Research Column

The Research Committee of SOAP, in an effort to assist members in conducting and evaluating research in obstetric anesthesia, presents this column. If you have ideas, suggestions, or questions for future topics, please write, phone, fax, or E-mail me:

Philip Hess, MD
Dept. of Anesthesiology
Beth Israel Deaconess Medical Center
330 Brookline Ave. East/St-308
Boston, MA 02215
Coordinator, SOAP Research Column
Phone: (617) 667-3112
Fax: (617) 667-7849
E-mail: phess@bidmc.harvard.edu

Measuring Motor Block

Edward T. Riley, MD

Department of Anesthesia
Stanford University School of Medicin
Stanford, CA

The perfect labor analgesic technique would provide complete pain relief with no motor block. Why is maintenance of motor function important? Adequate muscle strength will enable the parturient to push effectively to facilitate spontaneous vaginal delivery. Several studies have shown that epidural techniques utilizing higher concentrations of local anesthetic are associated with an increased incidence of instrumented vaginal delivery and a longer second stage of labor.[1, 2] It is commonly assumed that decreased motor function associated with higher concentrations of local anesthetics causes these adverse outcomes. In addition, patients prefer less motor block; many parturients complain if they cannot move their legs and some find it quite distressing. Some patients want to ambulate during labor or get up to use the bathroom without assistance. Those who do so may succeed in voiding, thus avoiding placement of a urinary catheter. For these reasons, obstetric anesthesiologists have developed and continue to refine analgesic techniques that minimize motor block.

One of the difficulties facing the researcher studying labor analgesic techniques is how to measure motor block. Obviously, if one is interested in a specific outcome, that outcome should be measured. For instance, mode of delivery is the ultimate outcome of interest relative to the effects of motor block. If we wanted to determine which of two techniques had the higher rate of spontaneous vaginal deliveries, we could randomize a series of patients and measure the outcome of labor. The problem with this type of study is that it requires a prohibitively large sample size. For example, if the expected vaginal delivery rates in the two groups were 70% and 80% and one wanted the study to have a power of 0.80 with a significance level of 0.05, 293 subjects would need to be enrolled in each group (586 subjects total)(GB-STAT™ v.6.5, Dynamic Microsytems, Inc, Silver Springs, MD).

The reality of enrolling hundreds of spontaneously laboring, nulliparous women makes such a study difficult, both in cost and in labor for the researchers. However, these types of studies are invaluable in demonstrating important clinical outcomes. Recently the Comparative Obstetric Mobile Epidural Trial study group in the UK reported an investigation in which they randomized 1054 nulliparous women to 3 treatment groups. They demonstrated that women who received labor analgesia that caused less motor block had a significantly higher rate of spontaneous vaginal delivery.[3] Given adequate time and resources, a study like this is the best way to evaluate the effects of motor block in laboring women.

Because it is difficult to generate the resources to conduct such large clinical trials, we often measure surrogate outcomes. One surrogate outcome for the mode of delivery is the length of the second stage of labor. Lesser degrees of motor block have been associated with a shorter second stage of labor. By measuring a continuous variable (time), instead of the proportional variable (mode of delivery), the sample size can be reduced significantly. For example, if the expected durations of the second stage of labor in a two-sample study are 90 min and 120 min with a standard deviation of 60 min, a sample size of only 31 per group (total 62) would have a power of 0.80 at a significance level of 0.05.

Of course motor block can be measured directly. The most frequently used measure of motor block is the Bromage scale.[4] In this scale the intensity of motor block is assessed by the patient's ability to move their lower extremities (Table 1). The most significant shortcoming of the Bromage score in studies of labor analgesia is that it was designed to measure differences in surgical blocks, and is somewhat irrelevant to measuring motor block due to dilute local anesthetic solutions for labor analgesia.

When using the Bromage scale for research in labor analgesia, it is important to measure motor block intermittently throughout labor, as the degree of block will change. For most analyses I document the greatest degree of motor block measured during the period of the study. It is also important to measure motor block in both legs since the block may be asymmetrical.

Several modification of the Bromage scale have been described, including the use of more gradations of motor block. For example, Breen et al. used a six-point scale to assess motor block (Table 2). The value is to differentiate patients in the Bromage score IV.

Table 1. Description of the Bromage score.[4] Grade Criteria Degree of Block

Table 2. Modified Bromage score as used by Breen et al. [5].

Several investigators have measured motor block on a scaled score by quantifying the force of isometric muscle contraction or by using average rectified electromyography.[6-8] Most of these methods are not easy to perform on the labor ward. However, an adaptation of these methods by Graham and McClure may be a viable and useful measure of motor in laboring women.[9] A force transducer in the shape of a dumbbell was placed between the subject's legs with the ends of the dumbbells resting on the medial epicondyles. The subject then squeezed her legs together and this maximal force was recorded. When compared with a modified Bromage score, the correlation coefficient was -0.58, whereas the correlation coefficient between the modified Bromage score and the original Bromage score in these same subjects was only 0.4. Therefore, this methodology is as consistent a measure as the Bromage score, and it is a scaled measure, and therefore should have increased power over the categorical type scores.

Recently, several studies have used the ability of women to walk after the induction of labor analgesia as an outcome variable. Walking is an important outcome variable since many women desire to ambulate during labor. However, there is no proven association between walking and an improved outcome of labor.[2] In addition, the ability to walk is again proportional data. Therefore, large sample sizes will be needed to assess differences among techniques.

The fundamental problem with the Bromage scale, walking studies, and other measures of lower extremity strength for assessing a woman's ability to push in labor, is that a woman does not push a baby through her pelvis with her legs! A better surrogate measure of a woman's ability to push would be to measure intra-uterine or intra-abdominal pressure generated during a contraction.[10] I have not seen any published studies in which intra-uterine or intra-abdominal pressure were used to assess the ability to push as affected by labor analgesic techniques. The reason for this may be the difficulty involved with enrolling laboring women in a study in which they are required to have an intra-uterine or rectal catheter placed. However, I believe this methodology could be useful in labor analgesia studies.

In summary, motor block is assessed in labor analgesia studies because of the assumption that the outcome of labor and maternal satisfaction is affected. The best measure is to assess the outcome of interest. Other measures such as the Bromage scale and isometric muscle contraction strength are easy, non-invasive measures of motor block that may be predictive of the effect a labor analgesic technique has on the outcome of labor. Intra-uterine and intra-abdominal measures of motor strength could be useful ways to assess the effect of labor analgesia.

References

Campbell, D.C., et al. Anesth Analg, 2000. 90(6): p. 1384-9.
Nageotte, M.P., et al. N Engl J Med, 1997. 337(24):p. 1715-9.
UK, C.O.M.E.T.S.G. Lancet, 2001 358(9275): p. 19-23.
Bromage, P.R. 1978, Philadelphia: WB Saunders. 144.
Breen, T.W., et al. Anesth Analg, 1993. 77(5): p. 919-24.
Zaric, D., et al. Reg Anesth, 1996. 21(1): p. 14-25.
Zaric, D., et al. Anesth Analg, 1991. 72(4): p. 509-15.
Liu, S., et al. Anesth Analg, 1995. 80(4): p. 730-4.
Graham, A.C. and J.H. McClure. Anaesthesia, 2001. 56(5): p. 470-6.
Buhimschi, C.S., et al. Lancet, 2001. 358(9280): p. 470-1.