The second drug paralyses the muscles, so you need a ventilator to breathe. The percentage of oxygen in the air is controlled by the ventilator. An alarm will sound if it drops too low.
The heart still works under general anaesthetic, and your heart rate is recorded by an electrocardiogram . the anaesthetist also monitors the percentage of haemoglobin in the blood and level of carbon dioxide. Body temperature is constantly measured, because hypothermia can occur under general anaesthetic.