I couldn't help it. I'm a research freak. I e-mailed some Paramedic and M.D. friends. Their universal response was along the lines of "do as much pressure as it takes to get the job done."
So I did a web search and received three completely different answers. I've included the URLs in case anyone wants to see the source.....
Nat. Registry EMS exam tips http://www.medtrng.com/emttips.htm
"Judging how much pressure to apply during Sellick's maneuver may be difficult. This exercise will help you to understand how much pressure is needed. Pinch your thumb and index finder together. Push on the bridge on your nose, or your partner's nose, until it hurts. Pressure applied during Sellick's maneuver is about equal to the pressure it takes to cause pain."
Society of Anesthesiologists page quotes Sellick himself... http://www.asahq.org/Newsletters/199...llick0999.html
Sellick's seminal paper shows lateral X-rays of the neck with the esophagus containing a latex tube full of contrast medium, and the effect of cricoid pressure is wonderfully demonstrated. "Cricoid pressure must be exerted by an assistant. Before induction, the cricoid is palpated and lightly held between the thumb and second finger; as anaesthesia begins, pressure is exerted on the cricoid cartilage mainly by the index finger. Even a conscious patient can tolerate moderate pressure without discomfort but as soon as consciousness is lost, firm pressure can be applied without obstruction of the patient's airway. Pressure is maintained until intubation and inflation of the cuff of the endotracheal tube is complete." The diagrams and photographs of this application of pressure are excellent. He goes on to echo the thoughts of Kite and Curry saying, "During cricoid pressure the lungs may be ventilated by intermittent positive pressure without risk of gastric distension."
Med. Univ South Carolina RSI page http://www.musc.edu/emergmed/RSI/rsi14.htm
"The procedure is best performed with the use of three fingers - the thumb, index, and middle fingers. The index is placed directly over the cricoid notch to facilitate location, while the thumb and middle fingers are placed to either side of the cricoid for stabilization. All three fingers hold the pressure equally. How much pressure to apply? This amount has been determined to be around 20 - 40 newtons 64,65, but I hardly see how this fact is helpful. It has been suggested that this equals that pressure which results in about 1/4 of the nail-bed blanching (reference not available - I was told this by a colleague at an airway seminar). However, Sellick's maneuver has been reported to cause airway obstruction in up to about 10% of cases 66 - due to excessive force occluding the trachea. Caution is urged! "
So it looks like WntrMute is right, depending on the source. Kavi