Non-invasive monitoring of lung function in intensive care
The survival of patients in intensive care depends on the continued adequate perfusion of critical tissues by blood with appropriate gas partial pressures. The part played by the lung in this process is the maintenance of the appropriate gas tensions, and if these are preserved, the lung may perhaps be disregarded. However, the continued maintenance of normal partial pressures is not achieved without some data to inform the clinician as to the current ventilatory status so that any changes may be made that are necessary with the changing clinical condition of the patient. Since the data must be obtained by non-invasive means, the first question that arises is how to monitor arterial blood gases, particularly oxygen and carbon dioxide. Whilst large numbers of physical principles might be applied to the problem, the currently available solutions use either transcutaneous gas sampling or optical methods in a transilluminated appendage. Both methods depend crucially on an adequate blood supply at the sampling site so that capillary blood preserves the arterial partial pressures; as blood flow slows, the capillary bed loses oxygen and gains carbon dioxide over an increasing capillary volume, yielding subcutaneous values intermediate between arterial and local venous. With substantial impairment of local blood flow, local venous oxygen saturation can approach zero, a result clearly of no value in determining ventilatory status.
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