To the Editor:—
I wish to congratulate Dr. Karmakar on his very thorough and informative review on paravertebral anesthesia and analgesia. 1
Dr. Karmakar has overlooked one technique of paravertebral blockade that, in my opinion, has shown particular merit and is worthy of further investigation—the use of a nerve stimulator. To my knowledge, the use of a nerve stimulator as a guide to the performance of paravertebral blockade was first alluded to by Drs. J. J. Bonica and F. P. Buckley. 2I have used and refined this technique for more than 5 yr now. As with any technique it has advantages and limitations. Some of its advantages include the following:
A nerve stimulator can be used in a supramaximal mode (Braun, 5.0 mamp) to help identify the paravertebral space. This is especially useful when the anatomy is distorted (e.g. , ankylosing spondylosis, previous surgery, local pathology), unusually challenging (e.g. , morbidly obese), or when the risk of pneumothorax is increased or its potential occurrence particularly undesirable (e.g. , severe chronic obstructive pulmonary disease, ambulatory patient). The diligent use of a nerve stimulator may warn of the “impending danger of a pneumothorax.”
If desired, the motor or sensory end-point can be fine-tuned to allow for successful blockade of a spinal nerve with as little as 1 ml of local anesthetic. For this reason, this technique may be particularly valuable, in conjunction with imaging (i.e. , fluoroscopy), when neurolytic procedures are performed.
As outlined by Dr. Karmakar in his excellent review article 1, paravertebral blocks have several potential limitations including the unpredictability of the “multisegmental single injection technique.” This technique has been popularized presumably because of reluctance to perform a multiple injection technique “that may incur more patient discomfort and risk.” The use of a nerve stimulator allows the precise identification of only those nerves that need to be blocked and provides an additional element of safety. This same principle can be applied to the performance of multiple injection paravertebral blockade when there is uncertainty about the identification of the exact levels targeted (poor correlation between surface landmarks and actual anatomic level) or where the actual levels that need to be blocked are not known with certainty (e.g. , a rib fracture) as the exact levels can be ascertained by the motor responses elicited.
The nerve stimulator is an excellent teaching and research device. It allows precise correlations to be made between anatomy, physiology (motor responses, electrically elicited paresthesiae, and reproduction of pain in the targeted dermatome/s) and clinical effect. There are many more potential advantages but limited space prevents a more extensive discussion.
Potential disadvantages may include the expense of the nerve stimulator and associated insulated needle, and the inability to easily observe a motor response in obese patients. Therefore, it may be prudent to have an assistant that can palpate a motor response. It is always prudent to use all of your senses to guide the needle, as with any technique.
The technique is simple and can be used on either an awake or heavily sedated or anesthetized patient. Any of the approaches described in Dr. Karmakar's review article can be used. 1I sedate the patient with intravenous ketamine (2.5–5.0 mg), versed (0.5–1 mg), and sufentanil (2.5–5.0 μg), administer oxygen by nasal prongs (2 to 3 l/min) and monitor with a pulse oximeter. The nerve stimulator (Braun) is set to deliver a supramaximal current (5.0 mamp). I use an insulated 5 cm Stimuplex needle and have found that it is the ideal length for virtually all adult patients in the thoracic region (T2–T12). The patient is warned that they may feel a pulsating “buzz” or feel some movement in their chest or abdomen. They are asked to report these phenomena as soon as they are perceived. The technique that is chosen determines how the needle is advanced. If the transverse process is encountered the needle is redirected either above or below the transverse process and advanced until a motor or sensory response is elicited. If the transverse process is not encountered the needle is advanced slowly until either a sensory or a motor response is elicited in the distribution of the “ventral” ramus of the spinal nerve. It does not seem necessary to refine the motor end-point although it is my practice to do so. I have found that when the needle is positioned in this manner a motor response will be elicited at a current of between 0.2–2.0 mamp. Approximately 30–40% of the patients will report a simultaneous electrically induced paresthesiae. The technique can also be used to facilitate difficult intercostals nerve blocks.