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COMPARATIVE STUDY OF THE EFFECTS OF AIR OR SALINE TO IDENTIFY THE EXTRADURAL SPACE
British Journal of Anaesthesia 1991; 66: 224-227
COMPARATIVE STUDY OF THE EFFECTS OF AIR OR SALINE TO IDENTIFY THE EXTRADURAL SPACE S. J. VALENTINE, A. P. JARVIS AND L. E. SHUTT
Fifty women in labour were allocated randomly to receive either air or saline to assist in the identification of the extradural space by the loss of resistance technique. A study volume of 4 ml of air or saline was used before 0.5% bupivacaine 8 ml and the spread of analgesia was followed for 30 min. The first segment blocked, time of onset, number of blocked segments and height of block were comparable in the two groups. At 30 min, there were eight patients with an unblocked segment in the air group, compared with two in the saline group (? < 0.07). All unblocked segments were blocked subsequently by further doses of bupivacaine. We conclude that air is more likely than saline to produce unblocked segments in the initiation of extradural analgesia in labour. KEY WORDS Anaesthesia: obstetric. Anaesthetic techniques: extradural. Complications unblocked segments.
Since extradural analgesia was popularized by Dogliotti in the 1930s, a variety of techniques have been used to identify the extradural space. Today, the loss of resistance technique using either an air- or saline-filled syringe is the most popular method. The proponents of these two methods disagree about the relative advantages and disadvantages of each medium. However, there have been no prospective comparative studies on the possible effects of the injected air or saline on the subsequent analgesia. The aim of this study was to determine if the choice of either air or saline in the loss of resistance technique would affect the onset, spread or character of the initial extradural injection of local anaesthetic in the patient in labour.
Fifty primiparous patients in early labour who had requested extradural analgesia gave informed verbal consent to the study, which was approved by the local Ethics Committee. Patients who had previously experienced an extradural or spinal procedure, patients in whom urgent delivery was anticipated and those who had a contraindication to extradural analgesia were excluded from the study. The patients were allocated randomly to one of two groups to receive either air or 0.9 % sodium chloride (saline) in equivalent volumes (4 ml) as part of the loss of resistance method of locating the extradural space. The blocks were undertaken by a group of five anaesthetists, including the investigators, all of whom had more than 3 years experience of extradural technique. Under aseptic conditions and following local infiltration with 1 % lignocaine 2-4 ml, extradural puncture was performed with a 16-gauge Tuohy needle (Portex) at either the L2-3 or the L3-4 interspace, with the patient lying in the left lateral position. Following puncture of the ligamentum flavum and satisfactory loss of resistance, 4 ml of the study medium was injected slowly into the space. A catheter was threaded through the Tuohy needle, leaving 2—4 cm in the extradural space. Before the extradural filter was attached, it was flushed with 0.5 % bupivacaine in order to avoid (apart from the small deadspace of the catheter) injection of air on subsequent dosing. As soon as the extradural catheter had been secured, a test dose of 0.5 % bupivacaine 4 ml was S. J. VALENTINE*, M.B., CH.B., F.C.ANAES.; A. P. JARVIS, M.B., CH.B., F.C.ANAES.; L . E. SHUTT, M.B., CH.B., F.C.ANAES.;
Sir Humphry Davy Department of Anaesthesia, Bristol Maternity Hospital, Southwell Street, Bristol BS2 8EG. Accepted for Publication: August 21, 1990. * Present address, for correspondence: Department of Anaesthesia, Queen Alexandra Hospital, Cosham, Portsmouth, Hants PO6 3LY.
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Fransisco on May 27, 2015
PATIENTS AND METHODS
SUMMARY
IDENTIFICATION OF THE EXTRADURAL SPACE
RESULTS
Twenty-five patients received an injection of air and 25 saline, in the loss of resistance technique. There was no difference between the two groups in age, weight and height (table I). There were no untoward cardiovascular side effects in either group, and no dural taps or other complications of extradural insertion. There were a total of eight unblocked dermatomal segments in eight patients in the air group and two unblocked segments in two patients in the saline group (P < 0.01) (table II). All unblocked segments occupied either the T12 or LI dermatome. Three unblocked segments occurred on the patient's left (dependent) side and seven unblocked segments on the right (uppermost) side. Two patients in the air group had blocks which did not extend above LI on one side (one right, one left). All unblocked segments were relieved after the 30-min study period by the injection of 0.5 % bupivacaine 4 ml with the patient lying on the unblocked side.
TABLE I. Patient characteristics (mean (SD or range))
Age (yr) Weight (kg) Height (m)
Air group
Saline group
25.3(17-35) 75.2 (3.9) 1.67(0.29)
24.7 (17-37) 73.8(5.1) 1.65(0.25)
TABLE II. Number of patients with unblocked segments at time 1 + 30 min. * P <0.0l between groups
Right Left Total
Air group
Saline group
6 2 8
1 1 2*
TABLE III. Onset of block (mean (SD))
Right Left
Air group (min)
Saline group (min)
10.5 (5.0) 8.0 (3.3)
9.8 (3.3) 8.5 (2.9)
TABLE IV. First segment to be blocked {median (range))
Right Left
Air group
Saline group
T11(T6-L3) T12 (T6-L3)
T i l (T8-L1) T i l (T8-L1)
There was no significant difference in the time to onset of the block between the groups on either the right or left side (table III), or in the first segment to be blocked on either side (table IV). The range of first segment blocked was wider in the air group compared with the saline group, but this was not significant. There was also no significant difference between the groups on comparing the right and left sides in the number of segments blocked at any of the times tested although, as expected, there was a difference within the groups in the number of blocked segments on the right or left side (table V). The height of the block was comparable also between the two groups (table VI). DISCUSSION
Since the advent of extradural analgesia, many methods have been proposed to identify the extradural space. The majority of methods rely on
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Fransisco on May 27, 2015
given (time t = 0) and, in the absence of any complications, a further 4 ml was given after 5 min (r + 5). All patients remained in the left lateral position for the 30-min duration of the study. An investigator who had no knowledge of the medium used to identify the extradural space assessed onset of sensory loss and dermatomal spread, using sterile 21-gauge needles to test pinprick at 5-min intervals up to 30 min (r + 30). At this time, patients were asked if they felt any discomfort and unblocked dermatomal segments were noted. An unblocked segment was denned as one remaining sensitive to pinprick whilst adjacent segments above and below were pain free. Maternal heart rate and arterial pressure were recorded at 5-min intervals throughout the study. After the 30-min study period, routine management was resumed. In patients with unblocked segments, measures were taken to ensure adequate extradural analgesia (vide infra). Data on the onset and extent of sensory block between and within the two groups were analysed for statistical significance using Student's t test. The incidence of unblocked segments was assessed using chi-square tables with Yates' correction. Data on height of the block between groups were compared using the Wilcoxon rank sum test. P < 0.05 was taken as statistically significant.
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BRITISH JOURNAL OF ANAESTHESIA
226
TABLE V. Number of segments blocked at times t+15, t + 25 inability and t + 30min (mean (SD)). *P<0.05; **P<0.01 within difficulty groups [3]-
to detect a sticky plunger [2] and and delay in detecting a dural puncture
Air is compressible, so that detection of the extradural space is more difficult and false positives are possible [4]. In practised hands, however, t+15 Right 5.7 (3.2) 4.9 (2.5) it does provide an easy and reliable technique, Left 7.3 (3.7) 8.1 (3.7)* particularly if glass syringes are used [5], and t + 20 there is no delay or difficulty in detecting dural 7.3 (2.7) Right 7.3 (3.3) puncture. Disadvantages of using air include the Left 9.4(3.2) 9.8 (3.7) possibility of missed segments [6], venous air t + 25 embolism [7] or cervical subcutaneous emphy8.6 (3.4) Right 8.6 (2.5) sema if large volumes of air are injected into the 10.6(3.2)** 10.9 (3.5)** Left extradural space [8]. t + 30 The frequency of missed or unblocked dermaRight 9.4 (2.6) 9.3(3.5) tomal segments during established extradural Left 10.9 (3.3) 11.5(3.4) analgesia for obstetric patients is thought to be about 6-8% [9]. This is similar to the frequency TABLE VI. Height of block established at times t + 15, t + 20, of missed segments in our saline group. There was t + 25 and t + 30 min (median (range)) a much greater incidence in our air group (32%) following the initial dose of bupivacaine. In every Time Air group Saline group subject in our study, the unanaesthetized dermatome was blocked by additional doses of bupivt+15 T10 (T6-L2) Right acaine. The mechanism of unblocked segments T10 (T7-T12) Left T9 (T6-L2) T9 (T6-T12) is unknown, but thought to be caused by either t + 20 failure of the local anaesthetic solution to reach a Right T9 (T6-T12) T8 (T5-L2) given segment in an adequate concentration, or a Left T8 (T5-T12) T8 (T4-L1) quantitative increase in afferent input through one t + 25 or more segmental nerves, possibly by stretching Right T8 (T4-L2) T9 (T5-T12) of the round ligament [6]. There are several Left T8 (T4-L1) T8 (T4-T12) potential factors to impede the local anaesthetic t + 30 solution reaching segmental nerves, including Right T8 (T4-L2) T9 (T5-T12) malposition of the extradural catheter tip [10], the Left T8 (T4-L1) T8 (T4-T12) type of catheter used [11], the presence of extradural membranes and adhesions [12] and distended extradural veins [6]. the identification of the negative pressure in the extradural space or the loss of resistance enIn this prospective study, our technique was countered on entering the space. The technique standardized; closed-end (three lateral holes) used most widely today involves loss of resistance extradural catheters were used and the patient was detection with either a saline or air filled syringe. maintained in the left lateral position throughout There are advantages and disadvantages to each so as to minimize the known experimental varitechnique. ables. A volume of 4 ml of air or saline was chosen The advantages of using saline include the fact following a consensus view amongst local obstetric that a rigid liquid-filled system is ideal for anaesthetists that 4 ml may be injected easily providing a crisp and unequivocal end-point to during identification of the extradural space the loss of resistance test. Liquid is incompressible although this volume was unlikely to be exceeded and so the transition from complete resistance to in experienced hands. loss of resistance is immediate and convincing. The results of this study indicate that air The distension of the extradural space with saline introduced into the extradural space may sigmay aid the passage of a catheter, but an excess of nificantly increase the incidence of local anaessaline may also dilute the local anaesthetic solution thetic failing to reach the nerve roots, presumably and result in inadequate block [1]. The other by the formation of bubbles. The range of first potential disadvantages of using saline include segment to be blocked was wider in the group of Time
Air group
Saline group
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Fransisco on May 27, 2015
IDENTIFICATION OF THE EXTRADURAL SPACE
extradural space in labouring patients increased the incidence of unblocked segments following the initial injection of local anaesthetic, but that there was no significant difference in the spread of analgesia. REFERENCES 1. Davidson JT. Identification of the cpidural space. Anesthesiology 1966; 27: 859. 2. Bromage PR. Epidural Analgesia. Philadelphia: WB Saunders, 1978; 192. 3. Moore DC. Regional Block, 4th Edn. Springfield. Illinois: Charles C. Thomas, 1965; 419. 4. Sharrock NE. Recordings of, and an anatomical explanation for, false positive loss of resistance during lumbar extradural analgesia. British Journal of Anaesthesia 1979; 51: 253-258. 5. Macintosh R. Lumbar Puncture and Spinal Analgesia, 5th Edn. London: Churchill Livingstone, 1985; 211-215. 6. Bromage PR. Epidural Analgesia. Philadelphia: WB Saunders, 1978; 558-559. 7. Naulty JS, Ostheimcr GW, Datta S, Knapp R, Weiss JB. Incidence of venous air embolism during epidural catheter insertion. Anesthesiology 1982; 57: 410-412. 8. Thomas JE, Schadner S, Reynolds A. Subcutaneous emphysema as a result of loss-of-resistance identification of epidural space. Regional Anesthesia 1982; 7: 44-45. 9. DucrowM. The occurrence of unblocked segments during continuous lumbar epidural analgesia for pain relief in labour. British Journal of Anaesthesia 1971; 43: 11721174. 10. Beck H, Brassow F, Doehn M, Bause H, Dziazka A, Shulte AM, Esch J. Epidural catheters of the multi-orifice type: dangers and complications. Ada Anaesthesiologica Scandinavica 1986; 30: 549-555. 11. Michael S, Richmond MN, Birks RJS. A comparison between open-ended (single hole) and closed-end (three lateral holes) epidural catheters. Anaesthesia 1989; 44: 578-580. 12. Shanks CA. Four cases of unilateral epidural analgesia. British Journal of Anaesthesia 1968; 40: 999. 13. Dalens B, Bazin JE, Haberer JP. Epidural air bubbles as a cause of incomplete analgesia during epidural anesthesia. Anesthesia and Analgesia 1987; 66: 679-683. 14. Philip BK. Effect of epidural air injection on catheter complications. Regional Anesthesia 1985; 10: 21-23. 15. Crawford OB. Comparative evaluation in peridural anesthesia of lidocaine, mepivacaine and L-67, a new local anesthetic agent. Anesthesiology 1964; 25: 321-329. 16. Bromage PR. Mechanism of action of extradural analgesia. British Journal of Anaesthesia 1975; 47 (Suppl.): 199S211S. 17. Duggan J, Bowler GMR, McClure JH, Wildsmith JAW. Extradural block with bupivacaine: Influence of dose, volume, concentration, and patient characteristics. British Journal of Anaesthesia 1988; 61: 324-331.
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Fransisco on May 27, 2015
patients in which air was used. It is also interesting to note the effect of gravity, with more unblocked segments occurring on the right (uppermost) side. The possibility of bubbles of air collecting within the extradural space following use of air with the loss of resistance technique was realized by Macintosh [5] and confirmed recently by Dalens, Bazin and Haberer [13]. They correlated the formation of air bubbles (as seen in peridurograms) with unanaesthetized segments in extradural block in children. It is debatable if this is relevant to obstetric analgesia, as the extradural space in children extends along spinal nerves which may facilitate significant bubble trapping and, while the volume of air used was unspecified, it is likely that relatively more air was used compared with the size of the infant extradural space. A study in which 12 ml of air was injected into the extradural space of patients in labour reported no segmental defect in subsequent analgesia, but no details were given [14]. The results of our study are not in disagreement, as they do not show that using air in the loss of resistance technique leads to persistently unblocked segments. We have demonstrated that use of 4 ml of air rather than of saline leads to a greater incidence of patchy analgesia following initial injection of local anaesthetic. It may be argued that the addition of saline 4 ml had the effect of increasing the volume of local anaesthetic by 50% whilst maintaining the concentration greater than that required to provide sensory block (0.33 %), and that this was the cause of the smaller incidence of unblocked segments in this group. However, the fact that the characteristics of the spread of the block in the two groups did not differ significantly militates against this. The first segment blocked, the time of onset, the total number of blocked segments and the height of the block were the same. This supports the long held view [15, 16], corroborated recently [17], that the dose of local anaesthetic injected, rather than the volume or concentration, is the important determinant of the subsequent spread of extradural block. In conclusion, we have demonstrated that the use of air rather than saline to identify the
227
COMPARATIVE STUDY OF THE EFFECTS OF AIR OR SALINE TO IDENTIFY THE EXTRADURAL SPACE S. J. VALENTINE, A. P. JARVIS AND L. E. SHUTT
Fifty women in labour were allocated randomly to receive either air or saline to assist in the identification of the extradural space by the loss of resistance technique. A study volume of 4 ml of air or saline was used before 0.5% bupivacaine 8 ml and the spread of analgesia was followed for 30 min. The first segment blocked, time of onset, number of blocked segments and height of block were comparable in the two groups. At 30 min, there were eight patients with an unblocked segment in the air group, compared with two in the saline group (? < 0.07). All unblocked segments were blocked subsequently by further doses of bupivacaine. We conclude that air is more likely than saline to produce unblocked segments in the initiation of extradural analgesia in labour. KEY WORDS Anaesthesia: obstetric. Anaesthetic techniques: extradural. Complications unblocked segments.
Since extradural analgesia was popularized by Dogliotti in the 1930s, a variety of techniques have been used to identify the extradural space. Today, the loss of resistance technique using either an air- or saline-filled syringe is the most popular method. The proponents of these two methods disagree about the relative advantages and disadvantages of each medium. However, there have been no prospective comparative studies on the possible effects of the injected air or saline on the subsequent analgesia. The aim of this study was to determine if the choice of either air or saline in the loss of resistance technique would affect the onset, spread or character of the initial extradural injection of local anaesthetic in the patient in labour.
Fifty primiparous patients in early labour who had requested extradural analgesia gave informed verbal consent to the study, which was approved by the local Ethics Committee. Patients who had previously experienced an extradural or spinal procedure, patients in whom urgent delivery was anticipated and those who had a contraindication to extradural analgesia were excluded from the study. The patients were allocated randomly to one of two groups to receive either air or 0.9 % sodium chloride (saline) in equivalent volumes (4 ml) as part of the loss of resistance method of locating the extradural space. The blocks were undertaken by a group of five anaesthetists, including the investigators, all of whom had more than 3 years experience of extradural technique. Under aseptic conditions and following local infiltration with 1 % lignocaine 2-4 ml, extradural puncture was performed with a 16-gauge Tuohy needle (Portex) at either the L2-3 or the L3-4 interspace, with the patient lying in the left lateral position. Following puncture of the ligamentum flavum and satisfactory loss of resistance, 4 ml of the study medium was injected slowly into the space. A catheter was threaded through the Tuohy needle, leaving 2—4 cm in the extradural space. Before the extradural filter was attached, it was flushed with 0.5 % bupivacaine in order to avoid (apart from the small deadspace of the catheter) injection of air on subsequent dosing. As soon as the extradural catheter had been secured, a test dose of 0.5 % bupivacaine 4 ml was S. J. VALENTINE*, M.B., CH.B., F.C.ANAES.; A. P. JARVIS, M.B., CH.B., F.C.ANAES.; L . E. SHUTT, M.B., CH.B., F.C.ANAES.;
Sir Humphry Davy Department of Anaesthesia, Bristol Maternity Hospital, Southwell Street, Bristol BS2 8EG. Accepted for Publication: August 21, 1990. * Present address, for correspondence: Department of Anaesthesia, Queen Alexandra Hospital, Cosham, Portsmouth, Hants PO6 3LY.
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Fransisco on May 27, 2015
PATIENTS AND METHODS
SUMMARY
IDENTIFICATION OF THE EXTRADURAL SPACE
RESULTS
Twenty-five patients received an injection of air and 25 saline, in the loss of resistance technique. There was no difference between the two groups in age, weight and height (table I). There were no untoward cardiovascular side effects in either group, and no dural taps or other complications of extradural insertion. There were a total of eight unblocked dermatomal segments in eight patients in the air group and two unblocked segments in two patients in the saline group (P < 0.01) (table II). All unblocked segments occupied either the T12 or LI dermatome. Three unblocked segments occurred on the patient's left (dependent) side and seven unblocked segments on the right (uppermost) side. Two patients in the air group had blocks which did not extend above LI on one side (one right, one left). All unblocked segments were relieved after the 30-min study period by the injection of 0.5 % bupivacaine 4 ml with the patient lying on the unblocked side.
TABLE I. Patient characteristics (mean (SD or range))
Age (yr) Weight (kg) Height (m)
Air group
Saline group
25.3(17-35) 75.2 (3.9) 1.67(0.29)
24.7 (17-37) 73.8(5.1) 1.65(0.25)
TABLE II. Number of patients with unblocked segments at time 1 + 30 min. * P <0.0l between groups
Right Left Total
Air group
Saline group
6 2 8
1 1 2*
TABLE III. Onset of block (mean (SD))
Right Left
Air group (min)
Saline group (min)
10.5 (5.0) 8.0 (3.3)
9.8 (3.3) 8.5 (2.9)
TABLE IV. First segment to be blocked {median (range))
Right Left
Air group
Saline group
T11(T6-L3) T12 (T6-L3)
T i l (T8-L1) T i l (T8-L1)
There was no significant difference in the time to onset of the block between the groups on either the right or left side (table III), or in the first segment to be blocked on either side (table IV). The range of first segment blocked was wider in the air group compared with the saline group, but this was not significant. There was also no significant difference between the groups on comparing the right and left sides in the number of segments blocked at any of the times tested although, as expected, there was a difference within the groups in the number of blocked segments on the right or left side (table V). The height of the block was comparable also between the two groups (table VI). DISCUSSION
Since the advent of extradural analgesia, many methods have been proposed to identify the extradural space. The majority of methods rely on
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Fransisco on May 27, 2015
given (time t = 0) and, in the absence of any complications, a further 4 ml was given after 5 min (r + 5). All patients remained in the left lateral position for the 30-min duration of the study. An investigator who had no knowledge of the medium used to identify the extradural space assessed onset of sensory loss and dermatomal spread, using sterile 21-gauge needles to test pinprick at 5-min intervals up to 30 min (r + 30). At this time, patients were asked if they felt any discomfort and unblocked dermatomal segments were noted. An unblocked segment was denned as one remaining sensitive to pinprick whilst adjacent segments above and below were pain free. Maternal heart rate and arterial pressure were recorded at 5-min intervals throughout the study. After the 30-min study period, routine management was resumed. In patients with unblocked segments, measures were taken to ensure adequate extradural analgesia (vide infra). Data on the onset and extent of sensory block between and within the two groups were analysed for statistical significance using Student's t test. The incidence of unblocked segments was assessed using chi-square tables with Yates' correction. Data on height of the block between groups were compared using the Wilcoxon rank sum test. P < 0.05 was taken as statistically significant.
225
BRITISH JOURNAL OF ANAESTHESIA
226
TABLE V. Number of segments blocked at times t+15, t + 25 inability and t + 30min (mean (SD)). *P<0.05; **P<0.01 within difficulty groups [3]-
to detect a sticky plunger [2] and and delay in detecting a dural puncture
Air is compressible, so that detection of the extradural space is more difficult and false positives are possible [4]. In practised hands, however, t+15 Right 5.7 (3.2) 4.9 (2.5) it does provide an easy and reliable technique, Left 7.3 (3.7) 8.1 (3.7)* particularly if glass syringes are used [5], and t + 20 there is no delay or difficulty in detecting dural 7.3 (2.7) Right 7.3 (3.3) puncture. Disadvantages of using air include the Left 9.4(3.2) 9.8 (3.7) possibility of missed segments [6], venous air t + 25 embolism [7] or cervical subcutaneous emphy8.6 (3.4) Right 8.6 (2.5) sema if large volumes of air are injected into the 10.6(3.2)** 10.9 (3.5)** Left extradural space [8]. t + 30 The frequency of missed or unblocked dermaRight 9.4 (2.6) 9.3(3.5) tomal segments during established extradural Left 10.9 (3.3) 11.5(3.4) analgesia for obstetric patients is thought to be about 6-8% [9]. This is similar to the frequency TABLE VI. Height of block established at times t + 15, t + 20, of missed segments in our saline group. There was t + 25 and t + 30 min (median (range)) a much greater incidence in our air group (32%) following the initial dose of bupivacaine. In every Time Air group Saline group subject in our study, the unanaesthetized dermatome was blocked by additional doses of bupivt+15 T10 (T6-L2) Right acaine. The mechanism of unblocked segments T10 (T7-T12) Left T9 (T6-L2) T9 (T6-T12) is unknown, but thought to be caused by either t + 20 failure of the local anaesthetic solution to reach a Right T9 (T6-T12) T8 (T5-L2) given segment in an adequate concentration, or a Left T8 (T5-T12) T8 (T4-L1) quantitative increase in afferent input through one t + 25 or more segmental nerves, possibly by stretching Right T8 (T4-L2) T9 (T5-T12) of the round ligament [6]. There are several Left T8 (T4-L1) T8 (T4-T12) potential factors to impede the local anaesthetic t + 30 solution reaching segmental nerves, including Right T8 (T4-L2) T9 (T5-T12) malposition of the extradural catheter tip [10], the Left T8 (T4-L1) T8 (T4-T12) type of catheter used [11], the presence of extradural membranes and adhesions [12] and distended extradural veins [6]. the identification of the negative pressure in the extradural space or the loss of resistance enIn this prospective study, our technique was countered on entering the space. The technique standardized; closed-end (three lateral holes) used most widely today involves loss of resistance extradural catheters were used and the patient was detection with either a saline or air filled syringe. maintained in the left lateral position throughout There are advantages and disadvantages to each so as to minimize the known experimental varitechnique. ables. A volume of 4 ml of air or saline was chosen The advantages of using saline include the fact following a consensus view amongst local obstetric that a rigid liquid-filled system is ideal for anaesthetists that 4 ml may be injected easily providing a crisp and unequivocal end-point to during identification of the extradural space the loss of resistance test. Liquid is incompressible although this volume was unlikely to be exceeded and so the transition from complete resistance to in experienced hands. loss of resistance is immediate and convincing. The results of this study indicate that air The distension of the extradural space with saline introduced into the extradural space may sigmay aid the passage of a catheter, but an excess of nificantly increase the incidence of local anaessaline may also dilute the local anaesthetic solution thetic failing to reach the nerve roots, presumably and result in inadequate block [1]. The other by the formation of bubbles. The range of first potential disadvantages of using saline include segment to be blocked was wider in the group of Time
Air group
Saline group
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Fransisco on May 27, 2015
IDENTIFICATION OF THE EXTRADURAL SPACE
extradural space in labouring patients increased the incidence of unblocked segments following the initial injection of local anaesthetic, but that there was no significant difference in the spread of analgesia. REFERENCES 1. Davidson JT. Identification of the cpidural space. Anesthesiology 1966; 27: 859. 2. Bromage PR. Epidural Analgesia. Philadelphia: WB Saunders, 1978; 192. 3. Moore DC. Regional Block, 4th Edn. Springfield. Illinois: Charles C. Thomas, 1965; 419. 4. Sharrock NE. Recordings of, and an anatomical explanation for, false positive loss of resistance during lumbar extradural analgesia. British Journal of Anaesthesia 1979; 51: 253-258. 5. Macintosh R. Lumbar Puncture and Spinal Analgesia, 5th Edn. London: Churchill Livingstone, 1985; 211-215. 6. Bromage PR. Epidural Analgesia. Philadelphia: WB Saunders, 1978; 558-559. 7. Naulty JS, Ostheimcr GW, Datta S, Knapp R, Weiss JB. Incidence of venous air embolism during epidural catheter insertion. Anesthesiology 1982; 57: 410-412. 8. Thomas JE, Schadner S, Reynolds A. Subcutaneous emphysema as a result of loss-of-resistance identification of epidural space. Regional Anesthesia 1982; 7: 44-45. 9. DucrowM. The occurrence of unblocked segments during continuous lumbar epidural analgesia for pain relief in labour. British Journal of Anaesthesia 1971; 43: 11721174. 10. Beck H, Brassow F, Doehn M, Bause H, Dziazka A, Shulte AM, Esch J. Epidural catheters of the multi-orifice type: dangers and complications. Ada Anaesthesiologica Scandinavica 1986; 30: 549-555. 11. Michael S, Richmond MN, Birks RJS. A comparison between open-ended (single hole) and closed-end (three lateral holes) epidural catheters. Anaesthesia 1989; 44: 578-580. 12. Shanks CA. Four cases of unilateral epidural analgesia. British Journal of Anaesthesia 1968; 40: 999. 13. Dalens B, Bazin JE, Haberer JP. Epidural air bubbles as a cause of incomplete analgesia during epidural anesthesia. Anesthesia and Analgesia 1987; 66: 679-683. 14. Philip BK. Effect of epidural air injection on catheter complications. Regional Anesthesia 1985; 10: 21-23. 15. Crawford OB. Comparative evaluation in peridural anesthesia of lidocaine, mepivacaine and L-67, a new local anesthetic agent. Anesthesiology 1964; 25: 321-329. 16. Bromage PR. Mechanism of action of extradural analgesia. British Journal of Anaesthesia 1975; 47 (Suppl.): 199S211S. 17. Duggan J, Bowler GMR, McClure JH, Wildsmith JAW. Extradural block with bupivacaine: Influence of dose, volume, concentration, and patient characteristics. British Journal of Anaesthesia 1988; 61: 324-331.
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patients in which air was used. It is also interesting to note the effect of gravity, with more unblocked segments occurring on the right (uppermost) side. The possibility of bubbles of air collecting within the extradural space following use of air with the loss of resistance technique was realized by Macintosh [5] and confirmed recently by Dalens, Bazin and Haberer [13]. They correlated the formation of air bubbles (as seen in peridurograms) with unanaesthetized segments in extradural block in children. It is debatable if this is relevant to obstetric analgesia, as the extradural space in children extends along spinal nerves which may facilitate significant bubble trapping and, while the volume of air used was unspecified, it is likely that relatively more air was used compared with the size of the infant extradural space. A study in which 12 ml of air was injected into the extradural space of patients in labour reported no segmental defect in subsequent analgesia, but no details were given [14]. The results of our study are not in disagreement, as they do not show that using air in the loss of resistance technique leads to persistently unblocked segments. We have demonstrated that use of 4 ml of air rather than of saline leads to a greater incidence of patchy analgesia following initial injection of local anaesthetic. It may be argued that the addition of saline 4 ml had the effect of increasing the volume of local anaesthetic by 50% whilst maintaining the concentration greater than that required to provide sensory block (0.33 %), and that this was the cause of the smaller incidence of unblocked segments in this group. However, the fact that the characteristics of the spread of the block in the two groups did not differ significantly militates against this. The first segment blocked, the time of onset, the total number of blocked segments and the height of the block were the same. This supports the long held view [15, 16], corroborated recently [17], that the dose of local anaesthetic injected, rather than the volume or concentration, is the important determinant of the subsequent spread of extradural block. In conclusion, we have demonstrated that the use of air rather than saline to identify the
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