ANAESTHESIOLOGY
Article sponsored by Petcam ® the stylet , and inspect the hub for blood or CSF .
• If you see blood a venous sinus has been penetrated and the needle should be withdrawn and repositioned .
• If CSF is present , you have entered the subarachnoid space and can withdraw or reposition the needle or , alternatively , inject one-fourth to onehalf of the calculated volume of the drug . The maximum dose of local anaesthetic that can be administered spinally ( intrathecally ) is < 1 ml / 10 kg , and only preservative-free drugs should be injected spinally . 4
• If no blood or CSF is observed in the hub , inject a test dose of 0.5 to 1 ml of air or saline solution with a sterile syringe to check for resistance to injection ( Figure 4 ). In our practice , we use glass syringes because they provide less resistance to injection ; however , plastic syringes may be used . If the epidural space has been correctly entered , there should be no resistance to injection of the air or saline solution . If the plunger pushes back , the needle has not been correctly positioned .
Figure 5 . Aspirate an air bubble ( ½ to 1 ml ) into the syringe containing the drug or drug mixture . Minimal compression of the bubble during injection indicates lack of resistance to injection and correct needle placement in the epidural space .
Another method to verify correct placement in the epidural space is to use the hanging drop technique . To perform this technique , remove the stylet before the needle penetrates the ligamentum flavum , and place a drop of saline solution or local anaesthetic into the needle hub . As the needle penetrates the ligamentum flavum , the fluid is aspirated into the needle because of the subatmospheric pressure within the epidural space . Other ways of identifying the epidural space include the use of a nerve stimulator or identification of appropriate pressure waveforms , though these techniques are rarely necessary . 2-4
Prepare the syringe Once correct needle positioning is confirmed , aspirate a small ( ½ to 1 ml ) bubble of air into the syringe with the drug or drug mixture , and attach the syringe to the needle . Gently aspirate to ensure the needle tip is not in a vein since placement in a vein does not always bleed back and would provide no resistance to air injection ( Figure 5 ). If blood is aspirated , the needle must be removed , and a new spinal needle should be obtained before placing the needle again .
Figure 4 . After positioning the needle , inject 0.5 to 1 ml of air or sterile saline solution to test for resistance to injection . If you encounter no resistance , the needle is correctly positioned in the epidural space .
Inject the drug When you are confident the needle tip is in the epidural space , inject the drug slowly over one to two minutes . vet360
Issue 06 | FEBRUARY 2017 | 16