The Paradoxical Alternative

Cardiovascular Curveball 006

Consider a 49 year-old female with a history of smoking and two weeks of increasing shortness of breath. She is being treated for pneumonia on the ward for three days but getting worse. An ICU review is performed on the ward and the following ECG is obtained.

Q1. Describe the ECG findings. What investigation is indicated?

The most significant finding on this ECG is the presence of electrical alternans.
The rhythm is sinus tachycardia at 100/min and the axis is normal.

The investigation indicated is an echocardiogram to confirm the presence of a pericardial effusion and to look for echocardiographic evidence of  pericardial tamponade.

Q2. You ring the cardiologist to arrange the investigation in question urgently and he asks you to assess the degree of pulsus paradoxus.

What is pulsus paradoxus and what are the potential causes of this phenomenon?

Pulsus paradoxus is defined as an inspiratory drop in blood pressure of 10mmHg or more during normal breathing.
It is caused by:

  1. pericardial tamponade
  2. hypovolaemia (especially during positive pressure ventilation)
  3. acute asthma
  4. massive pulmonary embolism

On the cardiologist’s advice you assess the degree of pulsus paradoxus and no significant respiratory variation in systolic pressure is present.

Check that the R-R interval on the ECG is regular to rule out arhymthmia as the cause of a fluctuating systolic blood pressure.

Q3. You run into difficulty getting the investigation you have requested in a timely fashion because the cardiologist argues that the absence of pulsus paradoxus is reassuring.

Is he right?

In this particular case, no.
Electrical alternans is usually associated with tamponade and there are many reasons why pulsus paradoxus may be absent in the presence of cardiac tamponade including:

  1. Pericardial adhesions (particularly over the right heart)
    • impede volume changes
  2. Severe left ventricular failure or marked left ventricular hypertrophy
    • in these circumstances the pericardial pressure effectively equilibrates only with the right heart pressures with the much less compliant left ventricle resisting phasically changing pericardial pressure
  3. Right ventricular hypertrophy without pulmonary hypertension
    • causes right-sided resistance to the effects of breathing
  4. Atrial septal defects
    • increased venous return balanced by shunting to the left atrium
  5. Severe aortic regurgitation
    • produces sufficient regurgitant flow to damp down respiratory fluctuations


  • Spodick DH. Acute cardiac tamponade. N Engl J Med. 2003 Aug 14;349(7):684-90. PMID: 12917306.
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