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Instructors' Collection ECG of the WEEK: Electrolyte Effects

Tue, 07/12/2016 - 16:00 -- Dawn

This ECG is from a 46-year-old woman with diabetes mellitus.  She presented to the Emergency Dept. with a complaint of weakness.  Her BP was elevated at 186/102.  Her blood glucose was 936 mg/dL (normal 70-105). 

So, what does her ECG show?

1) Sinus bradycardia at 55 bpm.  The rhythm is regular, with no extrasystoles.

2) A slightly prolonged PR interval at 218 ms (.218 seconds). Normal is 120-200 ms. 

3) A “wide side of normal” QRS duration at 109 ms. Normal is 70-100, but can be a little longer in normal individuals.

4) A prolonged QTc interval at 520 ms.  Normal QTc is 460 ms or less in women. A helpful rule of thumb is that the QT should be less than half the preceding RR. 

5) Normal P waves.

6) Normal axes of P, QRS, and T waves.

7) A large Q wave in Lead III which is not repeated in aVL.  This can be inconsequential when confined to Lead III, or can be a pathological Q wave, especially if also seen in aVF. 

8) Flat ST segments.  Normal ST segments are convex upward, like a smile.

9) Tall, peaked T waves in precordial leads V2 through V6, and in Lead II. 

What does it mean?

Unfortunately, we do not have complete labs for this patient, or any information about her outcome.

We know that patients who have uncontrolled diabetes are at risk for renal failure, so we should consider electrolyte imbalances when we see abnormalities in the ECG. 

The prolonged, flat ST segment is suggestive of HYPOCALCEMIA.  The low-amplitude P waves and tall, peaked T waves suggest HYPERKALEMIA You will find a discussion of simultaneous hypocalcemia and hyperkalemia on this website HERE.  

We are looking forward to your comments.

Ask the Expert

Wed, 06/15/2016 - 00:57 -- Dawn
Dr. Jerry Jones

Question:

Dr. Jones, can you help me understand refractory periods better? I find that a difficult topic to teach, and there are so many different terms used to describe refractory periods.

Today’s expert is Dr. Jerry W. Jones, MD, FACEP, FAAEM

Jerry W. Jones, MD FACEP FAAEM is a diplomate of the American Board of Emergency Medicine who has practiced internal medicine and emergency medicine for 35 years. Dr. Jones has been on the teaching faculties of the University of Oklahoma and The University of Texas Medical Branch in Galveston. He is a published author who has also been featured in the New York Times and the Annals of Emergency Medicine for his work in the developing field of telemedicine. He is also a Fellow of the American College of Emergency Physicians and a Fellow of the American Academy of Emergency Medicine and, in addition, a member of the European Society of Emergency Medicine. 

Dr. Jones is the CEO of Medicus of Houston and the principal instructor for the Advanced ECG Interpretation Boot Camp and the Advanced Dysrhythmia Boot Camp. 

 

Answer:

 

                          Refractory Periods: Absolute/Relative and Effective/Functional

        

 

 

 If you do any reading of the vast amount of literature regarding ECG interpretation, you have certainly encountered the terms effective refractory period and functional refractory period. In introductory courses, we learn about the absolute refractory period and the relative refractory period, but no one ever teaches the effective and functional refractory periods. Most definitions are confusing and incomplete, so I have written a short monograph on this topic.

 

The effective refractory period  is basically the same as the absolute refractory period – but there is a slight difference!

The absolute refractory period is a physiologic state – it begins with the onset of the action potential at Phase 0 and represents the period involving all of depolarization and that part of repolarization during which no amount of stimulus can result in another action potential.

The effective refractory period tries to define the absolute refractory period in more realistic, practical terms. The effective refractory period also represents the period during which a typical impulse cannot produce another action potential. So how does this differ from the absolute refractory period? To understand this, we must now jump to a better understanding of the relative refractory period.

The relative refractory period begins at the point that a maximal stimulus is able to initiate another action potential. The key phrase here is maximal stimulus. If a maximal stimulus occurs one-millionth of one millisecond after the end of the absolute refractory period, another action potential will be generated. But for that action potential to occur, that stimulus will have to be at its maximum amplitude because the threshold potential will be much closer to zero potential than it usually is, thus requiring much greater amplitude to initiate the action potential.

But most Phase 0 depolarizations do not result in maximal voltage. Many times, the arriving impulse does not have a full complement of sodium channels to open, so even though the “all-or-nothing” threshold is reached and an action potential is generated, that action potential has less-than-maximal amplitude. When that is the case, the impulse will have to occur further and further into the relative refractory period before threshold is reached and an action potential is generated. So while there is most definitely an absolute refractory period with a definite end to it, as far as an individual patient is concerned, the “absolute refractory period” may not end until some point well into the relative refractory period. So, for that patient, his/her effective refractory period may be a bit longer than the actual absolute refractory period.

The absolute and relative refractory periods are real phenomena. They are also observable phenomena: we can see that an atrial impulse arrived during the absolute refractory period of the AV node or His bundle because it failed to conduct in spite of more than adequate voltage. We can see that an atrial impulse arrived during the relative refractory period of the AV node or His bundle because it conducted with a prolonged PR interval. But observing these phenomena doesn’t really tell us exactly where the absolute refractory period ends and the relative refractory period begins.

The effective refractory period begins with a programmed stimulus (S1) and ends with a programmed stimulus (S2). S1 marks the beginning of Phase 0 of the action potential and S2 marks the longest interval from S1 that fails to result in a depolarization. Note that I did not restrict the longest interval to “within the absolute refractory period.”

In the diagram above, there are 5 equal, vertical lines representing paced impulses (S2) following Phase 0 (S1) of the action potential. Only a line crossing through the curved line representing the relative refractory period has reached threshold and will result in an action potential. Here, only the 5th line breaches the relative refractory curve, so it conducted.

The length of the effective refractory period depends on the strength of the stimulus being used and the length of the coupling interval (number of msec between S1 and S2). If the stimulus is not very strong, the effective refractory period will be measured well into the actual relative refractory period before a depolarization appears (as in the diagram above). If a stronger stimulus is used, a depolarization will be produced earlier and the effective refractory period will be shorter and more representative of the absolute refractory period. Also, if the coupling interval is rather long, the last non-conducted S2 may occur well before the end of the absolute refractory period. So it’s possible that the effective refractory period may actually be measured as being longer or shorter than the absolute refractory period. Although the effective refractory period and the absolute refractory period are not always exactly equal, for practical purposes they can be considered almost the same (since the electrophysiologist’s attempts to measure the effective refractory period are obviously much more precise than depicted in the diagram above). The terms are frequently used interchangeably in the literature but now you understand the subtle difference.

Many people make the mistake of thinking that if the effective refractory period is “basically” the same as the absolute refractory period, then the functional refractory period is the same as the relative refractory period. Nothing could be further from the truth! The functional refractory period and the relative refractory period are not at all the same, though they both relate to the point during the action potential in which an extra-strong stimulus can result in a depolarization. The functional refractory period is the electrophysiologist’s attempt to measure the distance from the onset of the action potential to the onset of the relative refractory period – not the duration of the relative refractory period! It actually represents the shortest interval between two consecutively conducted, paced impulses (S1 and S2). The relative refractory period begins at the point during repolarization that an exceptionally strong stimulus can initiate a depolarization and it ends (usually, but not always) with the onset of Phase 4. This is not what the functional refractory period measures!

The electrophysiologist, however, finds it more practical to measure from a programmed stimulus (S1) that initiates an action potential to the earliest point at which a sufficiently strong stimulus (S2) is able to initiate another depolarization. Thus, the functional refractory period is a measurement between two programmed stimuli – once again, S1 and S2 – and covers all the same territory as the effective refractory period. But again, this determination is voltage- and time-dependent with the strength of the stimulus and the coupling interval (the interval between S1 and S2) affecting where the first depolarization occurs.

But note that while the effective refractory period and the absolute refractory period are virtually the same, the functional refractory period and the relative refractory period are measurements of different sections of the action potential. What we think of as the relative refractory period begins, basically, where the functional refractory period ends. 

Note:

1.       The effective refractory period is – by definition –shorter than the functional refractory period if the same stimulus strength and the same coupling intervals of S1 and S2 are used in both measurements.

2.       The effective refractory period is (presumably) completely overlapped by the absolute refractory period while the functional refractory period and relative refractory periods overlap very little!

3.       Both the effective refractory period and the functional refractory period begin and end with a programmed stimulus. The absolute refractory period and the relative refractory period are surmised based on the duration of the action potential (QT interval) and the response of the heart to the following sinus or ectopic impulse.

4.       The effective refractory period essentially determines the end of the absolute refractory period while the functional refractory period determines the beginning of the relative refractory period. 

 

The absolute refractory period ends around the time the membrane potential has returned to about -60°. Likewise, that is approximately where the relative refractory period begins.

 

ECG Basics: Sinus Tachycardia vs. PSVT

Thu, 04/21/2016 - 00:13 -- Dawn

Narrow-complex tachycardias can be very confusing to students of basic-level ECG.  There are very many rhythms that fall into the broad category of narrow-complex tachycardia.  We usually further divide them into sinus tachycardia and other "supraventricular tachycardias".  The basic student will want to make this distinction, as well as be able to differentiate atrial fib and atrial flutter from the other SVTs.  The more advanced student will want to go into more detail about which mechanism for supraventricular tachycardia is present.

Just the basics, please.   When the tachycardia is regular, it is most important to determine whether it is a SINUS TACHYCARDIA or a SUPRAVENTRICULAR TACHYCARDIA.  (Yes, we are aware that sinus rhythms are supraventricular, but the term "supraventricular tachycardia" or "SVT" is usually reserved for the fast, regular rhythms that are not sinus.)  So, what clues will be most helpful to our beginner students?

Rate    SVTs tend to be faster than sinus tachycardia.  More importantly, they are fast regardless of the patient's situation.  Sinus tachycardia almost always is reacting to the patient's situation.  For instance, a 22-year-old woman resting in a chair with a heart rate of 150 is likely to have an SVT.  A 22-year-old woman who is running in a 10 k marathon race and has a heart rate of 160 is responding appropriately to an increased need for oxygen and nutrients to her cells. Sinus tachycardia will ususally be 160 or less, and have an obvious reason for being, such as fever, pain, anxiety, exercise, hypovolemia, hypoxia, or drugs.  Unfortunately, many beginning students are told that any narrow-complex tachycardia with a rate of 150 or less is sinus, and over 150 is SVT. While they may be right most of the time, or on the written test they are about to take, this rule should not be applied in "real life".  Sinus rhythms can go over 150, and SVTs can be slower than 150.  So, what other clues should we be teaching beginners?

Consider the clinical situation    Look for an obvious cause for sinus tachycardia.  If none is found, strongly consider SVT.  Remember that pediatric patients have faster heart rates, especially infants.  If the strip is on a test, with no clinical information, consider these:

Onset and offset   Since we develop sinus tachycardia as a reaction to some other condition, the onset of the faster rate will be gradual.  That is, each beat will be closer to the last until maximum rate is reached.  This may take only a few beats, but there will be a gradual lengthening of the R-to-R intervals.  SVT, on the other hand, will usually begin very abruptly, with a premature atrial contraction (PAC).  From that beat forward, there is a fast, regular rhythm.  We call this paroxysmal supraventricular tachycardia, because it begins paroxysmally.  These rhythms usually END abruptly, as well.  If we are fortunate to see the onset or offset of the tachycardia, we will know whether it is sinus or ectopic in origin.

P waves     Sometimes, a tachycardia is so fast that P waves are buried in the preceding T waves and we can't evaluate them.  This can make it difficult to differentiate between sinus tachycardia and PSVT.  It helps to have multiple leads, especially a 12-lead ECG, because P waves show up better in some leads than in others.  Suggest to your students that they check Leads II and V1 if they have the option.  PSVT rhythms are often REENTRANT rhythms, caused by a circular conduction pathway that allows one impulse to circle around and reenter the ventricles.  These rhythms often have retrograde P waves, which will be negative in the inferior leads (II, III, aVF).  SVTs may also have P waves that are after the QRS.  Also, the P waves in an ectopic tachycardia will usually look different than the sinus P waves.  So, if we catch the onset of the tachycardia, and it is sudden, with a change in the appearance of the P waves, we are certain to have a PSVT.

Response to treatment.   Sinus tachycardia may respond temporarily to a Valsalva maneuver, or bearing down, but it will return because the cause of the sinus tachycardia is still present.  Supraventricular tachycardia often is stopped by a Valsalva maneuver or carotid sinus massage.  Sinus tachycardia usually responds promptly to addressing its cause - relieving pain, reducing fever, calming anxiety, etc.

It helps to give the students factual information, even when it is necessary to simplify.  That way, when they go on to more advanced training, they do not have to "unlearn" factoids they have memorized.  I have had to help students "unlearn" the 150 per minute "rule" hundreds of times.  And, thanks to the widespread use of rhythm generators for training, many people firmly believe that "sinus tach has a P wave and a T wave and SVT has only a T-P".  

Coronary Arteries Anterior View Labeled

Click to open: 
Anterior view of coronary arteries

This is an original illustration by Dawn Altman.  It is free for your use in an educational setting.  For other uses, please contact Dawn at [email protected]

 

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