Factors Affecting Epidural Anaesthesia

Site of injection

• After lumbar injection, analgesia spreads both caudally and, to a greater extent, cranially, with a delay at the L5 and S1 segments, due to the large size of these nerve roots.
• After thoracic injection, analgesia spreads evenly from the site of injection. The upper thoracic and lower cervical roots are resistant to blockade due to their larger size. The epidural space in the thoracic region is usually smaller and a lower volume of local anaesthetic is needed.

Dosage

The dose required for analgesia or anaesthesia is determined by several factors but generally, 1-2ml of local anaesthetic is needed per segment to be blocked. The spread of local anaesthetic in the epidural space is unpredictable as the size of the epidural space is variable, as is the amount of local anaesthetic that leaks into the paravertebral space.

The dose (in milligrams) is a function of the volume injected and the concentration of the solution, and the response is not necessarily the same if the same dose is used but in a different volume and concentration. A higher volume of a low concentration of local anaesthetic will result in a larger number of segments blocked but with less dense sensory block and less motor block. It is important to remember that sympathetic nerve fibres have the smallest diameter and are most easily blocked (see below), even with low concentrations of local anaesthetic, and the degree of sympathetic block is related to the number of segments blocked. With an epidural catheter, incremental dosing is possible and this is important in preventing excessively high sympathetic block with hypotension.

The need for repeat or "top-up" doses of local anaesthetic is dependent on the duration of action of the drug. Repeat doses should be given before the block regresses to the extent that the patient experiences pain. A useful concept is the "time to two-segment regression". This is the time from injection of the first dose of local anaesthetic to the point where maximum sensory level has receded by two segments. When two-segment regression has occurred, approximately one half of the original dose should be injected to maintain the block. The time to two-segment regression for lignocaine is 90-150 minutes, and for bupivacaine it is 200-260 minutes.

Age, height & weight

There is an age related decrease in the volume of local anaesthetic needed to achieve a given level of block, presumably due to a decrease in the size and compliance of the epidural space. The patient's height appears to correlate to some extent with the volume of local anaesthetic needed, so that an adult of 5ft should receive a volume of local anaesthetic at the lower end of the range (i.e. 1ml per segment blocked), while volumes up to 2ml per segment may be required for taller patients. The safest approach is to inject incremental doses and monitor the effect carefully. There is little correlation between the weight of a patient and the volume of local anaesthetic needed, although in morbidly obese patients the epidural space may be compressed due to the effect on intra-abdominal pressure, and a smaller volume of local anaesthetic is needed. Furthermore, venous engorgement of the epidural space due to compression of the azygos venous system may further reduce the volume of the epidural space, and increase the risk of puncture of an epidural vein. The same applies to patients with ascites, large intra-abdominal tumours and in the latter stages of pregnancy.

Posture

The effect of gravity during placement of the block has traditionally been assumed to have an effect on the spread of local anaesthetic and thus the area blocked, i.e. in the sitting position the lower lumbar and sacral roots are preferentially blocked, while in the lateral decubitus position, the nerve roots on the dependent side are more densely anaesthetised. Although there is very little scientific evidence that this is the case, the clinical experience of most practitioners suggests that gravity may have some effect.

Vasoconstrictors

Although the addition of vasoconstrictors to local anaesthetic drugs has been shown to prolong anaesthesia with other regional techniques and local infiltration, their effect on epidural anaesthesia is less consistent. With bupivacaine, the addition of adrenaline has not been shown to prolong anaesthesia, while with lignocaine; the addition of adrenaline (usually 1:200 000) does prolong the duration of action. However, vasoconstriction does reduce the amount of systemic absorption of local anaesthetic drugs, and reduces the risk of toxicity.

Alkalinisation of local anaesthetics

Commercially available solutions of local anaesthetics have a pH between 3.5 and 5.5, for chemical stability and bacteriostasis. Most local anaesthetics are weak bases and exist in their ionised (hydrophilic) form at this pH. Since nerve blockade is dependent on penetration of the lipid nerve cell membranes, and the non-ionised (lipophilic) form crosses membranes more easily, it follows that raising the pH of the solution will increase the proportion of drug in the non-ionised form and thus enhance nerve membrane penetration and speed up the onset of blockade. The addition of 8.4% sodium bicarbonate (0.5ml per 10ml of local anaesthetic solution) has become popular in achieving more rapid onset of blockade with, for example, emergency Caesarean Section. 

Physiological Effects of Epidural Blockade

The segmental nerves in the thoracic and lumbar region contain somatic sensory, motor and autonomic (sympathetic) nerve fibres. Sensory and autonomic fibres have a smaller diameter and are more easily blocked than larger, more rapidly-conducting motor fibres. The relationship between sensory and autonomic outflow is complex, but sympathetic block usually extends 1-2 levels higher than sensory block.

Effects on organ systems

• Cardiovascular system. Vasodilatation of resistance and capacitance vessels occurs, causing relative hypovolaemia and tachycardia, with a resultant drop in blood pressure. This is exacerbated by blockade of the sympathetic nerve supply to the adrenal glands, preventing the release of catecholamines. If blockade is as high as T2, sympathetic supply to the heart (T2-5) is also interrupted and may lead to bradycardia. The overall result may be inadequate perfusion of vital organs and measures are required to restore the blood pressure and cardiac output, such as fluid administration and the use of vasoconstrictors. Sympathetic outflow extends from T1 - L2 and blockade of nerve roots below this level, as with, for example, knee surgery, is less likely to cause significant sympathetic blockade, compared with procedures requiring blockade above the umbilicus.
• Respiratory system. Usually unaffected unless blockade is high enough to affect intercostal muscle nerve supply (thoracic nerve roots) leading to reliance on diaphragmatic breathing alone. This is likely to cause distress to the patient, as they may feel unable to breathe adequately.
• Gastrointestinal system. Blockade of sympathetic outflow (T5-L1) to the GI tract leads to predominance of parasympathetic (vagus and sacral parasympathetic outflow), leading to active peristalsis and relaxed sphincters, and a small, contracted gut, which enhances surgical access. Splenic enlargement (2-3 fold) occurs.
• Endocrine system. Nerve supply to the adrenals is blocked leading to a reduction in the release of catecholamines.
• Genitourinary tract. Urinary retention is a common problem with epidural anaesthesia. A severe drop in blood pressure may affect glomerular filtration in the kidney if sympathetic blockade extends high enough to cause significant vasodilatation.
• Effects on cardiovascular physiology during pregnancy. Aortocaval compression by the gravid uterus in the supine position leads to hypotension due to compression of the inferior vena cava, which results in diminished venous return and a drop in cardiac output. Epidural blockade, with its attendant sympathetic blockade, exacerbates the hypotension by causing peripheral vasodilatation. Compression of the aorta also reduces uterine blood flow, and it is thus clear that the combination of aortocaval compression and epidural blockade can have a profound effect on uterine and therefore placental blood flow. The supine position should be avoided in pregnant women undergoing epidural analgesia and anaesthesia, and the patient should be in a lateral (preferably left) or tilted position at all times. Hypotension should be corrected promptly with fluid replacement in the first instance. Alpha-adrenergic drugs, such as methoxamine or phenylephrine, have traditionally been avoided as they cause constriction of uterine vessels and may worsen uterine hypoperfusion. Ephedrine is the drug of choice, as it is primarily a b-agonist and increases blood pressure by increasing cardiac output. However, should profound hypotension occur, a pure vasoconstrictor may be more effective in raising the blood pressure and therefore the uterine perfusion pressure. 

(Continued ...)

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