Beck et al studied 113 patients with three-holed catheters in situ, and reported 14 catheters (12%) to be misplaced, with 13 invading veins and the other located epiduro-subarachnoid. Their incidence of dual compartment block would seem to be excessively high, compared to other published figures (eg 0.2% (14)), and may be largely attributed to the rigid catheters they used. It is of interest that blood could only be aspirated in 5 of the 13 cases of proven vascular invasion and that signs of toxicity were only obvious in 6 of the 9 patients in those who were given a local anaesthetic.
It has been demonstrated with computerised tomographic scanning that it is possible to insert an epidural catheter with the eyes in three spaces. The clinical picture produced when a drug is injected into more than one compartment has been named "Multicompartment Block" (19) (Figure 11.1).
The current multi-holed catheter design with eyes 3 to 4mm apart may predispose to the occurrence of multicompartment block especially when one bears in mind that the lumbar dura has a mean thickness of 0.5 mm.
Catheter misplacements and multicompartment block may be defined as "primary", when the catheter is initially inserted incorrectly (1, 2), or "secondary" when a correctly positioned catheter subsequently migrates into a blood vessel or through the dura into subdural or subarachnoid spaces (3) (Figure E.2).
Attempts to assess the clinical significance of multicompartment block are complicated by the finding that, in many reports of misplaced catheters, the eye configuration was not stated and it is difficult to know whether the misplacement was primary or secondary.
The development of multicompartment block may also be determined by the injection technique as it has been suggested that, when multi-holed catheters are used, the differential outflow through the individual eyes may be a function of both the injection pressure and the speed of injection (4).
Several cases of convulsions caused by inadvertent intravascular injection of local anaesthetic secondary to vascular invasion by catheters have been reported (Chapter 36). This has occurred with both a two-hole (after the first top-up) (5) and a three-hole (after the sixth top-up) catheter (6).
Invasion of the subarachnoid space has also been described. Hartley (7) and Ward et al (9) both recorded perforation of the dura with a two-hole catheter, with the latter proving fatal. Intrathecal spread of a second top-up was reported by Philip and Brown (10) with a terminal eye catheter and by Barnes (11) with a lateral eye catheter. Robson and Brodsky (12) were able to aspirate CSF only at the time of the third top-up.
Multicompartment block is seen not only with lateral eye catheters, but also with the Tuohy needle itself where mixed epidural-subdural block developed during a single injection (13).
A terminal eye catheter can also produce a mulitcompartment block. A single top-up of local anaesthetic produced classical symptoms of intravascular injection (Table 36.4) followed by an effective epidural block 5 minutes later (14) (Chapter 7). Test aspiration for blood had been negative. Two mechanisms can be proposed to explain this sequence of events. Either, the terminal eye was in both the vein and the epidural space, or, the catheter had previously punctured an epidural vein allowing entry into the circulation of part of the subsequent dose of local anaesthetic.
A worrying aspect of these cases of multicompartment block is that late collapse can develop following previously "normal" top-ups when the anaesthetist may have left the immediate vicinity of the patient. Delayed collapse is of even greater concern in those obstetric units where the top-ups are performed by midwives and the arrival of medical assistance may be delayed.
A recent strategy designed to reduce the occurrence of inadvertent multicompartment block has been the introduction of closer eye epidural catheters. This new catheter has 3 lateral eyes, with a 1mm eye separation, close to its tip (Figure 9.1) (19).
While the incidence of accidental multicompartment spread is low, the use of planned multicompartment block (as in a combined spinal-epidural technique) (Chapter 62) is growing rapidly. Even with deliberate multicompartment block there have been some unexpected complications (16, 17, 18).
References:
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3. GREGORETTI S. Uneventful extradural analgesia after unrecognized perforation Canadian Anaesthetists Society Journal 1978; 25:509-5
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17. BOUGHER R J, RAMAGE D. Spinal subdural haematoma following combined spinal-epidural anaesthesia. Anaesth Intens Care 1995 23:111-113.
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19. Collier CB; Gatt SP A new epidural catheter. Closer eyes for safety? Anaesthesia. 1993 48(9): 803-6