disease, and a range of drugs
Tall or peaked T waves are classically associated with hyperkalemia, and T-wave changes also follow heart rate, myocardial disease, and a range of drugs. Because normal polarity varies so much between patients, a T wave that has changed since the last recording is far more meaningful than one that merely looks unusual.
Common abnormalitiesPeaked T wave
Tall, narrow and peaked
The T wave becomes noticeably taller and more sharply pointed than the patient's usual shape. Because normal T polarity varies so much between animals, this is far more convincing as a change from a previous tracing than as a one-off appearance.
QT interval
Onset of ventricular depolarization to the end of repolarization.
Key takeawayThe QT only means something alongside the heart rate, because it shortens as the rate rises. Record the rate with the measurement, or the number cannot be interpreted later.
What it representsThe QT interval covers the entire electrical life of one ventricular beat - depolarization and repolarization together
The QT interval covers the entire electrical life of one ventricular beat - depolarization and repolarization together. It is strongly rate-dependent: it shortens as the heart speeds up and lengthens as it slows. That dependence is not a nuisance to be corrected away casually; it means a QT measurement is meaningless unless you report the heart rate alongside it.
How to measure itMeasure from the onset of the QRS to the point where the T wave returns to the baseline
Measure from the onset of the QRS to the point where the T wave returns to the baseline. The start is easy and the end is not, so use the tangent method on the T-wave downslope for consistency, and always record the rate with the measurement.
What changes itCalcium and potassium disturbances, several antiarrhythmic drugs, and autonomic tone all move the QT interval
Calcium and potassium disturbances, several antiarrhythmic drugs, and autonomic tone all move the QT interval. Because rate moves it too, a QT that has changed between two recordings only means something once you have checked whether the rate changed with it.
Common abnormalitiesProlonged QT interval
Prolonged for the rate
Repolarization is taking longer, so the T wave finishes later relative to the QRS that started it. The interval can only be judged alongside the heart rate, since it naturally shortens as the rate rises.
See it moveSinus tachycardia · Sinus bradycardia
RR interval
R peak to R peak - the cycle length, and the basis of rate and rhythm.
Key takeawayThe RR interval gives you both the rate and the rhythm. Whether the intervals are regular, regularly irregular, or irregularly irregular is the question that organizes the whole interpretation.
What it representsThe RR interval is the time from one beat to the next
The RR interval is the time from one beat to the next. Two of the most important things on any tracing come out of it: the heart rate, which is derived from its length, and the rhythm assessment, which comes from how it varies. Whether the RR intervals are regular, regularly irregular, or irregularly irregular is the question that organizes the whole interpretation.
How to measure itMeasure peak to peak between consecutive R waves, since the R peak is the sharpest and most repeatable landmark on the complex
Measure peak to peak between consecutive R waves, since the R peak is the sharpest and most repeatable landmark on the complex. Dividing 60 by the RR interval in seconds gives an instantaneous rate for that pair of beats. When the rhythm is irregular, counting complexes over a fixed length of strip gives a mean rate, and the mean is the more honest number.
What changes itThe pattern of variation is the diagnosis in miniature
The pattern of variation is the diagnosis in miniature. RR intervals that lengthen and shorten in step with breathing are sinus arrhythmia, which is normal and vagally mediated in dogs. Beat-to-beat variation with no discernible pattern and no P waves points to atrial fibrillation. A single unexpectedly long RR is a pause, and what ends the pause - a sinus beat or an escape beat - is the next thing to look at.
Common abnormalitiesSinus arrhythmia · Atrial fibrillation · Sinus pause & arrest · Sinus bradycardia · Sinus tachycardia
Lengthening and shortening in step with breathing
The spacing between beats waxes and wanes in a smooth cycle while every complex stays normal and every P wave is present. In dogs this is a normal, vagally mediated finding rather than an arrhythmia to treat.
🫀 See it move
Irregularly irregular, with no P waves to explain it
No two cycles are the same length and there is no pattern to the variation, and the baseline shows no organized P waves. The ventricles are responding to chaotic atrial activity, so their timing is effectively random.
📄 Read the guide🫀 See it move
A single unexpectedly long gap
The rhythm runs regularly and then one cycle is conspicuously longer before it resumes. What ends the pause matters as much as the pause itself - a sinus beat and an escape beat mean different things.
🫀 See it move
Regular, but uniformly long
Every cycle is the same length and every one is long, with normal P waves and normal complexes throughout. The rhythm is intact; it is simply running slowly.
🫀 See it move
Regular, but uniformly short
Every cycle is the same length and every one is short. At faster rates the P wave can start to merge into the preceding T wave, which is the first thing to check before calling the rhythm anything other than sinus.
🫀 See it move
PP interval
P peak to P peak - the atrial cycle length.
Key takeawayMarching the P waves out is what separates the AV blocks. When PP stays regular while RR does not, the failure is below the atria rather than in them.
What it representsThe PP interval is the time between consecutive atrial depolarizations, so it is to the atria what the RR interval is to the ventricles
The PP interval is the time between consecutive atrial depolarizations, so it is to the atria what the RR interval is to the ventricles. When conduction is intact the two are identical, because every atrial impulse produces a ventricular one. They come apart the moment conduction fails, and that is exactly why PP is worth measuring in its own right rather than assuming it follows RR.
How to measure itMeasure between the same point on consecutive P waves - peak to peak, for the same reason RR uses the R peak: it is the most repeatable landmark
Measure between the same point on consecutive P waves - peak to peak, for the same reason RR uses the R peak: it is the most repeatable landmark. The technique that matters is marching them out. Set calipers to one PP interval and step along the strip, and you can see at a glance whether the atria are firing regularly even when the ventricles are not.
What changes itComparing PP against RR is how the AV blocks are read
Comparing PP against RR is how the AV blocks are read. In second-degree block the P waves march out regularly straight through a dropped beat, so PP stays constant while RR suddenly lengthens. In third-degree block PP and RR are each regular but bear no relationship to one another, which is what atrioventricular dissociation looks like. Where PP itself varies cyclically with breathing, that is sinus arrhythmia rather than a conduction problem.
Common abnormalitiesSecond-degree AV block · Third-degree (complete) AV block · Sinus arrhythmia
Regular and unchanged straight through a dropped QRS
March the P waves out and they keep perfect time across the gap, while the RR interval around it is obviously longer. The atria never faltered - the failure is below them, at the AV node.
📄 Read the guide🫀 See it move
Regular, but bearing no relationship to the RR intervals
PP is regular and RR is regular, but they run at different rates and drift against each other. Two independent pacemakers are running at once, one above the block and one below it.
📄 Read the guide🫀 See it move
Varying cyclically with respiration
The atrial rate itself speeds up and slows down with the breathing cycle, and the ventricles follow it beat for beat. The variation is in the sinus node, not in conduction.
🫀 See it move
