Invasive direct arterial monitoring

Blood pressure is most usefully monitored from larger limb arteries, e.g. femoral or brachial. However, the potential for damage to these arteries is considerable and most consider it safer to use the radial or dorsalis pedis arteries, the pressure in which is higher. The arterial cannula is connected to an appropriate transducer system via a short length of non-compliant manometer tubing. The transducer should be matched to the monitor, i.e. as recommended by the manufacturer of the monitor. The transducer must be zeroed to atmospheric pressure. The transducer should be positioned at the level of the 4th intercostal space in the mid-axillary line. The transducer, manometer tubing and cannula should be continuously flushed with 3ml/h heparinised saline (1000IU/l).

Damping errors

It is important that the monitoring system is correctly damped. An underdamped system will overestimate systolic and underestimate diastolic blood pressure. The converse is true for an overdamped system. Moreover, it is not possible to correctly interpret waveform shape if damping is not correct. A correctly damped system will return immediately to the pressure waveform after flushing. Return is slow in an overdamped system and there is often resonance around the baseline before return to the pressure waveform in an underdamped system.

Interpretation of waveform

The shape of the arterial pressure waveform gives useful qualitative information about the state of the heart and circulation:

Short systolic time Hypovolaemia High peripheral resistance Marked respiratory swing Hypovolaemia Pericardial effusion Airways obstruction High intra-thoracic pressure Slow systolic upstroke

Poor myocardial contractility High peripheral resistance

Limitations of blood pressure monitoring

It is important not to rely on arterial blood pressure monitoring alone in the critically ill. A normal blood pressure does not guarantee adequate organ blood flow. Conversely, a low blood pressure may be acceptable if perfusion pressure and blood flow is adequate for all organs. Measurement of cardiac output, in addition to blood pressure, is necessary where there is doubt about the adequacy of the circulation.


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