Epidural Route (Chapter 1):
The variables which are responsible for the spread of anaesthetics are:
1. volume,
2. mass of solution,
3. speed of injection,
4. age,
5. body weight,
6. height, and
7. position of patient (1, 2, 3, 4).
The evidence for an effect of posture on the spread of epidurally-administered local anaesthetics is inconclusive and conflicting. Studies have shown a variable effect of posture with regard to speed of onset, extent of spread and duration of anaesthesia (5, 6, 7, 8).
Nishimura (8) found that posture had no significant effect on the extent of blockade but Bromage (5) showed that there was a slight, but significant, difference in the spread between the supine and the sitting positions.
It is probably fair to say that gravity does play a part in the distribution of anaesthetic solutions with the sitting position favouring caudad spread and the Trendelenberg position favouring cephalad spread. The magnitude of this difference in the extent of blockade is not great.
Spinal Route (Chapter 98):
The spread of local anaesthetics in the subarachnoid space is dependent on:
1. gravity,
2. volume,
3. mass,
4. baricity,
5. speed of injection, and
6. size of needle (9, 10, 11).
Hyperbaric solutions (SG >1.009) will tend to spread caudad in patients in the sitting position and cephalad in patients in the Trendelenberg position (Figure 30.1). As a result of the intrinsic curves of the spine an injection made at L4 or lower with the patient in the supine position will tend to spread caudad and that made at L3 or higher will spread cephalad. Hypobaric solutions (SG < 1.003) will spread in the opposite direction.
The distribution of isobaric solutions (SG between 1.003 and 1.009) is, in the main, unaffected by gravity and, therefore, by patient posture.
References:
1. Bonica JJ. Principles & Practice of Obstetric Analgesia & Anesthesia. Williams & Wilkins, 1995
2. Bromage PR. Epidural Analgesia. Philadelphia. WB Saunders, 1978: 131-148.
4. Hodgkinson R, Husain FJ. Obesity, Gravity and Spread of Epidural Anesthesia. Anesth Analg 1981: 60: 421-424.
5. Bromage PR. Spread of Analgesic Solutions in the Epidural Space and their site of action: A statistical study. Br J Anaesth 1962: 34: 161-178.
6. Apostolou GA, Zarmakoupis PK, Mastrokostopoulos GT. Spread of epidural anesthesia and the lateral position. Anesth Analg 1981: 60: 584-586.
7. Park WY, et al. The sitting position and anesthetic spread in the epidural space. Anesth Analg 1984: 63: 863-864.
8. Nishimura N, Kitahara T, Kusakabe T. The spread of lidocaine and I-131 solution in the epidural space. Anesthesiology 1959: 20: 785-788.
9. Bonica JJ. Principles & Practice of Obstetric Analgesia & Anesthesia. Williams & Wilkins, 1995