Assessment of block height for satisfactory spinal anaesthesia for caesarean section

Authors


Correspondence to: A. M. Cyna
Email: allan.cyna@health.sa.gov.au

Summary

We investigated block heights that anaesthetists considered adequate for caesarean section to proceed under spinal anaesthesia. During 3 months, 15 obstetric anaesthetists recorded block height to touch, pinprick or cold when spinal anaesthesia was considered satisfactory for caesarean section to proceed. Median (IQR [range]) block height for touch, pinprick, first cold and icy were: T10 (T7–T12 [T3–L1]); T5 (T4–T6 [C7–L1]); T5 (T4–T6 [C7–L1]); and T3 (T2–T4 [C7–L1]), respectively. Modalities were significantly correlated for: touch and cold, p = 0.0001; touch and icy, p = 0.0007; touch and pinprick, p = 0.0018; cold and icy, p < 0.0001; cold and pinprick, p = 0.0001; icy and pinprick, p < 0.0001. Pairwise comparisons showed differences between all modalities (p < 0.001) apart from pinprick and first cold (p = 0.94). All women had satisfactory anaesthesia despite 76 (81%) having a block to touch below T6. Single modality assessment of block height, particularly using touch, may erroneously indicate inadequate anaesthesia for caesarean section.

The assessment of block height following the administration of a regional anaesthetic block for caesarean section is said to be important for determining whether there is satisfactory anaesthesia to allow surgery to proceed and to practise to an appropriate standard [1]. Pain during caesarean section is one of the more common reasons for a successful medicolegal claim [2]. However, assessments of block height rely on the subjective experience and perceptions of the patient, and the relationship between block height as assessed by touch, pinprick or cold is variable [3, 4]. A number of methods are used to assess block height [5] and some workers have argued that the assessment of adequacy of regional block with cold or sharp pinprick is at best unhelpful, and at worst misleading, and that these modalities should be abandoned in favour of touch. Furthermore, it has been stated that, ‘If a block fails in mid-surgery, even with cold or pinprick level at or above T4, and there is no assessment indicating an adequate level of block to touch preoperatively, then difficulties for the anaesthetist lie ahead should litigation ensue’ [6]. Although this view was expressed in 2001 and based on older data mostly obtained before the routine use of lipid soluble opioids, other authorities have also stated that ‘we should be using touch rather than cold’ for testing block height [1]. However, the reliance on touch alone could lead to unnecessarily increasing the dose of local anaesthetic injected during spinal anaesthesia, delaying the start of caesarean section, or even abandoning perfectly adequate anaesthesia [7]. Modalities available to assess block height include the Neurotip™ (Owen Mumford, Oxford, UK), ethyl chloride spray and light pressure with a finger to assess touch; an 18-G needle to assess pinprick; and ethyl chloride spray, alcohol swab and ice to assess temperature [5, 8–10]. There are problems with some of these methods of assessment. For example, the Neurotip may not be commonly available; an 18-G cannula can cause trauma to the skin; and ethyl chloride spray is relatively costly, unavailable in many countries, and unfriendly to the environment. Ice, touch and pinprick are commonly utilised methods of determining block height after spinal anaesthesia [5] perhaps due in part to low cost, ready availability and clinicians’ familiarity.

Although various modalities have been tested in a controlled way at various time intervals after injection of intrathecal injectate [3, 9], there appears a paucity of literature reporting the assessment of block height at the time point when the anaesthetist considers that it is satisfactory for the obstetrician to proceed with caesarean section. We therefore aimed to investigate the block height levels recorded by anaesthetists in our institution when spinal anaesthesia was assessed as clinically satisfactory for caesarean section to proceed.

Methods

The study was an observational study conducted in the largest tertiary referral centre for maternity care in South Australia and ran from 22 September until 21 December 2009. Following local human research ethics committee approval, anaesthetists were asked to use a modified anaesthetic chart for recording assessments of block height when they considered that spinal anaesthesia was adequate for caesarean section to proceed. This form included a diagram of dermatomes and a table in which to record the block height for each modality tested (Fig. 1). As such assessments were part of routine anaesthetic care in our institution, the requirement for written patient consent was waived by our ethics committee.

Figure 1.

 Diagram of dermatomes and table used to record the block height assessment for each modality tested.

All women presenting for elective caesarean section under spinal anaesthesia were potentially eligible for inclusion. Patients were excluded if they required an interpreter, were unable to understand the distinction between modalities of block, were deaf, or had a neurological disorder such as stroke. Information regarding spinal anaesthesia was documented in, and later retrieved from, the patient’s anaesthetic chart, including time of injection of drug; drug dose; spinal technique; time and block height when anaesthesia was considered satisfactory for surgery to proceed; and any interventions recorded during surgery such as supplementation or conversion to general anaesthesia.

Assessment of block height by the anaesthetist was performed on both sides using touch, pinprick and cold in a standardised manner on each patient, using a standardised explanation to patients. Ice was applied above the patient’s clavicle as a reference point and then applied to the L1 dermatome, asking the patient ‘Can you feel me touch you?’ If the woman stated that they could not feel anything, women were asked to report when they first felt touch as the ice was moved continuously cephalad in the mid-clavicular line. Once the patient stated at which dermatome level they were able to feel touch, they were then asked to report when it felt cold. Once patients stated that they could feel cold, they were then asked to report when it felt icy cold without change from the reference point. Pinprick was assessed using an Interlink® vial access cannula (BD™, North Ryde NSW, Australia; designed for needleless access of rubber-topped vials) applied initially to the patient’s clavicle. Then beginning at L1, the vial access cannula was applied to the skin to successively more cephalad dermatomes until the woman perceived the sensation to be as sharp as that felt at the reference point. The block heights for each modality were assessed and recorded when the anaesthetist considered that the woman had a satisfactory spinal anaesthesia block for caesarean to proceed. Anaesthetists also recorded whether the patient reported discomfort or needed an analgesic/anaesthetic intervention at any time during surgery, as well as the time and stage of the intervention. Postoperatively, either in the recovery room or during routine anaesthetic follow-up the next day, patients were questioned in a standardised manner regarding whether they were comfortable and satisfied with their anaesthesia care during surgery and if they would have preferred a general anaesthetic. Those patients missing assessments before discharge from hospital were followed up over the telephone at home.

The values analysed were the lower block height level on the left or right sides for each modality for: first sensation of touch; first sensation of cold; icy cold; and pinprick when felt the same as the reference point. The spinal segments from C7 or above to L2 were numbered 1–15 for data analysis. The Kruskal-Wallis test was used to compare block levels between modalities, followed by pairwise comparisons for significant differences. Friedman’s test was used to determine whether there were differences between right and left heights of block. Associations between the different modalities were determined using Spearman’s correlations. Values of p ≤ 0.05 were considered significant.

Results

Fifteen anaesthetists participated in the study: seven consultants completed between one and 10 anaesthetic charts each and eight registrars completed between one and 11 charts each. Ninety-four women were included in the study. All the anaesthetic charts had data suitable for analysis, although two patients’ records had missing data for touch block height and three for pin prick. Indications for surgery were previous caesarean section 57 women (61%); breech or transverse presentation 10 women (11%); previous vaginal trauma/surgery 4 (4%); twins 4 (4%); pre-eclampsia 4 (4%); maternal request 3 (3%); intrauterine growth restriction 2 (2%); placenta praevia 2 (2%); postdates 2 (2%); and one each for macrosomia, fetal abnormality, gestational diabetes and pelvic fracture. The indications in two cases were unclear. The median (IQR [range]) age of the women was 34 (28–37 [20–45]) years and their booking weight was 77 (68–93 [48–130]) kg. The majority of women spoke English as their primary language and only 26 (28%) spoke it as a second language. Spinal anaesthesia was with heavy bupivacaine 0.5%, with a median (IQR [range]) dose of 2.2 (2.2–2.3 [2.0–2.6]) ml with 15 (15–15 [10–20]) μg fentanyl and/or 125 (125–150 [100–150]) μg morphine. Eighty-seven women received both fentanyl and morphine, four women received morphine alone and three received fentanyl alone. The time from injection to final assessment of adequate block height was documented in 64 charts (68%). The mean (SD [range]) time from injection to the decision to proceed was 9.5 (3.0 [5.0–20.0]) min.

Pairwise comparisons of block heights are shown in Table 1 where differences were seen between all modalities (p < 0.001) apart from pinprick and first cold (p = 0.94). Modalities were significantly correlated for: touch and cold, p = 0.0001; touch and icy, p = 0.0007; touch and pinprick, p = 0.0018; cold and icy, p < 0.0001; cold and pinprick, p = 0.0001; and icy and pinprick, p < 0.0001. No significant differences were found between the two sides for touch, p = 0.51; pinprick, p = 0.56; cold, p = 0.11; and icy, p = 0.38. Overall, there were significant differences in block level between modalities (p < 0.001) with pairwise comparisons showing differences between all modalities p < 0.001, apart from pinprick and first cold p = 0.94 (Table 1).

Table 1. Block heights and number of patients assessed as having satisfactory anaesthesia for surgery to proceed in whom block height was below T6, in 94 women undergoing caesarean section under spinal anaesthesia. Values are median (IQR [range]).
 TouchPinprickFirst coldIcy cold
  1. *p < 0.001; †p = 0.001; ‡p = 0.001; §p = 0.94; ¶p = 0.001; **p < 0.001.

  2. ap < 0.001; bp = 0.001; cp = 0.001; dp = 0.3981; ep = 0.0003; fp < 0.0028.

Block heightT10 (T7–T12 [T3–L1])*†‡T5 (T4–T6 [C7–L1])*§¶T5 (T4–T6 [C6–L1])†§**T3 (T2–T4 [C6–L1])‡¶**
Block height below T676 (81%)abc15 (16%)ade11 (12%)bdf1 (1%)cef
Block height below T6 and requiring intervention6 (6%)3 (3%)3 (3%)1 (1%)

Several women had blocks documented as lower than T6 despite the anaesthetists’ assessment of the block as adequate for surgery to proceed (Table 1). Ten women reported no loss of sensation to touch (i.e. recorded as below L1 bilaterally), and an additional three women had no loss of sensation to touch unilaterally. Of these 13 women, 11 (85%) had anaesthesia without needing any intervention, while two (Patients 3 and 5 in Table 2) required an intervention. Only 16 women had loss to touch at T6 or higher, with 76 (81%) maintaining a sensation of touch at this dermatome. One woman stated that she felt all modalities on testing despite having a profound motor block of the lower limbs and a painless urinary catheter insertion. She agreed that surgery could proceed provided that general anaesthesia was immediately available if required. Her surgery also proceeded satisfactorily without the need for intervention. Six women (6%) required an intervention for discomfort (Table 2). Although two women were dissatisfied with aspects of their birth experience, all had satisfactory anaesthesia to complete their caesarean section surgery. None of the women in our study required, or expressed a desire for, general anaesthesia at any time. Follow-up of women after surgery was obtained from 48 women (51%) in the recovery room, 29 (31%) on the first postoperative day, and 11 (12%) by phone call after discharge. Six women were lost to follow-up postoperatively and could not be contacted after discharge. However, all six were documented to have had an uneventful anaesthetic without intervention during their caesarean section.

Table 2. Patients requiring an intervention during caesarean section under spinal anaesthesia.
PatientLevel of blockComments
TouchPinprickFirst coldIcy cold
  1. CSF, cerebrospinal fluid; iv, intravenous.

1T10T3T3T2Discomfort due to loss of proprioception. Fentanyl 75 μg given iv after the baby was born. Patient reported at postoperative follow-up that she was satisfied and comfortable during her anaesthetic
2L1L1L1L1Previous failed spinal. Difficult to assess. Fentanyl 50 μg given iv towards end of surgery. Patient reported being satisfied with, and comfortable during, her anaesthetic at postanaesthesia follow-up
3Below L1T3T3C7Fentanyl 50 μg given iv 30 min after first incision. Local anaesthetic required to complete suturing. Patient reported that she was satisfied with, and comfortable during, her anaesthetic at post anaesthesia review
4T9T6T7T4Complaints of headache and neck pain. Fentanyl given iv. Satisfied and comfortable with anaesthetic
5Below L1T8T7T5Slow onset of block despite free flowing CSF. Fentanyl 100 μg given iv immediately before delivery and 20 min after delivery. Not satisfied but stated on postoperative review that she was comfortable during anaesthetic
6T10T8T6T4Chest pain, no abdominal pain. Reassurance given. Satisfied and comfortable with anaesthetic

Discussion

This is the first study to report modality assessments of block height at the time point when spinal anaesthesia was assessed by the anaesthetist as satisfactory for surgery to proceed, rather than at various stipulated time intervals after the administration of intrathecal injectate.

We have found a significant correlation of block heights between modalities suggesting that all provide potentially useful information about block height albeit at different levels. We found a wide variation in the assessments of block height between modalities and as previously reported [9]; these were lower for touch than for cold or pinprick, which in turn were lower than that measured as icy without change from the reference point (Table 1). More than three quarters of the women in our study failed to have a block to touch that reached T6 at the time anaesthesia was thought to be adequate to commence surgery, and of these, only five (5%) required an additional pharmacological intervention (Table 2). None of these women, when questioned, reported that they would have preferred conversion to a general anaesthetic. While situations will always exist in which a general anaesthetic is the most prudent management, premature or unnecessary abandonment of an adequate regional anaesthetic block, based on a single method of assessment, may result in the mother’s being unable to participate in the birth experience, and risks maternal morbidity and even mortality. The inherent inaccuracy of assessment of block height has been well documented and considerable variation may exist between individual anaesthetists [10–14]. The critical importance of communication and the way anaesthetists question patients about their block have been emphasised [12].

Our results suggest that the traditional belief that there is one modality where a particular level of block height ensures adequate anaesthesia in every patient, is an unrealistic goal. Defining a dermatome level above which anaesthesia is always satisfactory and below which it is always inadequate denies the reality that patients’ reports of their experiences are subjective and can be modulated by suggestion and expectations both of the anaesthetist and the patient [15]. It may be more helpful to assess whether the progression of the block is behaving as would be expected, with the modality of choice, and the way it is tested, the one that is most familiar and provides most reassurance to the anaesthetist.

Fewer than 10% of patients in our study required an intervention during surgery and these were largely unrelated to assessments of block height but due to symptoms such as anxiety, loss of sensation and chest discomfort. Exactly why anaesthetists administered an intervention is unknown as they were not specifically questioned in this regard. Although it might be assumed that this would be for pain, this may not necessarily have been the case as all of these patients, when questioned postoperatively, stated that they were comfortable during their caesarean section, satisfied with their anaesthesia and would not have preferred general anaesthesia (Table 2). Patients’ ability to communicate perceptions accurately in the highly stressed setting of undergoing a caesarean section under regional anaesthesia also needs to be taken into account [16].

When last surveyed, the most common method of assessing block height in the UK was the use of temperature change [5], and anaesthetists more recently have advocated its continued use [17]. In contrast to strong opinions to the contrary [3, 4, 12], our study findings suggest that height of block assessments to cold may be reliable in most cases and appear much more likely to avoid assessing an adequate block as one that is unsatisfactory for surgery to proceed. Whilst some may argue that ‘most’ isn’t enough, Russell’s own definition of an adequate block height to touch for caesarean section to start is when T6 is ‘either blocked to touch, or expected to be blocked to touch before surgery had reached the peritoneal cavity’ [3]. This definition relies on the subjective expectation of the anaesthetist that a level of T6 during surgery will be achieved during, rather than reporting an assessment before, surgery. In contrast, we have not used an ‘expectation’ component in the assessment but the actual block level achieved before commencing surgery. In addition, where other researchers have assessed ‘normal touch’ against a reference point before surgery, they report that a number of patients required intervention despite a block height to normal touch above T6 [12]. While a more standardised assessment of block has been recommended [1, 4, 18], the subjective influences that inevitably occur when assessing block height will remain. Interestingly, the wide variations seen in our study are reflected in the findings of strong advocates for the use of touch, where block levels have been found on occasion to reveal up to eight dermatomes difference when touch is compared with pinprick [3]. As previously stated, one patient reported feeling cold, pinprick and touch at all levels despite having complete bilateral lower limb block and a painless urinary catheter insertion. On skin testing over the incision site, the patient was unaware of pain and the patient reported postoperatively that she was comfortable during, and satisfied with, her anaesthesia, neither needing nor desiring general anaesthesia at any stage.

There were some limitations of this study. First, the height of some of the blocks was likely to have increased by the time of first incision. However, it was clear that in a minority of women, a block height of T6 to touch was never going to be achieved despite clear evidence of subsequent satisfactory anaesthesia during surgery. This suggests that abandoning such adequate blocks would expose women to the risks associated with general anaesthesia in pregnancy. Use of the plastic spike of the vial access cannula to assess pinprick is not a widely described validated method of testing height of block. However, this cannula is the standard way we assess pinprick in our institution, and minimises the risk of penetrating the patient’s skin with a metal needle. Although we assessed pinprick and icy cold according to a reference point, it may have also been useful to test at what level the ice was first perceived as icy. The use of an ice cube to assess touch and not a separate standardised tool such as the Neurotip may be criticised, but we find that our use of ice, to assess the first sensation of touch, cold and icy cold, is an easy and inexpensive way for most patients to communicate to the anaesthetist the height of block, and reflects current practice within our institution. Our height of block assessment using the first sensation of touch rather than normal touch against a reference point may be questioned, but the first sensation of light touch is reported to be ‘the best predictor of the likely efficacy of a spinal or epidural block’ [12]. Interestingly, there seems to be ‘a close association between the levels of block to touch assessed by the ethyl chloride spray and the Neurotip’ and ‘in normal clinical practice ethyl chloride spray touch sensation can be considered equivalent to Neurotip touch in predicting the likely adequacy of spinal anaesthesia’[3]. It would be interesting to investigate whether light touch with ice is similarly equivalent. Unfortunately, the time from final testing to commencing surgery was not collected; although this would not have affected our results or conclusions, it would be of interest to collect this information in future research on this topic. We did not ask anaesthetists to change their usual practice and assess maximum block height, as once there was a decision for surgery to proceed, further assessments are rarely performed. Although anaesthetists in this study used only a figure on the anaesthetic chart as a guide for anatomical reference points, it has been shown recently that even when dermatomes are clearly marked by tape fixed to the midline of the patient’s torso, individual anaesthetists show a wide variation in their assessment of block height levels [18]. The study investigators collated data and transcribed it onto a spreadsheet, which could potentially be a source of bias. However, all data were checked by a second investigator (RO/CE) to ensure that they were accurately transcribed from the anaesthesia charts. The use of multiple anaesthetist assessors, while potentially reducing the likelihood of a standardised assessment, has the advantage of being a more accurate reflection of clinical practice, taking into account the variability in assessments made by different practitioners. All five patients administered a pharmacological intervention during their surgery reported that they were comfortable and satisfied with their anaesthesia when questioned postoperatively about their experience (Table 2). This suggests that the anaesthetist’s use of supplemental analgesia may not always be because of pain or that any pain experienced when managed satisfactorily will subsequently lead to an experience that is remembered postoperatively as being comfortable. Either way, the need for intervention during surgery in our study does not appear to have been of any significance to these patients. Finally, our research ethics committee approved the study without the requirement for obtaining written consent from patients. However, we recognise on reflection that best practice would have been to obtain written consent.

Further areas for research in this field might include auditing current practice of anaesthetists in other institutions within Australia and elsewhere. It would also be useful to identify which modalities or combinations are likely to provide the most clinically useful information to anaesthetists, and how touch from a continuous movement of an ice cube relates to a Neurotip assessment or how continuous movement relates to single-point placement. Also, it would be of interest to determine how the perception of first touch to ice compares with perceived first touch and normal touch by other methods. It is reported that the use of the term ‘touch’ and how this particular sensory modality is assessed may represent different meanings to different clinicians and researchers [12]. Future studies may also wish to investigate how the way anaesthetists communicate with the patient affects her responses about block height. A comparison of which modality anaesthetists and patients found most reassuring in the assessment of their block, and other factors apart from height of block that may affect the decision to proceed with caesarean section following spinal anaesthesia, would also be of interest. Outcomes such as the use of an intervention during surgery may reflect anaesthetists’ perceptions and expectations rather than patient’s pain or distress. Future studies should consider reporting why interventions were administered and whether these were requested by the patient with and without prompting. In addition, it would be useful to know how the administration of an intervention related to measures of pain experienced. Our study findings suggest that height of block is only one, albeit important, factor determining patient satisfaction and perception of comfort.

In summary, our study has highlighted the large variation and complexity that exists within this area. Although the majority of assessments of block height are straightforward, their subjective nature not infrequently leaves the anaesthetist facing a multitude of clinical data when assessing a balance of risks. We are not in any way suggesting that clinicians should proceed in the absence of a discernible or adequately assessed block. However, all women in our study reported being comfortable and satisfied with their anaesthesia despite over 80% having a block to touch below T6 when anaesthesia was considered adequate for surgery to proceed. Communicating with the patient and checking at regular intervals both before and throughout the operation may be more important in mitigating patient dissatisfaction and any potential for litigation than advocating one modality over another. It is likely that some patients will find a particular modality such as cold discrimination easier to discern than touch or pinprick and vice versa. Therefore, as long as the determination of block height has a subjective component dependent on patients’ reports and the anaesthetist’s expectations [3], the debate regarding the optimal technique to predict comfort during caesarean section will remain [17]. Our study findings suggest that cold and pinprick testing of block height correlate well with touch and can provide useful information during the assessment of adequacy of spinal anaesthesia for caesarean section. A low level of block height using any one modality alone, but particularly using the first sensation of touch, may erroneously indicate inadequate anaesthesia for caesarean in some women.

Acknowledgements

We thank the anaesthetists and nursing staff at the Women’s and Children’s Hospital in Adelaide for their assistance.

Competing interests

No external funding and no competing interests declared.

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