Measurement of blood flow has lagged behind pressure monitoring because of the inherent risks, complexity, and expense of pulmonary artery catheterization, the benchmark technique for cardiac output measurement in the intensive care unit (ICU). A variety of techniques are now available to assess cardiac output with either complete or relative non-invasiveness. These should be viewed as complementary to, rather than competing against, the invasive methods. While no technique can lay claim to absolute accuracy, technological improvements are continually raising the profile of non-invasive devices because of greater reliability and ease of use. Other than cardiac output, many provide additional information on circulatory status (e.g. preload, contractility, extravascular lung water). All have flaws, and data derived from different methods often correlate poorly. Some techniques may be better applied to monitoring trends rather than to absolute output.

The use of invasive flow monitoring in the ICU tends to be reactive, and it is often turned to when empirical therapy has failed to provide an adequate response to a cardiovascular disturbance. A reliable means of assessing global flow non-invasively would promote early identification of compensated derangements and faster correction.

Prospective users should familiarize themselves with both the theory and practice underlying a particular device, develop sufficient expertise to recognize unreliable or erroneous signals, and be aware of the limitations of both machine and technology. Poor equipment design and inadequate user education have contributed to many techniques being either discredited or used incorrectly.

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Sleep Apnea

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