As we inhale, the pressure in our chest cavity (thorax) falls to below atmospheric pressure. In response to this underpressure, air flows into our lungs and balances it out. The underpressure in the thorax is transferred to the organs in the thorax, such as the food pipe (esophagus). The upper esophageal sphincter normally prevents air from flowing into the stomach and the stomach becoming inflated as we inhale. If the larynx is removed surgically and the upper esophageal sphincter cut through (myotomy) so the patient will be able to speak using a voice prosthesis, then the stomach may fill with air during strenuous inspiration (e.g. during sporting activities).
Most esophagotracheal fistulas and voice prostheses are situated intrathoracic. This means that the valve of the voice prosthesis is also exposed to the thoracic underpressure on inspiration. If the underpressure in the thorax and esophagus exceeds the valve opening pressure of the voice prosthesis, then the valve will open when breathing (the valve flaps of the voice prostheses open in the direction of the esophagus), which can be seen in some patients. As laryngectomees can swallow and breathe at the same time, the prosthesis may leak. However, the sheer number of accidental openings of the flap valve when breathing (as well as when speaking) can result in premature material fatigue of the valve, resulting in its failure.
If a patient is constantly requiring his voice prosthesis to be changed at short intervals (1-5 weeks), then it is possible that the valve is opening as the result of underpressure during inspiration. These faulty voice prostheses usually show little or even no evidence of a biofilm, so it can be ruled out as the cause for the short service time of the voice prosthesis. Some affected patients report that their stomachs fill with air following physical strenuous activity, which is unpleasant for them.
The valve opening as the result of underpressure is visible by endoscopy, but also by the naked eye.
A voice prosthesis with increased flap opening resistance has to be used to stop the valve from opening due to underpressure. There are two options here: the Blom-Singer® Increased Resistance prosthesis, and the Provox® ActiValve prosthesis, which is available in three opening pressures (light, strong, extra strong). In vivo measurements of the inspiratorial underpressure in the esophagus are required to choose the optimum prosthesis. Until now, this has only been possible through experimentation. That is why the necessary opening pressure can only be estimated or optimized by approximation.