This invention relates to a respiration monitor, in which the respiration activity is monitored by measuring the variations of the thorax impedance or other suitable parameters of a patient caused by his respiration activity.
A sufficient gas exchange in the lung is assured only if the lung is sufficiently ventilated. This ventilation is dependent on the respiration frequency on the one hand and on the respiration "deepness" (breath volume) on the other hand. Respiration monitors are used for monitoring the respiration of patients exposed to danger, e.g., patients in intensive care stations or prematurely born children. Generally, such instruments register the respiration activity and produce an alarm signal in the case of cessation of respiration (apnea), if the apnea duration exceeds a predetermined alarm limit.
In most cases the respiration monitoring is based on an impedance measurement of the patient's thorax. However, other measuring methods are also known, e.g., measuring the temperature variations of the respiration air or pressure variations between body and support.
Effective respiration is possible with relatively low frequency and deep breathing as well as with relatively high frequency and flatter breathing. Some prior art respiration monitors comprise an automatic sensitivity control responsive to the amplitude of the respiration signals. This control is performed with a given delay corresponding to a given time constant. In order to prevent misinterpretation of noise and artifacts as respiration signals, a minimum amplitude threshold is provided, below which respiration signals are no longer detected. If this threshold is not reached for a duration exceeding the alarm limit an alarm signal is produced.
Since the minimum amplitude threshold must be so low that all effective breaths are detected, it may happen that spurious signals exceed this threshold and are misinterpreted as respiration signals although a possible apnea is present. Such spurious signals may have their origin in the heart activity, for example. A respiration monitor capable of suppressing such spurious signals is described in U.S. Pat. No. 3,976,052, issued to Gerhard Junginger and Helmut Zeeb and assigned to the assignee of this application. The respiration monitor described in this patent operates with two amplitude thresholds, one of which is a fixed minimum threshold and the other one of which is a variable threshold dependent on the respiration amplitude. The respiration monitor is connected to separately receive signals corresponding to the heart beat rate and compare the periods of these signals to the periods of the signals measured at the thorax of the patient. If the periods of both signals are substantially equal, the signals measured at the thorax are actually due to heart activity rather than to respiration. In this case, the variable threshold is raised so that the monitor no longer responds to heart beat related signals but indicates that no respiration signals are measured.
Although capable of good suppression of spurious signals, prior art respiration monitors do not take into account the fact that certain further conditions must be met in order to assure a gas exchange that is sufficient for the patient. For example, if the respiration frequency is low, a predetermined minimum deepness of breath must be achieved.