Central venous cannulation is used for:
• delivering irritant or vasoactive drugs
• CVP measurement
• as a conduit for example, pacing wires
The CVP is expressed in mmHg when transduced (in which case a mean value is taken) or cm H2O if measured by a manometer (in which case the value at end-expiration is used); 10 mmHg is equivalent to 13 cm H2O. Errors in measurement are commonplace in areas where staff are not familiar with the equipment.
The CVP is diminished by reduced venous return, which is caused by hypovolaemia or vasodilatation. Both of these require volume replacement. However, several factors cause the CVP to rise:
• raised intrathoracic pressure, for example IPPV
• right heart failure
• lung diseases with pulmonary hypertension, for example COPD
• compensatory vasoconstriction in healthy people, for example bleeding
• tension pneumothorax, constrictive pericarditis, or tamponade.
The most important concept is that the CVP is a pressure not a volume. Many things affect the pressure in the right heart that have nothing to do with volume:
• valve disease
• lung disease (afterload)
• vasoconstriction or dilation (preload)
• muscle compliance
So, although the CVP is being used as an estimate of left ventricular filling pressure, it has several limitations. Single readings cannot influence management. It is the response of the CVP to fluid challenges that can help to assess volume status - it is possible to have a high CVP and to be volume depleted.
As mentioned in the previous chapter, the "rule of threes" is useful guide to fluid therapy. Generally, if the CVP remains unchanged or rises but then falls back to the original value after 5 minutes, a further fluid challenge should be administered. If the CVP rises and remains elevated, no further fluid is indicated.
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