A MAGNETIC COUPLING TO IMPROVE

PLACEMENT OF GASTROENTERAL FEEDING TUBES

David Cronin, Tadhg Lambe

and Pádraig Cantillon-Murphy

School of Engineering, University College Cork, Cork, Ireland

Keywords: Permanent Magnets, Endoscopy, Gastroenteral Feeding Tube, PEG, Magnetic Coupling.

Abstract: Percutaneous Endoscopic Gastrostomy (PEG) is a non operative endoscopic technique to place a

transabdominal (from outside the abdomen through the gastric wall and into the stomach) gastric feeding

tube. It is the preferred method of eneteral feeding in patients who would otherwise have inadequate

nutritional intake due to a number of underlying illnesses. During the PEG procedure, the feeding tube can

deviate from its intended path, perforate organs and surrounding tissues leading to complications. We

propose a novel technique to alleviate or eliminate these concerns using magnetic coupling. This technique

forces the tube to pass through a specified path, compressing tissues between the gastric and abdominal

walls such that the tube cannot deviate from its intended path. This modified PEG procedure could secure a

safer tract for insertion, decrease procedural time and limit user variability, with hypothesised benefits

including shorter procedural times and lower complication rates. The magnetic coupling mechanism has

been modelled using analytical tools with experimental validation. The approach has been demonstrated in a

bench-top anatomical model and may be of use in applications beyond the PEG procedure including

endoscopic instrument positioning on the gastric wall.

1 INTRODUCTION

1.1 Enteral Nutrition Techniques

Enteral nutrition is a means of delivering nutrition to

patients who would otherwise be unable to feed

themselves for a variety of reasons: neurological

impairment, dysphasia (difficulty in swallowing)

after surgery, oral cavity tumours, anorexia, or as a

preventative for aspiration pneumonia. There are a

number of types of enteral feeding solutions used

but the most common are nasogastric tubes (NGT)

and gastroenteral tubes, placed using the

percutaneous endoscopic gastrostomy (PEG)

technique. While NGT nutrition is often preferred by

radiologists, PEG is the preferred technique amongst

gastroenterologists, endoscopists and surgeons

(Ponsky, 1981) This is because PEG tubes are easier

to tolerate, show better nutritional results and

patients with PEG tubes have higher survival rates

than those with NGT tubes, even when PEG tube

patients are in more advanced stages of illness

(Dwolatzky, 2001).

However, PEG tube placement is not without

complications including gastric perforation, tube

blockage, site infection, PEG tube dislocation and

inadvertent puncture of peripheral organs such as the

colon (Britton, 1997; Conlon 2004; Loser 1998). In

this work, we are particularly interested in

addressing the last of these complications: the

inadvertent puncture of organs that can become

sandwiched in between the gastric and abdominal

walls during the placement of the PEG tube.

Our solution is a simple magnetic coupling (i.e., two

apposing north/south magnetic surfaces) consisting

in two permanently-magnetized rings which are

initially coupled across the gastric and abdominal

walls to provide a safe tract for subsequent passage

of the gastroenteral feeding tube. Since coupling of

the two rings only occurs inside a predeterminable

distance, we hypothesise that this technique could be

used as to improve feeding tube placement using the

PEG technique by eliminating a significant

complication.

1.2 Magnetic Coupling in Surgery

The use of magnets in minimally-invasive

interventions has a long history. The magnetic

retrieval of foreign bodies in the esophagus, stomach

138

Cronin D., Lambe T. and Cantillon-Murphy P..

A MAGNETIC COUPLING TO IMPROVE PLACEMENT OF GASTROENTERAL FEEDING TUBES.

DOI: 10.5220/0003763401380142

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 138-142

ISBN: 978-989-8425-91-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

and deodenum was first proposed as early as 1957

(Equen, 1957). More recently, magnetic coupling

has been used for anchoring of magnetic instruments

(Scott 2007), retrieval of stents (Cantillon-Murphy,

2010) and magnetic NGT guidance (Gabriel 2001)

where an external, hand-held magnet guided the

feeding-tube through the esophageal tract to the

subject's duodenum. In this work, we extend that

work to the use of magnetic coupling for providing a

safe tract for transabdominal insertion of an enteral

feeding tube.

1.3 Current PEG Technique

(a)

(b)

(c)

(d)

(e)

Figure 1: Outline of the current PEG tube placement

procedure involving (a) endoscopic trans-illumination, (b)

transabdominal needle perforation, (c) guidewire-

introduced tube placement, (d) mechanical interlocking

and (e) fixation (reproduced from Ponsky 2004).

Currently, the technique most commonly used for

PEG tube placement is the so-called ‘pull’

technique, outlined in Figure 1. An endoscope is

introduced into the patient’s stomach and a

combination of trans-illumination (i.e., shining the

endoscopic lamp across the gastric and abdominal

walls) and finger pressure is used to determine the

site of closest contact (Figure 1(a)). The so-called

‘safe tract’ method involves insertion of a syringe of

local anaesthetic which is blindly guided across the

abdominal and gastric walls at the site of

transillumination (Figure 1(b)). The site is

catheterised and an endoscopic snare (introduced via

the endoscope’s instrument channel) is used to lasoo

a guidewire which is pushed through the catheterised

abdominal and gastric walls (Figure 1(c)). Removing

the endoscope leaves the guidewire extending across

the abdominal and gastric walls and out of the

patient’s mouth. The guidewire then serves as the

tram-line for the oral introduction of the feeding

tube in advance of reintroducing the endoscope for

inspection. The distal end of the feeding tube usually

has a round bumper to prevent its escape through the

gastrotomy (Figure 1(d, e)). The procedure ends

when the portion of inserted feeding tube outside the

abdominal wall is snipped, the nutrition sack is

attached and the endoscope is removed. The external

t-bar shown in Figure 1(d) sits above the skin and is

designed to stop excessive tension and “buried ring

syndrome” where the gastric wall grows over the

tube thereby causing obstruction

1.4 Magnetic Coupling and PEG

Figure 2: Magnetic coupling to aid in PEG tube

placement.

Our approach is to augment the ‘safe tract’ approach

shown in Figures 1(a) and (b) with a simple

mechanism of temporary magnetic coupling across

the abdominal and gastric walls, as shown in Figure

2. The magnetic coupling of two rings, one on the

abdominal wall (external to the patient) and one on

the gastric wall (endoscopically delivered) is the

second step (i.e., coming between Figures 1(a) and

1(b)) in a slightly modified PEG technique.

However, once the coupling is in place, the result is

that the relative positions of gastric and abdominal

walls are fixed and sandwiched in place at a known

minimum separation, as predicted by theory.

Furthermore, coupling only takes place at a

separation less than a critical maximum which can

A MAGNETIC COUPLING TO IMPROVE PLACEMENT OF GASTROENTERAL FEEDING TUBES

139

be used as a check for inadvertent sandwiching of

organs like the colon between the gastric and

abdominal walls during the procedure.

2 METHODS

2.1 Magnetic Coupling Design

The critical component of the approach is the

coupling of the two magnetic rings at a known and

predictable distance of separation. To investigate,

this dependence, the usual magnetic charge model

was used to simulate magnetic mating of two

permanently magnetised concentric rings (Furlani,

2001). Following the usual Coulombic Law

formulation, the force vector exerted by ‘magnetic

charge’, q

, on ‘magnetic charge’, q

is given by

(1) where μ

is the magnetic permeability of free

space (4πx10

-7

H/m) and r

is the displacement

vector between q

and q

= μ

/ (4π r

) (1)

One of the principal challenges in magnetic coupling

is the inverse square roll-off in the force of attraction

between magnetic components. We began by

simulating the coupling forces between concentric

mating magnetic rings using the open-source Radia

(ESRF, France) add-on to Mathematica 7 (Wolfram

Corp., Champaign, Illinois). The magnetic force is

found by discretisation of all the magnetic surfaces

and integration of (1) over the nearby surfaces which

are subject to the field (i.e., the adjacent ring). The

simulated results, shown in Figure 3 demonstrate the

familiar inverse square relation between force and

separation between the two magnetic rings (ignoring

gravitational forces) as well as the force associated

with coupling an external magnet to internal steel

rings (SR1-SR3) of varying dimensions. Based on

the investigations of Figure 3, we designed a

coupling capable of magnetic mating across

expected stomach wall thickness of 3-4 cm. Two

permanent magnetic N52 grade neodymium-iron-

boron (NdFeB) rings were purchased (26mm OD,

18mm ID and 25mm H) from HKCM Engineering,

Germany, for subsequent testing (Figure 4(a)). In

addition, a number of mild steel rings (EN3B grade

mild steel) were fabricated of various wall

thicknesses and lengths (Figure 4(b)). Because

EN3B grade steel has significant paramagnetic

properties (i.e., it behaves magnetically in the

presence of a magnetic field source such as a

permanent magnet with magnetic susceptibility, χ ≈

800), we also investigated the use of mild steel rings

for use as the gastric wall magnet in Figure 2.

Figure 3: Force-separation characteristics between the N52

grade ring magnet and (i) a second identical magnetic ring

(MR), and (ii) three stainless steel rings of varying

thicknesses and lengths; SR1 with 26mm OD, 20mm ID

and 25mm H, SR2 with 26mm OD, 20mm ID and 12.5mm

H and SR3 with 22mm OD, 20mm ID and 12.5mm H.

Gravitational forces are ignored.

Figure 4: The gold-plated N52 grade neodymium-iron-

boron ring (a) and stainless steel rings of various

thicknesses (b) which were used in subsequent testing.

2.2 Mechanical Design

The most significant challenge in implementing a

magnetic coupling across the gastric and abdominal

walls was the placement of the gastric wall magnet

within the patient’s stomach without any incision.

Since the PEG procedure already involves the oral

introduction of an endoscope, we used the

endoscope as the vehicle to carry the gastric magnet

into its final position. A number of approaches were

considered including introducing the magnet in

advance of the endoscope using a magnetised

catheter. However, the technique that was found

most satisfactory was spearing the ring with the

endoscope’s shaft with a radially-inflatable ring

balloon between the endoscope and magnet, as

shown in Figures 5 and 6. This approach had four

significant advantages; (i) the magnet could be held

in position on the endoscope’s shaft by inflation of

the balloon and released for coupling by deflation;

(ii) inflation of the balloon beyond the ring OD

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during oral insertion limited any possibility of

tearing to the oesophageal wall upon introduction of

the endoscope; and (iii) the magnet presented no

visual impediment to the scope’s field of view.

The ring balloon was constructed from a 51Fr

(17mm) veterinary endotracheal tube (Jorgensen

Laboratories, Colorado) which was chosen to fit

snugly over a standard 12mm diameter endoscope

(GIF Q20 by Olympus Inc., Japan). The balloon was

lure-lock connected to a standard endovascular

balloon indeflator (Boston Scientific Corp.,

Massachuetts) which was used to inflate the balloon

to a measureable pressure as shown in Figure 5(c).

Figure 5: (a) An modified endotraceal tube was used as the

ring balloon, fitting snuggly over the 12mm endoscope.

(b) When inflated, the balloon fixed the ring on the

endoscope’s shaft. (c) The balloon also minimised risk of

tissue tearing due to the magnet’s edges upon insertion of

the endoscope.

3 RESULTS

3.1 Magnetic Coupling

The magnetic coupling force predicted by Figure 3

was experimentally investigated by compression and

separation tests using a Stable Micro Texture

Analyser (Godalming, UK). The resultant force -

separation characteristic for the various rings are

shown as datapoints on Figure 3.

To accurately predict the required force to couple

the gastric and abdominal magnets in the presence of

the inflated balloon, the axial force required to slide

the gastric magnet off the inflated balloon was also

investigated as a function of various balloon

inflation pressures. This is an important parameter

because, in the modified procedure, the magnetic

coupling is mainly impeded by the frictional forces

between the balloon and gastric magnet, which

varies as a function of inflation pressure, and not

gravitational force The coupling distance (i.e.,

critical distance at which mating occurs) between the

various gastric magnets of Figure 3 and the NdFeB

abdominal ring magnet is shown in Table 1. These

results correspond to the worst-case scenario where

gravity acts axially against the magnetic coupling

force. This is not unrealistic in a clinical setting

where the PEG placement usually takes place while

the patient lies on their back.

Table 1: Coupling Distance to NdFeB Ring.

dFeB SR1 SR2 SR3

Coupling Dist (mm) 35±3 21±2 19±4 18±3

3.2 Procedural Testing

Figure 6: The procedure was demonstrated in a benchtop

anatomical model for (a) endoscopic navigation

imparment and (b) magnetic coupling before testing in the

scaled plasticine gastric model for (c) navigation and (d)

transgastric magnetic coupling.

The modified PEG procedure including magnetic

coupling was experimentally demonstrated in a

benchtop test using a simplified anatomical model,

as shown in Figure 6(a) and (b). The magnetic

coupling mechanism and ‘steerability’ of the

endoscope in the presence of the balloon and ring

magnet was then investigated using a plasticine

gastric model (Figure 6(c) and (d)). In this

investigation, the endoscope was advanced through a

2cm diameter rigid tube (simulating the oesophagus)

into the model stomach. The endoscope was then

flexed at approximately 30

○

to the horizontal to

provide coupling to the external magnetic ring.

Some manipulation of the endoscope’s position was

required before coupling was achieved. This was

primarily due to the thick inner wall of the ring

balloon. Coupling occurred at a separation of 3-4cm.

After coupling, the gastric magnet was removed by

reinserting the endoscope into the ring, inflating the

balloon and removing the external magnetic ring.

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141

4 DISCUSSION

In this study, we propose a simple yet novel

mechanism that may reduce complications in the

placement of gastroenteral feeding tubes. The

technique relies on the temporary magnetic coupling

of two rings, one in the stomach (which is

endoscopically delivered) and a second external to

the patient. We have successfully prototyped a

preliminary proof-of-concept design which we

investigated in the benchtop setting for technical

feasibility. As indicated in Figure 3, the coupling

compression forces are highly predictable. Also,

depending upon the coupling ring materials, we have

shown in Table 1 that the distance within which

coupling occurs can be predicted. Based on expected

gastric/abdominal wall separation of 3-4cm and the

results of Table 1, it is clear that two N52 NdFeB

rings represent the best opportunity for successful

coupling. This modification may represent a

significant advantage over current approaches where

there is no knowledge of gastric to abdominal wall

separation distance.

We have also identified a number of elements

that need attention in advance of a pilot animal

study, the most critical of which is the inner wall

thickness of the ring balloon. The balloon is an

excellent means to maintain the gastric ring magnet

in position until coupling is needed. However, in the

current embodiment, which uses a retrofitted

endotracheal tube, the balloon wall thickness

represents a significant enough impediment to

manoeuvrability of the endoscope to be problematic.

To alleviate this concern, we propose (i) to design

and construct a customised ring balloon with

minimal inner wall thickness and (ii) to consider the

use of a bronchoscope (6-8mm OD) rather than an

endoscope (12mm OD) for future investigations. A

second refinement will involve the integration of a

visual confirmation of mating (e.g., a light-emitting

diode which turns on upon coupling) attached to the

external magnetic ring unit. We are confident that

with these modifications, an acute porcine survival

study can soon be undertaken (Autumn 2011).

Finally, we note that this approach of transgastric

magnetic coupling may have implications beyond

that considered in this work. The use of

transabdominal magnetic coupling for positioning of

surgical instruments has already been elegantly

demonstrated (Scott 2007). In this work, we propose

a novel extension to that approach which uses the

endoscope as the vehicle for introducing the gastric

magnetic. This may have implications for recent

advances in minimally-invasive procedures such as

natural orifice surgery and single-site laparoscopy,

where a similar approach could be employed to

tether endoscopic instruments to the gastric wall

during procedures by means of gastric to abdominal

wall magnetic coupling (e.g., a magnetic camera

positioning system).

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