Present tense mixed with past tense in diaries?

The use of present tense in past narratives is called historical present tense. It serves multiple functions.

One possible reason people use historical past tense is to show contrast between two points of time in the past. In the second sentence of your example, there is the event of hearing the buzzer and rushing out, and there is the event of getting to the operating theatre. In the first event, you have sounded and crimped, and in the second event, you have ...is now unbearably... and ...while I'm scrubbed into theatre (scrubbed here is passive, not past).

Another reason people use historical past tense is when telling the an intense story. Telling intense stories forces speakers/writers to relive the memories of the event. When these feelings are very strong, the speaker/writer may feel more comfortable using present tense. It shows a stronger attachment to the intensity of the moment. This has been seen in narratives reflecting on near-death experiences [1]. This may explain why present tense is used to introduce the very intense memory in the first example sentence.

[1] Harvey, A. D. (1986). Evidence of a tense shift in personal experience narratives. Empirical Studies of the Arts, 4(2), 151-162.