In the fifties and early sixties, a very common practice was for the epidural enthusiast to cut a suitable length of industrial polyvinyl chloride (PVC) or nylon tubing of approximately 1mm external diameter from a large (100 foot) roll and to have this sterilized with the other epidural equipment. Some departments added their own markings prior to sterilization. This latter process usually increased the stiffness of the catheter. Furthermore, the cut end of the catheter was relatively traumatic to the tissues and more likely to penetrate vessels. Later, such catheters could be supplied sterilized and packaged, although they were often reused on 4 or 5 occasions! Lee's catheter (3) was one of the first with a smooth non-patent tip and a single lateral eye (Figure 9.1).
Later catheter developments (Figure 9.1) included two eyes in various configurations (5). By 1980, these had been superseded. Today, the two types of epidural catheter most commonly used world-wide are the terminal eye variant and the one with three lateral eyes (Figure 9.1). Whereas about 90% of the Australasian and European market consists of the three lateral-eye model, the reverse is true in the U.S.A. where some 90% of catheters have only the terminal eye. This difference in usage remains unexplained.
Some catheters
are supplied with stylets, others are reinforced (Figure 9.5) and most are
constructed of polyamide nylon. Silicone catheters have been introduced for
long-term use with multiple holes at the tip. Catheter types are manufactured in
the size range of 16-21G and may or may not be radio-opaque.
In Australia, the typical catheter has three lateral eyes arranged in a helix, with either 3 or 4mm between the eyes, and the distal eye being 5-8 mm away from the tip (2).
The reason for the introduction of three lateral eyes has never been explained in the literature, but the following information was supplied by one manufacturer (Portex): "In the late 1970's, the manufacturing technology was developed to allow the drilling of small holes in the catheters, without producing any debris. The diameter of the holes related to the maximum size that could be drilled without weakening the tube. The number of holes was then determined by that which gave equivalent to full-bore flow."
For the Portex 16G catheter, the eye-discharge area is larger than the cross-sectional area of the tube bore and the flow rate through the three eyes is the same as that through an open-ended catheter at the same injection pressure. The exact spacing of the eyes was not thought to be critical but was largely related to manufacturing practicalities, with the aim of siting the eyes as close together as possible to reduce the incidence of multicompartment block (Chapter 11). Current technology has allowed the holes to be positioned much closer together (1mm separation) and much closer to the tip (1) (Figure 9.1).
The choice of epidural catheter for non-obstetric work does not appear to be critical, but the lateral-eye catheter produces improved results in obstetric patients (1, 2, 4). In one series, unsatisfactory blocks requiring remedial action were recorded with 32% of the terminal eye catheters compared with 11% of the lateral eye catheters and 8% of the former had to be replaced compared to 2% of the latter (2).
References:
1. COLLIER CB, GATT
SP. A new epidural catheter. Closer eyes for safety? Anaesthesia 1993
48:803-806.
2. COLLIER CB, GATT SP. Epidural catheters for obstetrics; terminal hole or lateral eyes. Regional Anesthesia 1994 19:378-385.
3. LEE JA. A new catheter for continuous extradural analgesia. Anaesthesia 1962 17:248-250.
4. MICHAEL S, RICHMOND MN, BIRKS RJS. A comparison between open-end (single hole) and closed-end (three lateral holes) epidural catheters. Anaesthesia 1989 44:578-580.
5. SKINNER BS. A new epidural cannula. Canadian Anaesthetists' Society Journal 1966 13:622-623.
6. WARD CF, OSBORNE R, BENUMOF JL, SAIDMAN LJ. A hazard of double-orifice epidural catheters. Anesthesiology 1978 48:362-364.