The present invention relates to a measuring detector for the measurement of gas flow, especially for the measurement of the pressure and/or flow of a patient's respiration. Moreover, the invention relates to a system for the measurement of the pressure and/or flow of a patient's respiration.
In hospitals, during intensive care and operations, respiration apparatus must be used to take care of the patients' respiration. Unhindered flow of gases into and from the patient's lungs is naturally of vital importance. The condition of the gas channels can be monitored both by measuring the concentrations of the exhalation gases and by measuring the flow and pressure of the gases. Especially, monitoring of the carbon dioxide content of exhalation gas is widely used as a routine in operating theatres. However, measurement of flow and pressure are essential additional functions both in respect of safety and because they make it possible to calculate quantities descriptive of the mechanical operation and respiratory metabolism of the lungs.
In principle, there are many applicable types of flow detectors. However, measurements in clinical conditions involve many problems. The flow is measured from the end of a so-called intubation tube inserted into the patient's windpipe. The detector is therefore exposed to both humidity and mucous secretions coming from the windpipe. It is clear that such soiling is likely to affect the operation of especially the commonly used turbine and hot-wire detectors. Ultrasonic detectors are better able to tolerate soiling, but they are dependent on changes of the flow profile, temperature and gas composition, requiring sophisticated compensation. Differential pressure detectors are better suited for clinical use. The flow in the tube may be laminar or turbulent. In the case of laminar flow, the pressure difference across a flow restricting element placed in the tube is directly proportional to the flow. In the case of a turbulent flow, the pressure difference depends on the square of the flow. In addition, the pressure difference depends on the square of the cross-sectional area of the flow tube. The detectors currently used are generally made of plastic, and the concentration of water forms small drops on the interior walls of the flow detector because water has a large contact angle to a plastic surface. The problem is that the condensed water together with possible secretions gathered in it reduce the cross-sectional area of the detector, resulting in an increase in the measured pressure difference. If the measured pressure difference is too large, this also means that the calculated flow value is too high and therefore incorrect. A detector with a small cross-sectional area is the most sensitive in this respect. In short-time use of the measuring detector the resulting error is generally not too large, but if the detector is used continuously e.g. for one or more hours in humid conditions, the error produced in the measurement results will be considerable. One way to eliminate this problem is to heat the detector to a temperature sufficient to prevent condensation. However, this method requires a heating element and an electric connection, so it is difficult to use in practice and a detector with a heating element is also expensive to fabricate. Moreover, a hot element may involve a danger to the patient.