Methods of measurement of motor blockade
The degree of motor blockade is quantitated in a variety of ways including dynamometry and the Bromage scoring method (1, 2). Dynamometry has been shown to be more accurate than the Bromage method but the latter is simple and can be performed at the bedside.
BROMAGE SCORE (3)
0: No motor block: full flexion of knee and foot.
1: Inability to raise extended leg. Just able to move knee.
2: Inability to flex knee. Able to move foot only.
3: Inability to flex ankle joint. Unable to move foot or knee.
The assessment of sympathetic blockade
The degree of sympathetic blockade is difficult to quantify. It may be assessed indirectly from changes in maternal vital signs which result from the cardiovascular effects of neuraxial blockade.
Sympathetic blockade below the level of the cardiac accelerator fibres (T1 - T4) causes venodilatation, and to a lesser extent arterial vasodilatation, in the lower extremities and abdominal viscera. This may manifest as hypotension the degree of which will be influenced by the state of maternal hydration and the presence of aortocaval compression (Chapter 57).
Sympathetic blockade above the level of the cardiac accelerator fibres (T1 - T4) will cause a bradycardia. However, it should be noted that, even in the absence of a high block, significant decreases in right atrial pressure (decreased venous return) can also result in bradycardia which is mediated by intrinsic chronotropic stretch receptors located in the right atrium and great veins. This effect can be prevented by preservation of venous return. This can be achieved by prehydration, avoidance of aortocaval compression (Chapter 28) and Trendelenberg or leg lift manoeuvres. Blockade of the cardiac sympathetic supply above T4 may also result in hypotension secondary to a loss of myocardial contractile force (4).
Hypotension secondary to sympathetic blockade should be anticipated with every block. If the epidural is felt to be the cause of hypotension, therapy should be targeted at the specific mechanism using volume replacement, venoconstrictors, vagolytics, vasoconstrictors and inotropes as appropriate (Chapter 6). However, it is imperative to exclude other possible causes of hypotension (Table 6.2).
Sympathetic blockade has also been detected indirectly by measuring increases in skin temperature (5, 6). Over 50% of parturients will demonstrate a rise in foot temperature of 3 degrees within 30 minutes of onset of lumbar sympathetic blockade (Chapter 71). Temperature measurement in both feet is useful in detecting unilateral sensory blocks early in the establishment of blockade. A change of posture can then be effected to secure a reliable bilateral block (6) (Chapter 30).
References:
2. Van Zundert A, Vaes L, Soetens M, De Vel M, Maesen F. Measuring motor block during lumbar epidural analgesia for vaginal delivery. Obstet Anesth Digest 1984; 4:31-34
3. Bromage PR. Epidural Analgesia. p144 Philadelphia WB Saunders 1978:
4. Cousins MJ. Physiologic effects of epidural blockade. In: Cousins MJ, Bridenbaugh PO (eds). Neural Blockade in Clinical Anesthesia and Management of Pain, 2nd ed, JB Lippincott Co, Philadelphia,1988. pp 230-2.
6. Griffin RP, Reynolds F. The association between foot temperature and asymmetrical epidural blockade. International Journal of Obstetric Anesthesia 1994 3: 132-136.