Pulsus paradoxus

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"Pulsus paradoxus" refers to an abnormal decrease of over 10mmHg in pulse pressure and subsequently pulse volume on inspiration. It is caused by a decrease in intrathoracic pressure. 

  1. On inspiration the intrathoracic pressure decreases
  2. The pressure intrathoracically is lower than the pressure extrathoracically which leads to an increased venous return to the right ventricles
  3. The right ventricles bulges into the left side reducing left ventricular volume. 
  4. Reduced intrathoracic pressure also leads to dialation of the pulmonary vessels
  5. Blood from the right ventricle pools in the lungs leading to reduced blood returning to the left ventricles
  6. Reduced blood in the left ventricles leads to a reduced stroke volume according to starling's law. 
  7. Reduced thoracic pressure on the aorta also contributes to the reduced pulse pressure
  8. Reduction of pulse pressure leads to a weaker or absent pulse volume being felt whist a contraction is heard on auscultation. 
Pulsus paradoxus may occur in: 

  • Cardiac tamponade 
  • Cardiac effusion
  • Restrictive pericarditis

due to reduced left ventricular filling due to increased pericardial pressure without a proportional increase in pulmonary venous pressure, and increased proportional effect of intraventricular septal bulging in a limited space. 
  • Severe acute asthma 
  • Severe COPD
due to increased differences in inspiratory and expiratory intrathoracic pressure. 

  • Massive PE 
  • Myocardial infarction
due to right ventricular dysfunction and dilation.
References
Douglas G, Nicol F, Robertson C, Macleod's Clinical Examination, 12th ed, Edinburgh, Churchill Livingstone, 2009. 
http://www.jpgmonline.com/article.asp?issn=0022-3859;year=2002;volume=48;issue=1;spage=46;epage=9;aulast=Khasnis
http://en.wikipedia.org/wiki/Pulsus_paradoxus

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