Central venous pressure

A central line or central venous catheter (CVC; Figures 58.1 and 58.2), is a catheter placed into a large vein, usually in the neck (internal jugular vein), chest (subclavian vein) or groin (femoral vein). It is used to measure central venous pressure, administer drugs or fluids that need to be administered rapidly or would damage peripheral veins, or to take blood samples.

Triple-lumen CVCs have proximal, medial and distal ports. The distal end lies in the superior vena cava (unless placed femorally) and should always be monitored using a transducer. The monitor produces a numerical value and a waveform. An accurate central venous pressure (CVP) measurement needs to be taken with the patient lying supine and the transducer aligned with the phlebostatic axis. The number (normal CVP is 2–6 mmHg) indicates right ventricular function and systemic fluid status.

Reasons why CVP may be elevated are:

• Over hydration increases venous return
  
• Heart failure or pulmonary artery stenosis limiting venous outflow
  
• Positive pressure breathing due to straining.

A reason why CVP may be decreased is:

• Hypovolaemic shock.

CVCs have potentially serious complications:

• Pneumothorax
  
• Bloodstream infections
  
• Thrombosis
  
• Misplacement – placing the catheter usually requires the patient adopting a Trendelenburg or at least supine position. This may be difficult in pregnancy as it may cause aortocaval compression.
  
• Air embolus – lines attached to a CVC must be kept air free
  
• Haemorrhage and formation of a haematoma.

The use of CVCs is becoming increasingly rare in maternity care.

Pulse oximetry

Pulse oximetry provides continuous and non-invasive monitoring of the oxygen saturation of haemoglobin in arterial blood. It works by emitting light and then measuring the light after it has passed through capillaries, usually in the fingertip (Figure 58.3

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