I've nursed patients with all sorts of ailments and conditions who have had hip and knee replacements and been just fine throughout the surgery. Conditions would include ~ heart problems such as history of heart attacks, arrhythmias, pacemakers, stents, valve replacements, medicated high blood pressure etc. ~ diabetes, asthma and other lung disorders, sleep apnoea, epilepsy ~ obese, elderly, young, frail All patients are closely monitored throughout the procedure with constant EKG monitors, blood pressure every 3 minutes, respirations recorded all with a constant display and record kept on a machine like this You also have a probe on a finger called a pulse oximeter which gives a constant display of your pulse rate and blood oxygen level on the machine. So the anaesthetist has a constantly updating display on your health on a minute by minute basis plus his own observation to monitor the patient's health. Nothing catches him unaware. Another thing to remember is that anaesthetists (or anaesthesiologists as you call them in the US) are fully qualified doctors who have done the whole training including emergency room, general surgery, other speciality surgeries like orthopaedics, obstetrics and many others. If they then choose to specialise in anaesthetics, which generally includes ICU and pain management, they undergo another five year programme of training. These doctors are so trained they wouldn't miss a mosquito's heartbeat! They've encountered and dealt with all manner of events and are capable of dealing with anything. You're always in safe hands with them.