Cephalad angulation of epidural needle insertion may be an important factor for safe epidural space approach: a mathematical model

Inoue, Satoki; Kawaguchi, Masahiko; Furuya, Hitoshi

doi:10.1590/S0034-70942011000600008

Abstracts

BACKGROUND AND OBJECTIVES: Epidural approach angle is one of the determining factors for needle traveling length to reach the epidural space. Likewise, the margin of safety against accidental dural puncture should be affected by epidural approach angle because the space available for epidural needle tip movement in the epidural space is depending on the inserting angle to the epidural space. The purpose of this study is to conjecture the effect of the angle formed with the skin and an epidural needle on the margin of safety against accidental dural puncture using a mathematical model. CASE REPORT: Suppose that the width of the epidural space is A mm and that the force advancing the epidural catheter is C kgf and the diameter of the catheter is D mm. In this situation, assuming that the cephalad angle with the midline approach formed with the skin and an epidural needle is θ, the following parameters can be determined: the available distance for the needle tip in the epidural space = A/sinθmm; cephalad advancing force of catheter = C*cosθ kgf; and the pressure at the site of dura matter where the epidural catheter is pushing = 400*C*sinθ.πD-2 kgf.cm-2. The longer the distance available for the needle tip, the larger the margin of safety for dura injury by the epidural needle. It is supposed that θ should be set smaller to make the margin of safety larger. Likewise, smaller θ could decrease the pushing pressure and more effectively create a distinct cephalad advancing force. CONCLUSIONS: Acute cephalad angulation should be considered to increase the margin of safety for epidural approach and catheterization.

Epidural Space; Anesthesia; Models; Safety Management

JUSTIFICATIVA E OBJETIVOS: O ângulo de abordagem peridural é um dos fatores determinantes da profundidade de inserção da agulha para se atingir o espaço peridural. A margem de segurança contra punção acidental da dura-máter deve ser afetada pelo ângulo de abordagem porque o espaço disponível para o movimento da ponta da agulha no espaço peridural depende do ângulo de inserção. O objetivo deste estudo é investigar os efeitos do ângulo formado entre a pele e uma agulha peridural na margem de segurança contra a punção acidental da dura-máter, usando-se um modelo matemático. RELATO DO CASO: Suponha que a profundidade do espaço peridural seja A mm, que a força do cateter que avança seja C kgf e o diâmetro do cateter, D mm. Nessa situação, presumindo-se que o ângulo cefálico formado na linha média entre a pele e a agulha seja θ, os seguintes parâmetros podem ser determinados: a distância disponível para a ponta da agulha no espaço peridural = A/senθ mm; a força cefálica do cateter que avança = C*cosθ kgf; e a pressão no local da dura-máter onde o cateter peridural está empurrando = 400*C*senθ.πD-2 kgf.cm-2. Quanto maior a distância disponível para a ponta da agulha, maior será a margem de segurança para lesão da dura-máter pela agulha peridural. Supõe-se que θ deva ser o menor possível para que a margem de segurança seja maior. Da mesma maneira, θ pode reduzir a pressão de empuxo e criar uma força cefálica de avanço mais eficaz. CONCLUSÕES: Deve-se considerar o ângulo cefálico agudo para aumentar a margem de segurança na cateterização do espaço peridural.

ANESTESIOLOGIA; TÉCNICAS ANESTÉSICAS; TÉCNICAS ANESTÉSICAS

JUSTIFICATIVA Y OBJETIVOS: El ángulo de abordaje epidural es uno de los factores determinantes de la profundidad de inserción de la aguja para alcanzar el espacio epidural. El margen de seguridad contra la punción accidental de la duramadre debe ser afectado por el ángulo de abordaje, porque el espacio disponible para el movimiento de la punta de la aguja en el espacio epidural depende del ángulo de inserción. El objetivo de este estudio, fue investigar los efectos del ángulo formado entre la piel y una aguja epidural en el margen de seguridad contra la punción accidental de la duramadre, usando un modelo matemático. RELATO DEL CASO: Supongamos que la profundidad del espacio epidural sea A mm, que la fuerza del catéter que avanza sea C kgf y el diámetro del catéter, D mm. En esa situación, y presumiendo que el ángulo cefálico formado en la línea media entre la piel y la aguja sea θ, los siguientes parámetros pueden ser determinados: la distancia disponible para la punta de la aguja en el espacio epidural = A/senθ mm; la fuerza cefálica del catéter que avanza = C*cosθ kgf; y la presión en la región de la duramadre donde el catéter epidural está ejerciendo su presión = 400*C*senθ.πD-2 kgf.cm-2. Mientras mayor sea la distancia disponible para la punta de la aguja, mayor será el margen de seguridad para la lesión de la duramadre por la aguja epidural. Supongamos que debe ser el menor posible para que el margen de seguridad sea mayor. De la misma manera, θ puede reducir la presión de empuje y crear una fuerza cefálica de avance más eficaz. CONCLUSIONES: Debemos considerar el ángulo cefálico agudo para aumentar el margen de seguridad en la cateterización del espacio epidural.

ANESTESIOLOGIA; TÉCNICAS ANESTÉSICAS; TÉCNICAS ANESTÉSICAS

CLINICAL INFORMATION

^IMD; Assistant Professor

^IIMD; Associate Professor, Nara Medical University, Japan

^IIIMD; Professor, Nara Medical University, Japan

Correspondência para

SUMMARY

BACKGROUND AND OBJECTIVES: Epidural approach angle is one of the determining factors for needle traveling length to reach the epidural space. Likewise, the margin of safety against accidental dural puncture should be affected by epidural approach angle because the space available for epidural needle tip movement in the epidural space is depending on the inserting angle to the epidural space. The purpose of this study is to conjecture the effect of the angle formed with the skin and an epidural needle on the margin of safety against accidental dural puncture using a mathematical model.

CASE REPORT: Suppose that the width of the epidural space is A mm and that the force advancing the epidural catheter is C kgf and the diameter of the catheter is D mm. In this situation, assuming that the cephalad angle with the midline approach formed with the skin and an epidural needle is θ, the following parameters can be determined: the available distance for the needle tip in the epidural space = A/sinθmm; cephalad advancing force of catheter = C*cosθ kgf; and the pressure at the site of dura matter where the epidural catheter is pushing = 400*C*sinθ.πD^-2 kgf.cm^-2. The longer the distance available for the needle tip, the larger the margin of safety for dura injury by the epidural needle. It is supposed that θ should be set smaller to make the margin of safety larger. Likewise, smaller θ could decrease the pushing pressure and more effectively create a distinct cephalad advancing force.

CONCLUSIONS: Acute cephalad angulation should be considered to increase the margin of safety for epidural approach and catheterization.

Keywords: Epidural Space; Anesthesia, Epidural; Models, Theoretical; Safety Management.

INTRODUCTION

The rate of accidental dural puncture after epidural approach has been estimated and varying from 0.19% to 3.6% in several studies ^1-4. Once accidental dural puncture occurs it has been suggested that postdural puncture headache (PDPH) develops in more than 50% of these patients, which can cause significant morbidity ⁵. Several prevention methods against PDPH have been proposed, some of them may be promising options to prevent PDPH ⁶. However, decreasing the rate of accidental dural puncture is supposed to be a more promising option rather than any existing prevention methods for PDPH.

It is obvious that epidural approach angle is one of the determining factors for needle traveling length to reach the epidural space. Likewise, the margin of safety against accidental dural puncture should be affected by epidural approach angle because space available for epidural needle tip movement in the epidural space is depending on the inserting angle to the epidural space. The purpose of this study is to conjecture the effect of the angle formed with the skin and an epidural needle on the margin of safety against accidental dural puncture using a mathematical model.

MATHEMATICAL MODEL

First, suppose that the width of the epidural space is A mm and that the subcutaneous distance from the skin surface to the epidural space is B cm. In addition, assume that the force advancing the epidural catheter is C kgf and the diameter of the catheter is D mm. In this situation, assuming that the cephalad angle with the midline approach formed with the skin and an epidural needle is θ, the following parameters can be determined (Figure 1). Here, it is not supposed that the caudad angulation is used as another option.

1. The available distance for the needle tip in the epidural space = A/sinθ mm

2. The distance from the insertion point to the reaching point at the epidural space = B/sinθ cm

3. Cephalad advancing force of catheter = C*cosθ kgf

4. The pressure at the site of the dura matter where the epidural catheter is pushing = 400*C*sinθ.πD^-2 kgf.cm^-2

COMMENTS

Even at the moment the epidural needle tip reaches the epidural space, the needle tip can be advanced to some degree in spite of paying the greatest attention to prevent injury of the dura. Therefore, the longer the distance available for the needle tip, the larger the margin of safety for dura injury by the epidural needle can become (Figure 1). It is supposed that should be set smaller to make the margin of safety larger.

The epidural needle cephalad direction may be limited because of the bony configuration of the spine when midline approach is used, especially in case of lumbar epidural approach. Paramedian approach can provide a wide range of angles for epidural needle direction. Therefore, paramedian approach may be more suitable to set the cephalad angulation smaller. Actually, it has been suggested that the incidence of accidental dural puncture is lower when using the paramedian approach based on the study using epiduroscopy in cadavers ⁷. In addition, a clinical study showed that the paramedian approach was associated with a lower frequency of technical problems compared to the midline approach ⁸. These reported results might be attributed to the steeper angle in the paramedian approach. Take the case of needle direction at 45 degrees, it could increase the margin of safety against accidental dural puncture by 1.4-fold compared with vertical direction (Figure 1).

Incidentally, advancement of the epidural catheter can be effectively performed when is set small because the force advancing the epidural catheter is greatly directed toward cephalad in such conditions. There are several articles available showing that epidural catheterization using paramedian approach is associated with fast insertion and minimum risk of catheter coiling ^9-11. These researchers have also suggested that fast, safe, and effective catheterization may be attributed to paramedian approach with cephalad angulation. Recently, it has been suggested that the spinal dura matter is also sensitive to traction and stretch, which might be one of the causes of paresthesia during epidural catheterization ¹². The pushing pressure is also reduced by acute cephalad angulation, resulting in reduction of dural stretch (Figure 1). It might be a possible reason for reducing paresthesia during epidural catheterization using paramedian approach ^12,13. Although a recent report denies that catheter passage can occur in the presence of intact dura, stronger pushing pressure would let the catheter pass the dura in the presence of dural damage with the epidural needle ¹⁴. In such a situation, steeper cephalad angulation would provide weaker pushing pressure against the dura, which might reduce unintentional intrathecal catheterization. Similar to the previous paragraph, take the case of needle direction at 45 degrees compared with vertical direction, it could decrease the pushing pressure to the dura by 0.7-fold compared with vertical direction. In this condition, a noteworthy effect is that it could create a distinct cephalad advancing force that unlike the advanced direction cannot be determined at vertically directed catheterization (Figure 1). In clinical situations, this is not completely related to the above hypothesis because Tuohy needles are used, which can determine the direction of catheterization.

In clinical settings, it might be difficult to verify the validity of this proposed theory based on the mathematical model because of the nature of its low rates of incidence. For the statistical analysis, very large study population would be required to investigate this hypothesis as a randomized clinical trial. Besides, approach methods for the epidural needle insertion surely depend on the attending anesthesiologists' preference. This may imply that it is nearly impossible to compare several approach methods on equal terms. For these reasons, it is considered suitable to propose this hypothesis using this mathematical model.

In conclusion, the smaller the angle formed with the skin and an epidural needle becomes, the larger the margin of safety for epidural space approach can be. In addition, with this condition epidural catheterization would be safer and easier to advance toward cephalad direction. Therefore, the paramedian approach with steep cephalad angulation should be considered to increase the margin of safety for epidural approach and catheterization.

REFERENCES

1. Berger CW, Crosby ET, Grodecki W - North American survey of the management of dural puncture occurring during labour epidural analgesia. Can J Anaesth, 1998;45:110-114.
2. Gleeson CM, Reynolds F - Accidental dural puncture rates in UK obstetric practice. Int J Obstet Anesth, 1998;7:242-246.
3. Sprigge JS, Harper SJ - Accidental dural puncture and post dural puncture headache in obstetric anaesthesia: presentation and management: a 23-year survey in a district general hospital. Anaesthesia, 2008;63:36-43.
4. Stride PC, Cooper GM - Dural taps revisited. A 20-year survey from Birmingham Maternity Hospital. Anaesthesia, 1993;48:247-255.
5. Choi PT, Galinski SE, Takeuchi L et al. - PDPH is a common complication of neuraxial blockade in parturients: a meta-analysis of obstetrical studies. Can J Anaesth, 2003;50:460-469.
6. Apfel CC, Saxena A, Cakmakkaya OS et al. - Prevention of postdural puncture headache after accidental dural puncture: a quantitative systematic review. Br J Anaesth, 2010;105:255-263.
7. Blomberg RG - Technical advantages of the paramedian approach for lumbar epidural puncture and catheter introduction. A study using epiduroscopy in autopsy subjects. Anaesthesia, 1988;43:837-843.
8. Blomberg RG, Jaanivald A, Walther S - Advantages of the paramedian approach for lumbar epidural analgesia with catheter technique. A clinical comparison between midline and paramedian approaches. Anaesthesia, 1989;44:742-746.
9. Ryu HG, Bahk JH, Lee CJ et al. - The coiling length of thoracic epidural catheters: the influence of epidural approach angle. Br J Anaesth, 2007;98:401-404.
10. Leeda M, Stienstra R, Arbous MS et al. - Lumbar epidural catheter insertion: the midline vs. the paramedian approach. Eur J Anaesthesiol, 2005;22:839-842.
11. Takeyama K, Yamazaki H, Maeda M et al. - Straight advancement of epidural catheter. Tokai J Exp Clin Med, 2004;29:27-33.
12. van den Berg AA, Sadek M, Swanson S et al. - Epidural injection of lidocaine reduces the response to dural puncture accompanying spinal needle insertion when performing combined spinal-epidural anesthesia. Anesth Analg, 2005;101:882-885.
13. Jaucot J - Paramedian approach of the peridural space in obstetrics. Acta Anaesthesiol Belg, 1986;37:187-192.
14. Angle PJ, Kronberg JE, Thompson DE et al. - Epidural catheter penetration of human dural tissue: in vitro investigation. Anesthesiology, 2004;100:1491-1496.