Operational issues Zeroing

Zeroing the pressure-monitoring system is probably the most important step in establishing the system. Mistakes in zeroing can result in substantial errors in pressure measurement, particularly when measuring pulmonary artery pressures. The zeroing process involves an interaction between the position of the transducer and the stopcock in the plumbing system with the zero control of the monitoring system. For each 10 cm of misalignment of the zeroing point, there is a pressure offset of 10/1.36 = 7.3 mmHg. Such a misalignment may cause an error of as much as 100 per cent in measurement of central venous pressure. The vertical position of the transducer relative to the patient's mid-axillary line can change, particularly if it is mounted externally. As a result the monitoring system zero should be verified frequently, and always prior to initiation of treatment changes based on pressure data. Proper zeroing is done by opening the appropriate zeroing stopcock to atmosphere and aligning the resulting fluid-air interface point at the patient's mid-axillary line ( Fig 1).

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Sleep Apnea

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