This woman had recently undergone rotator cuff surgery and presented with L shoulder and chest pain. The pain was very reproducible with palpation and with range of motion of the shoulder. It seemed to be clearly musculoskeletal pain and my plan was to not even order an ECG (believe it or not). I was not planning to order any tests at all.
The physician assistant, however, did obtain this one:
Here is her previous ECG:
I could not believe that this new ST depression was due to ischemia.
What was causing the ST depression?
The differential diagnosis of ST depression in the context of a normal QRS (absence of LBBB, RBBB, LVH, RVH, WPW, hyperK, etc. etc. etc.) is very short:
1. Ischemia
2. Hypokalemia
3 Digoxin effect (associated with short QR interval)
4. Idiopathic/baseline.
Her medication list included a diuretic, so we checked the K. It was marginally low at 3.3 mEq/L. An hour after 40 mEq/L of oral potassium, we recorded this ECG (the pain was not resolved):
Troponins were negative and the patient was discharged.
Lesson:
1. Remember there are etiologies of ST depression other than ischemia.
2. If you order ECGs to investigate ischemia in situations of low pretest probability, you must have the skills to differentiate true positives from false positives.
3. Hypokalemia may have pathognomonic findings, such as prominent U-waves, as seen in many of these posts, but it may have no findings or only non-diagnostic ST depression, prolonged QT interval, or T-wave flattening.
Selected Literature on the ECG in Hypokalemia
1. http://circ.ahajournals.org/content/21/4/505.full.pdf
short version: http://circ.ahajournals.org/content/21/4/505.short
Electrocardiographic criteria of hypokalemia include various combinations of the following signs:
(1) T/U value of 1 or less in lead II or V3,
(2) U-wave amplitudes of greater than 0.5 mm. in lead II or greater than 1 mm. in V3, and
(3) S-T depression of 0.5 mm. or more in lead II or leads V1, V2, and V3.
It must be remembered that a normal electrocardiogram does not exclude hypokalemia and that an electrocardiogram which fulfills the established criteria does not necessarily indicate hypokalemia unless the factors discussed have been eliminated or minimized.
2. ECG Diagnosis: Hypokalemia
The earliest electrocardiogram (ECG) change associated with hypokalemia is a decrease in the T-wave amplitude.1 As potassium levels decline further, ST-segment depression and T-wave inversions are seen, while the PR interval can be prolonged along with an increase in the amplitude of the P wave.1 The U wave is described as a positive deflection after the T wave, often best seen in the mid-precordial leads (eg, V2 and V3). When the U wave exceeds the T-wave amplitude, the serum potassium level is < 3 mEq/L.2 In severe hypokalemia, T- and U-wave fusion with giant U waves masking the smaller preceding T waves becomes apparent on the ECG.1,2 A pseudo-prolonged QT interval may be seen, which is actually the QU interval with an absent T wave.1Severe hypokalemia can also cause a variety of tachyarrhythmias, including ventricular tachycardia/fibrillation and rarely atrioventricular block.3 Treatment of hypokalemia involves parenteral and oral potassium supplementation, as well as identification and treatment of the underlying cause.1
3. Our Study of the ECG in Significant Hypokalemia
We found that the ECG was very sensitive and specific for K less than or equal to 2.9 mEq/L, vs. 3.5 or greater, but this sensitivity depended on a subjective aspect of interpretation as well as on findings of abnormal T-waves, prominent U-waves, ST depression, or a computerzied QTc greater than 450 ms.
Smith, S. W., S. J. Bronner, and L. M. Hayden. Abstract 400: Derivation of a Rule for Diagnosis of Hypokalemia on the Electrocardiogram." Annals of Emergency Medicine 56.3 (2010): S129-S130.
Study Objectives: There is little data on the electrocardiographic (ECG) diagnosis
of hypokalemia (HypoK). We hypothesized the ECG to be an accurate predictor of
significant HypoK (K 2.9 mEq/L) or normoK (K 3.5 mEq/L) (NormoK).
Methods: Retrospective study. We searched the electronic medical record for
consecutive emergency department diagnoses of HypoK, then hand searched for
those in which there was an ECG recorded before administration of potassium (K),
and the K was less than or equal to 2.9 mEq/L. For controls, we searched for consecutive ECGs in
patients with K of at least 3.5 mEq/L. Abnormal QRS (bundle branch block or
intraventricular conduction delay) or extreme tachycardia (HR 130) were
excluded, as were redundant patients. One expert reader (ExR) and two resident
readers (RRs) who underwent a short tutorial interpreted the randomly sorted ECGs
while blinded to the K level and the Bazett-corrected QT interval (QTc-B). ECGs
were analyzed for computerized QTc-B, presence of U-waves [None (NUW), subtle
(SUW), or prominent (PUW)], T-Wave flattening (TF), and ST segment depression
(STD). Resident readers combined, and the expert reader, noted subjective
interpretations (SI, either ExR-SI or RR-SI) [definite HypoK (SI), or not]. Analysis
was by descriptive statistics, by Student’s t-test and by Chi-square (CS).
Results: There were 100 cases of HypoK with an ECG; 13 were excluded, leaving
87. There were 58 controls. QTc-B was the single best differentiator, with accuracy
(Acc) of 74%. Expert reader Accuracy was 72%, and Resident reader was 63% accurate (p .10 by
CS). Mean QTc-B for HypoK was 475 +/- 8.2 milliseconds (ms); for NormoK is was
429 +/- 5.5 ms (p = 0.0001 by Student’s t-test).
Prolonged QTc-B or a prominent U-wave was the best objective combination, with
Sens, Spec, and Acc of 80%, 78%, and 79%. These two criteria, if added to
subjective diagnosis by the ExR, versus none of the 3, had 86% Sens, 100% Spec and
92% Acc. If added to subjective diagnosis by the RRs, versus none of the 3, results
were 83% Sens, 98% Spec, and 89% Acc.
Conclusion: Computerized QTc is longer in HypoK than NormoK. Significant HypoK (less than or equal to 2.9) in the ED can be recognized on the ECG with high accuracy using QTc-B and presence of
prominent U-waves. HypoK can be recognized with very high Sens, Spec, and Acc,
using subjective interpretation of either the expert reader or the residents, plus QTc-B
and presence or absence of prominent U-waves.
The physician assistant, however, did obtain this one:
 |
| There are abnormal ST segments and T-waves in V2-V4, with ST segment depression in V3 and V4. |
Here is her previous ECG:
 |
| Normal, with some normal variant T-wave inversion in V2, but no ST depression |
I could not believe that this new ST depression was due to ischemia.
What was causing the ST depression?
The differential diagnosis of ST depression in the context of a normal QRS (absence of LBBB, RBBB, LVH, RVH, WPW, hyperK, etc. etc. etc.) is very short:
1. Ischemia
2. Hypokalemia
3 Digoxin effect (associated with short QR interval)
4. Idiopathic/baseline.
Her medication list included a diuretic, so we checked the K. It was marginally low at 3.3 mEq/L. An hour after 40 mEq/L of oral potassium, we recorded this ECG (the pain was not resolved):
 |
| The ST depression is resolved. |
Troponins were negative and the patient was discharged.
Lesson:
1. Remember there are etiologies of ST depression other than ischemia.
2. If you order ECGs to investigate ischemia in situations of low pretest probability, you must have the skills to differentiate true positives from false positives.
3. Hypokalemia may have pathognomonic findings, such as prominent U-waves, as seen in many of these posts, but it may have no findings or only non-diagnostic ST depression, prolonged QT interval, or T-wave flattening.
Selected Literature on the ECG in Hypokalemia
1. http://circ.ahajournals.org/content/21/4/505.full.pdf
short version: http://circ.ahajournals.org/content/21/4/505.short
Electrocardiographic criteria of hypokalemia include various combinations of the following signs:
(1) T/U value of 1 or less in lead II or V3,
(2) U-wave amplitudes of greater than 0.5 mm. in lead II or greater than 1 mm. in V3, and
(3) S-T depression of 0.5 mm. or more in lead II or leads V1, V2, and V3.
It must be remembered that a normal electrocardiogram does not exclude hypokalemia and that an electrocardiogram which fulfills the established criteria does not necessarily indicate hypokalemia unless the factors discussed have been eliminated or minimized.
2. ECG Diagnosis: Hypokalemia
The earliest electrocardiogram (ECG) change associated with hypokalemia is a decrease in the T-wave amplitude.1 As potassium levels decline further, ST-segment depression and T-wave inversions are seen, while the PR interval can be prolonged along with an increase in the amplitude of the P wave.1 The U wave is described as a positive deflection after the T wave, often best seen in the mid-precordial leads (eg, V2 and V3). When the U wave exceeds the T-wave amplitude, the serum potassium level is < 3 mEq/L.2 In severe hypokalemia, T- and U-wave fusion with giant U waves masking the smaller preceding T waves becomes apparent on the ECG.1,2 A pseudo-prolonged QT interval may be seen, which is actually the QU interval with an absent T wave.1Severe hypokalemia can also cause a variety of tachyarrhythmias, including ventricular tachycardia/fibrillation and rarely atrioventricular block.3 Treatment of hypokalemia involves parenteral and oral potassium supplementation, as well as identification and treatment of the underlying cause.1
3. Our Study of the ECG in Significant Hypokalemia
We found that the ECG was very sensitive and specific for K less than or equal to 2.9 mEq/L, vs. 3.5 or greater, but this sensitivity depended on a subjective aspect of interpretation as well as on findings of abnormal T-waves, prominent U-waves, ST depression, or a computerzied QTc greater than 450 ms.
Smith, S. W., S. J. Bronner, and L. M. Hayden. Abstract 400: Derivation of a Rule for Diagnosis of Hypokalemia on the Electrocardiogram." Annals of Emergency Medicine 56.3 (2010): S129-S130.
Study Objectives: There is little data on the electrocardiographic (ECG) diagnosis
of hypokalemia (HypoK). We hypothesized the ECG to be an accurate predictor of
significant HypoK (K 2.9 mEq/L) or normoK (K 3.5 mEq/L) (NormoK).
Methods: Retrospective study. We searched the electronic medical record for
consecutive emergency department diagnoses of HypoK, then hand searched for
those in which there was an ECG recorded before administration of potassium (K),
and the K was less than or equal to 2.9 mEq/L. For controls, we searched for consecutive ECGs in
patients with K of at least 3.5 mEq/L. Abnormal QRS (bundle branch block or
intraventricular conduction delay) or extreme tachycardia (HR 130) were
excluded, as were redundant patients. One expert reader (ExR) and two resident
readers (RRs) who underwent a short tutorial interpreted the randomly sorted ECGs
while blinded to the K level and the Bazett-corrected QT interval (QTc-B). ECGs
were analyzed for computerized QTc-B, presence of U-waves [None (NUW), subtle
(SUW), or prominent (PUW)], T-Wave flattening (TF), and ST segment depression
(STD). Resident readers combined, and the expert reader, noted subjective
interpretations (SI, either ExR-SI or RR-SI) [definite HypoK (SI), or not]. Analysis
was by descriptive statistics, by Student’s t-test and by Chi-square (CS).
Results: There were 100 cases of HypoK with an ECG; 13 were excluded, leaving
87. There were 58 controls. QTc-B was the single best differentiator, with accuracy
(Acc) of 74%. Expert reader Accuracy was 72%, and Resident reader was 63% accurate (p .10 by
CS). Mean QTc-B for HypoK was 475 +/- 8.2 milliseconds (ms); for NormoK is was
429 +/- 5.5 ms (p = 0.0001 by Student’s t-test).
Prolonged QTc-B or a prominent U-wave was the best objective combination, with
Sens, Spec, and Acc of 80%, 78%, and 79%. These two criteria, if added to
subjective diagnosis by the ExR, versus none of the 3, had 86% Sens, 100% Spec and
92% Acc. If added to subjective diagnosis by the RRs, versus none of the 3, results
were 83% Sens, 98% Spec, and 89% Acc.
Conclusion: Computerized QTc is longer in HypoK than NormoK. Significant HypoK (less than or equal to 2.9) in the ED can be recognized on the ECG with high accuracy using QTc-B and presence of
prominent U-waves. HypoK can be recognized with very high Sens, Spec, and Acc,
using subjective interpretation of either the expert reader or the residents, plus QTc-B
and presence or absence of prominent U-waves.
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