Tachycardias from emotion, exercise and pain, and drugs such as digoxin and beta stimulators, will all increase the outflow tract gradients and may considerably reduce the cardiac output to essential organs, such as the myocardium and brain.Preoperative beta blockers should be maintained.(Patients are often already taking beta blockers to prevent tachycardias.)
Hypotension from blood loss, regional anaesthesia, or vasodilator drugs, cause similar reductions in cardiac output and can worsen obstruction.Drugs and techniques that cause vasodilatation and hypotension must be avoided,contraindicated. However, with the greater ability to control the onset of epidural and by the use of continuous infusions and opiates, it has been suggested that regional blocks are no longer contraindicated. Hypotension should be treated by restoring vascular volume. If vasopressors are required, an alpha1agonist, such as phenylephrine or methoxamine, is the most suitable.
An adequate preload should be given and blood loss should be replaced promptly.