Monday, September 29, 2008

Explanation to medical instruments

Explanation to medical instruments

Pulmonary capillary wedge pressure

from bailey book

The pulmonary capillary wedge pressure (PCWP) is a better indicator of both circulating blood volume and left ventricular function. PCWP is obtained by a pulmonary artery flotation balloon catheter (Swan—Ganz). This can be used to differentiate between left and right ventricular failure, pulmonary embolus, septic shock and ruptured mitral valve, and can also be an accurate guide to therapy with fluids, inotropic agents and vasodilators. It may also be used to measure cardiac output by a thermodilution technique simply at the bedside.

Measurement of pulmonary capillary wedge pressure

This specialised procedure requires supervised training, practice, patience and experience in interpreting the values measured and waveforms indicated. Complications include arrhythmias, pulmonary infarction, pulmonary artery rup­ture, balloon rupture and catheter knotting, in addition to the complication from central venous cannulation. The catheter should not be left in situ for more than 72 hours; if further haemodynamic monitoring is required, a new catheter should be inserted.

Method. Strict aseptic central venous cannulation should be performed (e.g. via right internal jugular vein) and using the appropriate introducers, cannula and guidewire, the catheter, flushed and wiped with heparin saline, introduced into the right atrium. The balloon, inflated with 1.5 ml of air, should be advanced slowly via the right ventricle into the pulmonary artery, checked by x-ray and monitored by pressure tracing, which becomes characteristically flat when the balloon wedges in a small branch to give the capillary pressure (indicating left atrial pressure). When the balloon is deflated, the pulmonary artery pressure is obtained. The balloon must never be reinflated in the absence of a normal pulmonary artery waveform as this means that the tip alone is wedged and reinflation might therefore rupture the pulmonary artery. Withdrawal of 2—3 cm is mandatory until the waveform reappears and reinflation can be permitted.

The transducer should be placed at the midaxillary point (zero reference point); the normal PCWP is between 8 and 12 mmHg (10.5 and 15.5 cmH2O), and normal pulmonary artery pressure is 25 mmHg systolic and 10 mmHg diastolic.

Clinical monitoring

In summary, patient monitoring in shock should include:

• pulse;

• blood pressure (recording systolic and diastolic pressure, the pulse pressure, using an intra-arterial line if necessary);

• heart rate and rhythm (cardioscope);

• respiratory rate and depth;

• CVP;

• PCWP in severe shock when the diagnosis is in doubt;

• urine output;

• serial blood gases and serum electrolyte measurements.

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