MEDICAL TUBING APPLICATIONS

Portex Ltd., Hythe, UK 3-4 December 1980

Members of the Biomaterials Group of the BES met in Hythe, Kent for a series of talks organized by Dr W.F. Watson, Technical Director of Portex Ltd. This company has been associated for many years with the supply of endotracheal tubes and trache- ostomy appliances, and has been responsible for the development of major improvements in design.

The meeting began with an excellent dinner and the participants then joined in an informal dis- cussion on Technical Opportunities and Hindrance in the Medical Field introduced by Dr. Watson. He also opened the programme for the following day by tracing the history of the soft-seal cuffed tube. In reviewing the very early uses of tracheostomy and endotracheal devices he particularly referred to the work of Sir William Mackewan in Glasgow, who conducted the first intubation anaesthesia. For present day intubation, only plasticized PVC is really worthy of consideration, apart from natural rubber. Silicone rubber is not only expensive but also a poor choice as a material because gases diffuse rapidly through it. Natural rubber dominates short-term theatre use, but the advantage of PVC is that it softens on warming to body temperature, whereas rubber stiffens due to the predominance of entropy changes affecting chain conformation.

PVC for long-term use is therefore less damaging to the tissues. Studies of N,O permeability have explained the clinical observation that the cuff becomes more inflated during anaesthesia using this gas. (see Biomateriuls, 1980, 1, 41).

Mr M. Chandler explained the scientific principles relating to pressure difference maintenance and discussed the compromise necessary between effective sealing and tracheal damage. The impor- tance of the cuff and the different design concepts were discussed. Cuff pressure studies have demon- strated that the hard seal cuff distends the trachea and that the pressure in the trachea is consequently that exerted by the trachea in order to recover its original shape. The effect of inflating the soft seal is just to fill the trachea and is thus less damaging clinically.

Dr Parsons (National Institute for Medical Research) described their collaborative research related to ‘Slow infusion of insulin and other drugs.’ The physiological effects of digitalis and related drugs were described in the context of experiments to determine pharmaco-kinetic effects. These studies showed that by continuous infusion the required concentration could be maintained in viuo rather than production of peaking, following injection. In

a subsequent study hypercalcaemic effects of PTH were followed. With a motor driven infusor the dose rate was established which gave a plateau response. Using the technique in osteoporosis studies, there is evidence that trabecular bone formation increases with continuous infusion.

Continuous infusion of insulin was investigated because improved carbohydrate control is indicated to reduce the morbidity in treatment. The role and effect of insulin is physiologically very complex, but the importance has been shown of maintaining constant low levels in the blood. Clinical trials using an infusor have shown that blood Jucose profiles can be closely controlled and there is preliminary evidence of reversal of the pathologies that accompany the dis- ease e.g. vascularity in the eye. A miniaturized infu- sor, designed and developed at NIMR and incorpora- ting a radically new drive mechanism, was shown.

Mr W.W. Vincent described the various polymers employed, although few have been specifically developed for medical use, partly because only a low volume of production is required. In the com- plex matter of evaluation of materials, he considered that Council of Europe protocols and the US Pharma- copaeia were more assistance than Standards to the manufacturer. For medical tubing, PVC is most frequently the chosen material since it has a good range of properties. Plasticizer leaching may be eliminated with new developments. The advantages and limitations of ethylene-vinyl acetate (EVA), styrene butadiene-styrene (SBS), copolymers, thermoplastic polyurethane, polyethylene, PTFE,. polypropylene and silicones were discussed. Sterih- zation procedures are of importance and a brochure summarizing methods and effects will be available from Portex shortly.

The history of Portex following its development from Portland Plastics to the present day in UK and overseas was presented by Mr R.W. Duffey. Mr A.M. Parry described some of the manufacturing procedures. Most of the work is inevitably batch rather than long volume production. He outlined their process for compounding PVC for the many different formulations they require, emphasizing the role of quality control. Engineering support services are of vital importance and in the field of sterile packaging Portex have made innovations. The thanks of the group are due to Dr Watson and those members of Portex who contributed to such an interesting meeting. Mr A. Easton, Managing Director, is thanked for his assistance and hospitality.

G.W. Hastings

J. Biomed. Engng. 1981, Vol. 3, April 163