Perspective

Interpleural anesthesia is a technique developed in an attempt to simplify body wall and visceral anesthesia after upper abdominal or thoracic surgery. Although considerable research has been carried out on the technique, accurate stratification of the risks and benefits of interpleural anesthesia remains elusive, and it is primarily a technique of historical interest.

Placement

Anatomy

The pleural space extends from the apex of the lung to the inferior reflection of the pleura at approximately L1. The pleural space also relates to the posterior and anterior mediastinal structures, as illustrated in Figure 34-1.

Figure 34-1. Interpleural block: anatomy.

Position

The patient is most often turned to an oblique position with the side to be blocked uppermost, as illustrated in Figure 34-2. The anesthesiologist stands facing the patient’s back.

Figure 34-2. Interpleural block: position and technique.

Needle Puncture

Once the patient is positioned properly and supported by a pillow, a skin wheal is raised immediately superior to the eighth rib in the seventh intercostal space, approximately 10 cm lateral to the midline. If a continuous technique is selected, a needle allowing passage of a catheter (often epidural) is selected. If a single-injection technique is chosen, a short, beveled needle of sufficient length to reach the pleural space can be used. (The proponents of this block most often advocate intermittent injections by catheter; thus, a single-injection technique is unusual.) Before inserting the needle, a syringe containing approximately 2 mL of saline solution is inserted immediately superior to the eighth rib, using a loss-of-resistance technique much like that used during epidural anesthesia. When the needle tip is in the pleural space, it is very easy to inject local anesthetic solution.

Conversely, some clinicians are proponents of a modified hanging-drop technique to identify entry into the pleural space. These anesthesiologists suggest a new term, falling column, to describe this technique. If the syringe plunger shown in Figure 34-2 is removed and the column of solution in the syringe barrel is observed, entry of the needle tip into the pleural space is identified by a falling column of saline solution. The needle is then secured and the procedure continues as it does with the loss-of-resistance method.

Once the needle is in position, either the local anesthetic is injected, if it is to be a single-injection technique, or a catheter is threaded through the needle. If a catheter is used, it should be threaded approximately 10 cm into the pleural space, taking care to minimize the volume of air entrained through the needle. The catheter is then taped in a position that will not interfere with the surgical procedure and local anesthetic is injected. Typically, 20 to 30 mL of local anesthetic is injected, after which the patient is rolled into the supine position to allow distribution of the anesthetic.

Potential Problems

Although pneumothorax might seem to be associated with any technique that violates the pleural space, this complication is apparently infrequent with interpleural anesthesia. A second problem with interpleural anesthesia is the unpredictable nature of the analgesia accompanying what seems to be an otherwise acceptable technique. This may be a result of anesthesiologists’ varying levels of experience with the technique, or perhaps it is the result of overzealous promotion of the technique.

Pearls

The mechanism behind interpleural anesthesia remains uncertain. As illustrated in Figure 34-1, one mechanism proposed is that the local anesthetic diffuses from the pleural space through the intercostal membrane to reach the intercostal nerves along the chest wall. A second mechanism is that the local anesthetic is distributed through the pleura and into the region of the posterior mediastinum, at which point the local anesthetic provides visceral analgesia by contacting the greater, lesser, and least splanchnic nerves. When more data are available, we will probably find that interpleural anesthesia results from a combination of these two mechanisms, plus the absorption of enough local anesthetic from the pleural space to produce blood levels that promote systemic analgesia.