The force that drives the contents of the uterus into the canula is caused by the difference of the atmospheric pressure and the low pressure inside the pump multiplied by the surface of the opening in the canula. The average atmospheric pressure is 760 mm Hg which would exercise a force of one kilogram per square centimetre if the vacuum in the pump were absolute. The manometer of the pump runs from 0 to 100% vacuum or from 0 till 760 mm Hg. It must be realised that zero means equal to atmospheric pressure and 100 or 760 means absolute vacuum. Since the force is determined by the atmospheric pressure minus the pump pressure this force is lower by low barometer reading, an effects that becomes evident on high altitudes. As for other reasons it would be unwise to build an abortion clinic in the Himalayas. Below a certain pressure the boiling point of water comes under body temperature so that the contents of the receiving vessel may boil, resulting in foam production This reduces the vacuum. If foam enters the pump it will be arrested by an automatic switch and only after cleaning it can be switched on again. The pressure should therefore not be lower than manometer reading 80% or 600 mm Hg.

Since the force exercised on the tissues is proportional to the surface of the opening in the canula it is clear that a bigger canula is more effective. In the beginning, for fear of possible perforation, especially in an advanced pregnancy with a soft uterine wall, aspiration was done with narrow tubes making the procedure very time consuming. As far as I know this complication has never happened. Clearly the uterine wall is tougher than expected. And with a proper premedication of ergometrine there seems to be no risk at all. Using too small a canula in the advanced pregnancy results in frequent blocking of the canula. In the young pregnancy it results in retention of placental tissue or even failure of the termination, especially if an inadequate source of vacuum is used like a hand operated syringe in Menstrual Regulation or Overtime Treatment (Chapter 6).

We may conclude that for a successful aspiration a powerful, quick acting pump is necessary. The canula must have a proper diameter. To avoid unnecessary dilation in the young pregnancy a smaller canula is used. Forced dilation should preferably not exceed 13 mm to prevent cervical incompetence. Although 10 mm dilation is sufficient to terminate a 13 week pregnancy, the diameter of canula should be roughly 2 mm less than the to the duration of pregnancy in weeks amenorrhoea. The minimum bore is 6, the maximum 13. Of course, if dilation is difficult a smaller canula must be used. This leads to a prolonged procedure with a greater risk of tissue retention.

Only one type of canula appears to be most effective in aspiration: a rigid transparent canula with a terminal opening directed sidewards. These canulae are straight or bent and they come in sizes 6 till 16 mm. They give a proper 'feel' of the inside of the uterus like a curette. They are made by the French company Juneau, MEDGYN (US) and the British Rocket of London.

The original Karman canula is a flexible tube with two side openings nearly at the end and a closed and rounded tip. They are available in sizes from 4 till 12 mm. They are used mainly for Menstrual Regulation. The possibility to perforate the uterus with it is practically the same as with the Juneau type. Although the tube is soft and it can bend a bit when pressed to the uterine wall, it is not flexible enough to avoid perforation if handled by a careless practioner.

The canula acts as a curette which at the same time removes the tissues. Contrary to a metal curette, a canula is not pressed against the uterine wall but the wall is sucked against the canula. This definitely diminishes the force applied to the uterine wall. Therefore, perforation is rare and the uterine wall is not scraped bare. The basal layer of the mucosa is not damaged so that there is but little risk of such conditions as Asherman's syndrome (the forming of adhesions between the uterine walls) or placenta accreta .

last revision spring 2010

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