Methods for providing expiratory pressure relief in positive airway pressure therapy
A method of operating a CPAP apparatus in which the interface pressure is controlled to rapidly drop at the start of expiration by an expiratory relief pressure (ERP) that is independent of instantaneous respiratory flow, following which the pressure rises to an inspiratory level at or shortly before the end of expiration, or at the onset of an expiratory pause, if any. The ERP is an increasing function of the inspiratory pressure. The expiratory pressure follows a template that is a function of the expected expiration time, the magnitude of the template being equal to the ERP. The current estimated proportion of expiration is determined by comparing the expiration time of the breath in progress to low-pass filtered expiratory durations measured for a number of the preceding breaths.
1. A method of determining the expiratory proportion of a breath, p, comprising the steps of:
monitoring a respiratory flow of air of a patient;
calculating an expected duration of expiration, d, from at least one prior breath from said respiratory flow;
determining the start of exhalation of the patient within the breath;
calculating a time, t, since the start of exhalation of the patient; and
calculating p from the ratio of t and d.
2. A method in accordance with claim 1 wherein the expected duration of a breath is calculated by low pass filtering a time series of prior expiratory durations.
3. A method in accordance with claim 2 wherein the low pass filter is an Infinite Impulse Response (IIR) filter.
4. A method in accordance with claim 3 wherein the IIR filter is first-order.
5. A method in accordance with claim 1 wherein the expected expiratory duration, d m+1 , of the (m+1)th breath is calculated from the duration of the current breath d m , the previous breath d m−1 and a constant k in accordance with the following equation:
d m+1 =k d m +(1 −k ) d m−1 .
6. A method in accordance with claim 2 , wherein a low pass filter update T exp,LPF,m , being the expected expiratory duration to be used for the next breath, is calculated by the following equation:
T exp,LPF,m =kT exp,m +(1 −k ) T exp,LPF,m−1
where k is a constant, T exp,m is the expiratory duration measured for the current breath m, and T exp,LPF,m−1 is the previous filter update calculated for the preceding breath.
7. A method in accordance with claim 5 wherein the constant k is generally in the range of 0.1 to 0.2.
8. A method in accordance with claim 1 wherein the expected duration of a breath is calculated using a median filter.
9. A method in accordance with claim 8 wherein the median filter has a length in the range of 5 to 7 elements.
10. A method in accordance with claim 3 , wherein a low pass filter update T exp,LPF,m , being the expected expiratory duration to be used for the next breath, is calculated by the following equation:
T exp,LPF,m =kT exp,m +(1 −k ) T exp,LPF,m−1
where k is a constant, T exp,m is the expiratory duration measured for the current breath m, and T exp,LPF,m−1 is the previous filter update calculated for the preceding breath.
11. A method in accordance with claim 6 wherein the constant k is generally in the range of 0.1 to 0.2.