Previous studies have suggested the following are necessary for a successful Valsalva manoeuvre with maximum vagal effect:

• Supine posturing
• Duration of 15 seconds
• Pressure of 40 mmHg (with an open glottis)

One popular method of generating a Valsalva Manoeuvre is to get the patient to blow into a syringe in an attempt to move the plunger. Different syringe sizes were tested. A 10ml (Terumo) syringe was best
The 10 mL syringe is useful in generating the recommended standard of 40 mmHg intrathoracic pressure for the Valsalva manoeuvre
Emerg Med Australas. 2009 Dec;21(6):449-54