To the Editor:—
Various methods have been used to detect the onset of lumbar epidural blockade (LEB). A predictor of successful LEB not requiring any direct communication with the patient would be particularly useful. One possibility is the use of a plethysmographic pulse oximeter. Photoelectric digital plethysmography has been shown to detect successful sympathectomy, as evidenced by an increase in pulse amplitude resulting from an increased blood flow to the treated limb. 1,2We, therefore, investigated whether a pulse oximeter could be used to detect successful epidural blockade.
Eighty-eight adult patients, American Society of Anesthesiologists physical status I or II, aged 22–51 yr, weighing 57 ± 14 kg (mean ± SD), of both sexes undergoing lower abdominal procedures during epidural anesthesia (alone or in combination with general anesthesia) were prospectively studied. The protocol was approved by our institutional ethical committee for human research and informed consent was obtained from all patients. Patients with a history of hypertension, diabetes mellitus, or peripheral vascular disease were excluded from the study.
After arrival in operating areas, axillary temperature was measured and two pulse oximeter probes were attached to the big toes. Oximetry was recorded on a Siemens Sirecust 1261 monitor (Siemans Medical Electronics, Inc.), which incorporates plethysmographic pulse oximeter waveform (PPWF) monitoring in its display. Amplitude of the waveform was proportional to the analog signal, not scaled. An epidural catheter (Perifix LOR, B. Braun Melsungen AG, Germany) was inserted at the L2–L3 lumbar interspace and advanced 3 cm cephalad. With the patient supine, a mixture of 10 ml bupivacaine, 0.5%(ASTRA-IDL, Bangalore, India), and 5 ml lidocaine, 2%(ASTRA-IDL), was injected into the epidural space. Maximum height of the PPWF displayed on the two different monitors were measured separately with the help of calipers and a measuring scale before administration of epidural drug mixture (a mixture of lidocaine and bupivacaine; baseline value) and then at 5, 10, 15, 20, 25, and 30 min after epidural drug administration. Simultaneously, the sensory blockade was also tested using the loss-of-cold-sensation test by applying ice cubes from the thoracic to the sacral dermatomes bilaterally at 5-min intervals until 30 min after administration of the epidural drug mixture. The operative procedure began after 30 min of epidural drug administration with or without supplemental general anesthesia. Patients were kept warm throughout. Any change in the height of PPWF from baseline value was computed as a percent of baseline.
Epidural blockade was successful in 82 of 88 patients and failed in 4. In the four patients without evidence of blockade, we saw no change in amplitude. In the 82 with good blocks, we saw a rapid increase in amplitude (fig. 1). In one patient, failure of the pulse oximeter prevented study. In an another patient, one limb showed significant increase in the PPWF, whereas in other limbs no such change in the height of PPWF was observed, and subsequent sensory test (loss of cold sensation) confirmed a unilateral LEB.
Fig. 1. Graph showing amplitude (mean % of baseline) of plethysmographic pulse oximeter waveform in left and right foot at different time intervals.
Fig. 1. Graph showing amplitude (mean % of baseline) of plethysmographic pulse oximeter waveform in left and right foot at different time intervals.
Discussion
The study shows an increase (also visually appreciable) in the amplitude of PPWF after successful LEB. This correlated well with sensory examination (loss of cold sensation). Further, this method detected the onset of LEB earlier than the usual sensory test (via loss of cold sensation).
Different sensory examinations to confirm a successful LEB have been described. 3However, they can not be used in situations in which verbal communication with the patient is difficult. Our method could be useful in such situations. Further, because the amplitude changes are so large, our method may be easier to use than the manual assessment of peripheral temperatures, as described by Asato et al. 4Laser Doppler flowmetry has been used to detect epidural blockade. 5However, apart from its high cost, it is not widely available in the operating room, and its use and interpretation require more skill to learn. The widespread availability of the pulse oximeter in the operating room and its dependence on pulsatile blood flow makes it a simple and safe tool that is easy to interpret.
At 10 min after epidural injection, an amplitude increase of 200% was seen in 79 of 82 patients who underwent successful epidural anesthesia. In contrast, loss of cold sensation at this point was noted in only 14 of 82 patients. Because the sympathetic fibers are first to be blocked after epidural anesthesia, 3the plethysmographic pulse oximeter helps in early detection of onset of LEB, saving precious operating room time in addition to ensuring pain relief. Even failed blockade could be detected early, allowing early institution of appropriate management, such as catheter readjustment or replacement or changing to general anesthesia. Subsequently, the plethysmographic signals could be used to monitor the effect of a repositioned catheter or when a “top-up” dose is to be given.
In conclusion, plethysmography pulse oximetry is useful in predicting successful epidural blockade. Considering its simplicity and effectiveness, we recommend its use to predict successful LEB, especially when verbal communication with the patient is difficult.
The authors thank Mr. Kishore K. Kaul for his kind assistance in data collection.
