Central Venous Pressure Waveforms

 

Normal CVP Waveforms

            The central venous waveform seen on the monitor reflects the events of cardiac contraction; the central venous catheter “sees” these slight variations in pressure that occur during the cardiac cycle and transmits them as a characteristic waveform.  There are three positive waves (a, c, and v) and two negative waves (x and y), and these correlate with different phases of the cardiac cycle and EKG.

cvpwave

  • + a wave :  This wave is due to the increased atrial pressure during right atrial contraction.  It correlates with the P wave on an EKG.
  • + c wave :  This wave is caused by a slight elevation of the tricuspid valve into the right atrium during early ventricular contraction.  It correlates with the end of the QRS segment on an EKG.
  • - x descent :  This wave is probably caused by the downward movement of the ventricle during systolic contraction. It occurs before the T wave on an EKG.
  • + v wave : This wave arises from the pressure produced when the blood filling the right atrium comes up against a closed tricuspid valve. It occurs as the T wave is ending on an EKG.
  • - y descent : This wave is produced by the tricuspid valve opening in diastole with blood flowing into the right ventricle.  It occurs before the P wave on an EKG.

 

Note: Because the central line lies within the thorax, central venous pressures and waveforms are influenced by changes in intrathoracic pressure during respiration.  These changes are small, but may be observed on the waveform.  CVP decreases slightly with spontaneous inspiration, and increases slightly with forced exhalation as well as positive pressure mechanical inspiration.  A patient on PEEP (positive end expiratory pressure) greater than 7.5cmH2 0 may also have an artifactually elevated CVP.

 

Pathologic CVP Waveforms

     Variations on the normal central venous waveform can provide information about cardiac pathology. For example.....

 

  • In atrial fibrillation, a waves will be absent, and in atrioventricular disassociation, a waves  will be dramatically increased ("cannon waves") as the atrium contracts against a closed tricuspid valve.
  • In tricuspid regurgitation, the c wave and x descent   will be replaced by a large positive wave of regurgitation as the blood flows back into the right atrium during ventricular contraction.  This can elevate the mean central venous pressure, but it is not an accurate measurement.  A better way of estimating CVP in this case would be to look at the pressured between the regurgitation waves for a more accurate mean.
  • In cardiac tamponade, all pressure will be elevated, and the y descent will be nearly absent.

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