How to detect an event and ensure its the first (earliest event that occurred? - sql

Question

I am attempting to extract data from a large patient database using SQL. I need to detect when an event occurred. In this case, it will be a decrease in SpO2 by 10% from when treatment started. It needs to be the earliest time point when it decreased. Example 100%-95-92-97-88-92-89 only the 88 is what I will need. The tricky part begins as patients might have two treatments throughout care. I need to compare these treatments to observe which treatment is the best. An example of the data is below.
The patient will have treatment A until treatment B starts or until the final reading and vis versa. the SpO2 values with ** are the ones I need to extract
Date
Time
SpO2
Treatment
05/19/22
18:23
100
N/A
05/19/22
18:24
95
A
05/19/22
18:25
90
N/A
05/19/22
18:26
85
N/A
05/19/22
18:27
N/A
B
05/19/22
18:27
90
N/A
05/19/22
18:28
85
N/A
05/19/22
18:29
80
N/A
05/19/22
18:30
78
N/A
05/19/22
18:31
76
N/A
What I'm hoping the final table looks like is
Date
Time
SpO2
Treatment
05/19/22
18:23
100
N/A
05/19/22
18:24
95
A
05/19/22
18:26
85
N/A
05/19/22
18:27
N/A
B
05/19/22
18:27
90
N/A
05/19/22
18:29
80
N/A
If you can please help, that would be amazing and thank you very much.