Insertion forces and risk of complications during cricothyroid cannulation

Printed in the USA + Copyright 0 1992 Pergamon PressLtd.

TheJournal of Emergency Medicine, Vol 10, pp 417-426, 1992

INSERTION FORCES AND RISK OF COMPLICATIONS DURING CRICOTHYROID CANNULATION Peter H. Abbrecht,

Reprint

MD,

Pho,* Richard R. Kyle, MS,* William H. Reams, ss,t and John Brunette,

*Uniformed Services University of the Health Sciences, Bethesda, Maryland; tUS Army Biomedical Research and Development Laboratory, Fort Detrick, Maryland Address: Dr. Peter H. Abbrecht, Uniformed Services University of the Health Sciences, Department of Physiology, 4301 Jones Bridge Road, Bethesda. MD 20814-4799

Many designs for CT cannulas have been developed. These range from simple uncuffed tubes to more complicated devices incorporating features such as an inflatable cuff to prevent upper airway leakage during insufflation. Insertion of cricothyrotomy devices may result in serious complications such as tracheal-esophageal perforation (5), tracheal laceration (6), or paratracheal cannula placement (6). The objective of this research was to identify the types of complications that might occur during insertion of four different cricothyrotomy devices into cadaver and canine necks and to relate occurrence of complications to cannula design features such as diameter and curvature. Penetration force would be expected to affect the dexterity and precision with which the operator introduces the device (7) and thus to affect occurrence of complications such as laceration or penetration of the posterior tracheal wall. Therefore, we measured penetration and insertion forces during the placement of each cannula. Since some cannula manufacturers provide lubricant to facilitate insertion while others do not supply lubricant, we measured forces both with and without application of lubricant. To obtain a better understanding of the penetration dynamics for each cannula, tracheal endoscopic video recordings and photographs were made during insertion of each type of cannula.

Cl Abstract -Our purpose was to determine the forces required to insert several different styles of cricothyroid cannulas and to relate the magnitude of these forces and cannula design features to the incidence of complications during insertion. Tests were done on unembalmed cadavers and anesthetized dogs. Samples of 4 different commercial cricothyroid cannuhts were tested. Each cannula type was tested in 5 different cadavers and 10 different dogs. A lubricant was applied to the cannulas in half of the dogs tested. Major findings are 1) there is a linear correlation between insertion force and device diameter, 2) higher puncture force is associated with a greater incidence of complications, 3) posterior wall penetration occurs more frequently with a curved penetrating device, 4) using small pilot needles to guide insertioa of large cannulas minimizes complications, and 5) lubricant is less effective for cannulas having abrupt diameter changes. These findings provide guidelines for design of safer cricothyroid cannulas. 0 Keywords - cricothyroid cannula; tissue penetration force; cannula complications; lubrication INTRODUCTION

Acute upper airway obstruction is an emergency that requires rapid establishment of a patent airway. However, it may be impossible to attain a patent upper airway for a variety of reasons, including anatomical or pathological abnormalities or obscured visualization due to blood or edema (1). In such cases cricothyroid (CT) cannulation may be a lifesaving technique for providing a pathway for artificial ventilation (2-4).

MATERIALS

24 September 1991; ACCEPTED:

9 January

AND METHODS

The commercial cannulas tested were the Abelson (Gilbert Surgical Instruments, Inc., Bellmawr, NJ), the Nu-Trake (International Medical Devices, Northridge, CA), and two Pertrachs (Pertrach, Inc.,

This research was supported in part by the US Army Medical Research and Development Command. RECEIVED:

LFD*

1992 417

0736~4679/92

$5.00

+ JO.

418

P. H. Abbrecht,

Clarksburg, WV). The Pertrach cannulas are referred to in this paper as the small Pertrach (OD 7.3 mm) and the large Pertrach (OD 8.8 mm), which are identical except for size. Photographs of the cannulas are given in Figure 1. The Abelson has a curved stainless steel airway tube enclosing a solid trocar with a bayonet tip. A round face plate limits the penetration distance. The Nu-Trake consists of an introducer with two tapered blades that surround a 13-gauge needle stylet. After penetration into the trachea, the stylet is replaced by an airway. The Nu-Trake is unique in that airway insertion expands the two blades of the introducer that are already through the neck tissue, instead of pushing an airway directly through tissue. The small Pertrach and large Pertrach consist of a curved plastic airway cannula with an inflatable cuff. In use, a pilot opening is made in the trachea using a scored 1Cgauge penetration needle. During airway insertion, the cannula contains a tapered dilator with a guide wire. When the guide

R. R. Kyle, W. H. Reams, J. Brunette

wire has been passed through the needle, the needle is broken away, the dilator/cannula advanced into the trachea, the dilator removed, and the cuff inflated. Scalpel blades for incising the skin are provided with the Nu-Trake and the Pertrachs. Table 1 contains the dimensions of the different components of the 4 cannulas. “Introducer” refers to the part of the device that makes the initial penetration through the cricothyroid membrane, and “Airway” refers to the components that convey gas to the trachea. Since with the Abelson device the cannula serves as both the introducer and the airway, the cannula values are included in both sections of Table I. Penetration length is the extent that the device can be inserted beneath the skin. Curvature is the change in angle between the airway inlet and outlet. This angle was determined by both geometric and force measurements. All 4 cannulas have diameter step increases in the portion that must be forced through the tissue during

(4

08

6)

(4

Figur ‘8 1. Cannulas tested in this study: (a) AWeon cannub: trceaf on left and ahway on right, (b) t&kTrake cenfwk intredu eer mm ,8tylet (center), 7.2.mm airway (rtght), (c) Small Pevt@&t WWui*: akwey v&B intldRlci cuff and dbtor on left, needle Ion right, , (d) Large Pertrach cannula: airway with inflated cuff and &tetor on left, needle on right.

Cannula

419

Force and Complications Table 1. Cannula Dimensions Cannufa Dimensions

Nu-Trake

Abelson

Introducer Greater diameter, mm Penetration length, mm Curvature, degrees Measured Calculated’ Airway Lesser diameter, mm Greater diameter, mm Penetration length, mm Curvature, degrees Measured Calculated l

4.5 59 61 65 3.7 4.5 50

Small Pertrach

Large Pertrach

6.9 37

2.1 53

2.1 53

0 0

0 0

0 0

5.5 7.3 65

7.1 6.6 53

65 62

51 49

7.2 6.0 x 11.4 35

61 65

0 0

*from force vectors

insertion. For the Abelson and the Pertrach cannulas, this discontinuity occurs between the leading edges of the airways and the trocar or dilator, respectively. With the Nu-Trake, this discontinuity is between the tips of the introducer blades and the stylet. Table 2 contains these dimensions, with the greater diameter being the diameter of the cannula just after the discontinuity, and the lesser diameter the diameter of the cannula just before the discontinuity. Peak insertion force for each cannula was measured in 5 different cadavers and in 10 different anesthetized dogs (5 with lubrication and 5 without lubrication). Only one cannula insertion was done in each subject. Twenty nonembalmed frozen cadavers (61 to 79 years, 65 to 77 kg) were obtained through the anatomical gift program of the Uniformed Services University of the Health Sciences. The bodies were thawed to room temperature (20 to 22OC) before testing. Dog studies were done on 40 mongrel dogs (20 to 25 kg) anesthetized with 30 mg/kg sodium pentobarbital. The animals were obtained following their use in other acute protocols. In both the cadaver and the animal studies, cannula insertion was done with Table 2. Csnnula Dlmenslons

at the Location

the body in the supine position and the neck fully extended. All of the cadaver studies and half of the dog studies were done without application of a lubricant. In the lubrication studies, a lubricant (SURGILUBETM) was applied to the Abelson trocar/cannula, the Nu-Trake stylet/introducer, and the two Pertrach dilator/airways. The entire surface of the portion of the cannula that passed through tissue during insertion was thoroughly coated with lubricant. The Nu-Trake airway was not lubricated before insertion because it did not contact tissue. The Pertrach needles were not lubricated because their small diameter resulted in very little resistance to tissue penetration. Before insertion of the cannulas, a midline vertical skin incision 1 cm long was made using the surgical blades supplied with the Nu-Trake and Pertrach cannulas. No surgical blade was supplied with the Abelson cannula. To make the Abelson measurements comparable with those for the other cannulas, a #l 1 surgical blade was used for making a skin incision before inserting the Abelson. The incision was carried through the skin, but not through the subcutane-

of the Diameter Discontinuity Cannula

Dimension Greater diameter, mm* Lesser diameter, mmt Greater/LesserT

Abelson airwayltrocar 4.5 3.7 1.22

Nu-Trake introducerlstylet 3.7 2.4 1.54

Small Pertrach airway/dilator

Large Pertrach airway/dilator

7.3 5.5 1.33

8.8 5.5 1.60

*Greater diameter is either the outside diameter of the airway leading edge or, in the case of the Nu-Trake, the outside diameter of the introducer blade tips with the stylet inserted into the introducer. tLesser diameter is the outside diameter of either the trocar, the stylet, or the dilator. *The ratio of greater to lesser diameter is the fractional increase in diameter at the discontunity.

420

P. H. Abbrecht,

ous tissues. Despite the different scalpel blades, care was taken to insure that the length and depth of skin incision were the same for all tests. Peak insertion forces were measured using a handheld mechanical force gauge (model L-30-M, Hunter Spring, Hatfield, PA) with a range of 0 to 30-lb force. The gauge was attached securely to the cannula or introducer using specially machined fittings. All tests were done by the same operator using the force gauge as a handle for the cannula. The operator had extensive prior laboratory experience in the insertion of the different cannulas. The operator oriented the tip of the cannula at an angle estimated to allow entry into the trachea without posterior wall penetration. During insertion, the operator applied a force FA and a countering torque M, (Figure 2) to the cannula in the direction that the cannula tip was pointing (tangent to the cannula at its tip). The force gauge registered a maximum force (FG) along the axis of the cannula hub. The axes of the forces F, and FG differed by the angle (Y, the geometric angle of curvature (Figure 2). The maximum force applied to the tissue was calculated by dividing F, by the cosine of (11.For the straight devices, CYis zero and cosine CY is one. The validity of this method for estimating maximum force was tested for each cannula type by measuring forces registered by the mechanical force gauge as the cannula was pressed perpendicularly

R. R. Kyle, W. H. Reams, J. Brunette

against the surface of a load cell (Figure 3). A model SMIOO load cell (Interface, Inc., Scottsdale, AZ) was used. The cosine of the angle CYwas calculated by dividing the mechanical gauge reading by the load cell value (FR in Figure 3). The corresponding angle was compared to the cannula’s angle of curvature obtained by physical measurement. The angles of cannula curvature obtained by direct measurement and obtained by calculation from the load cell measurements are given in Table 1. There was good agreement between the two techniques, with the largest difference (7%) occurring with the Abelson cannula. The torque exerted by the operator on each cannula was calculated by multiplying the force FA by the moment arm L (Figure 2) for each device. The neck penetration angle p was set by direct observation of the cannula tip, and the gauge readings were corrected to give the axial force exerted on the tissue by the cannula tip. Thus, the use of the force gauge as a handle should not have altered the insertion force measurement. To determine the mechanism of any possible trauma caused by the cannulas as they entered the trachea, a fiber optic bronchoscope (model BR-30, Reichert Fiber Optics, Southbridge, MA) was passed through the nose or mouth and positioned in the glottic aperture. To minimize any potential effect of the scope on the force measurements, the tip was placed so that it just barely separated the cords. The bronchoscopic images were recorded with a video camera (Javalin Electronics, Torrance, CA) and video recorder (Panasonic Industrial Company, Secaucus, NJ). Following each insertion test, the trachea was incised at the site of cannula entrance, and the posterior tracheal wall examined for any trauma from cannula insertion. Two-way analysis of variance (SuperANOVA, vl.llD, Abacus Concepts, Inc., Berkeley, CA) was performed on the initial penetration force values and on the airway introduction force values to determine 1) the force variances between species, and 2) the force variances among cannulas. Animal use was approved by the Laboratory Animal Review Board and performed in accordance with the Guide for the Care and Use of Laboratory Animals, NIH Publication No. 86-23, revised 1985.

Tissue Figure 2. Force vectors during penetmtlon and insertion of cannulas. The gauge measures force along the axls that is at angle o to the tip of the cannula. Thus, the force applled to the cannula, F,, is equal to F&o8 CY.The tleeue ree@tbn force, FR, Is equal and opposlte to FA. The opemtw must apply a torque MA to counter the moment created by FR x L.

RESULTS Placement of the Pertrach and Nu-Trake cannula is a two-step process. There is an initial penetration of tissue using a pilot needle (Pertrach) or an intro-

421

Cannula Force and Complications

Figure 3. Method for validating the applied force computations. The force gauge reads force at the angle (Y from the applied force. The applied force, FI, is calculated as F&OS cxand should be equal to the load cell reading FR.

ducer/stylet (Nu-Trake) followed by insertion of the actual airway using the penetration device as a guide. The initial penetration and airway insertion of the Abelson are accomplished in a single operation with a cannula containing a trocar. Thus, there is no pilot device to guide the airway with the Abelson. Figure 4 shows the average peak forces measured for the initial penetration and airway insertion steps for the different cannulas in cadavers and canines without lubrication. Peak initial penetration forces are shown in Figure 4a. There were no significant differences (P = 0.24 by ANOVA) between cadavers and dogs in the forces required for initial penetration. In both cadavers and dogs, the penetration forces required for the Pertrach pilot needles were significantly less (P < 0.005) than those required for the Abelson airway/trocar or the Nu-Trake introducer/stylet. The forces required for insertion of the airway are shown in Figure 4b. The measured insertion force for the Abelson is also its measured penetration force. For any given cannula, there were no significant differences (P = 0.92) between the cadaver and the canine values. In either species, the Abelson required significantly less insertion force (P < 0.005) than the large Pertrach. Figure 5’ shows the relationship between average peak force in the dog and greater diameter for the device configurations in which the cannula punctures

40

Cadaver

1 Cl

8

a. Initial Penetration

Dog

2 a

0 Abelson

40

1

Cadaver

Abelson

Nu-Trake

b. Airway

Nu-Trake

S Pertrach

L Pettrach

Insertion

S Pertrach

L Pertrach

Figure 4. (a) Average peak apptfed forces required for inltlal tracheal penetra%n of the 4 different cannulaa In cadavers (shaded bars) and dogs (open bars). Each bar mpresents mean of values obtained In ffve diffemnt objects. Error bars show one SE. (b) Average p6ak m Woes required for airway insertion of the 4 different cannulas in cadavers (shaded bars) and dogs (open bars).

422

P. H. Abbrecht,

40,

I

-z 5 30Ei. $b 20LL B "alo2 0 0

I 2

Greair

I

Diamettlr,

1

I 8

10

mm Figure 5. Force required to puncture dog tissue as a function of greater dlameter. Tests were done wlthout lubricant. For each cannula, values are the mean obtained in 5 different subjects. Brackets show f one SE.

tissue. The data for insertion of the Nu-Trake airway is excluded from Figure 5 because this airway insertion does not involve tissue puncture. Figure 5 also shows the line obtained by linear regression, indicating that the force required for puncture increases as greater diameter increases. The Pearson product moment correlation is 0.97, and the intercept is not significantly different from zero (P < 0.05) (8). We postulated that the force required to introduce devices with large discontinuities in diameter would be related more to the tearing of tissue than to the overcoming of sliding friction. With that assumption, the amount of reduction of insertion force due to lubrication would be expected to be inversely related to the fractional increase in diameter at the point of discontinuity. Figure 6 shows the effect of lubricating the cannula in dog insertions. The lubrication tests were done for the same devices included

LL K1.51

R. R. Kyle, W. H. Reams, J. Brunette

in Figure 5, with the exception of the Pertrach needle. The ratio of lubricated to nonlubricated force is plotted as a function of the fractional increase in diameter at the discontinuity (greater diameter divided by lesser diameter). The cannula with the smallest diameter increase, the Abelson, had the greatest reduction of insertion force due to lubrication. The cannula with the greatest diameter increase, the large Pertrach, had no reduction of force with lubrication. The straight line in Figure 6 obtained by linear regression is a good representation of the data over the range studied (? = 0.923). However, since the lubricated to nonlubricated force ratio would not be expected to be greater than unity, the line should not be extrapolated beyond the point for the large Pertrach. Several complications occurred during initial tracheal penetration and airway insertion. Table 3 lists the observed frequency complications in the cadaver and dog studies. With the Abelson cammla, there was a high incidence of posterior tracheal wall penetration in both cadavers and dogs. Figure 7a is a bronchoscopic view in a cadaver showing the tip of the Abelson trocar embedded in the posterior tracheal wall. Figure 7b is a photograph of the resulting wound in the posterior wall mucosa. In some instances the cannula tip penetrated the entire posterior wall and impinged on the esophagus. In the dog studies, there was also a tendency for the Abelson to slip down a lateral wall of the trachea during attempts to penetrate the cricothyroid membrane. This produced damage to paratracheal structures. With the NuTrake, there was one incidence of posterior wall penetration in a cadaver (Figure 8a, b) during introducer/stylet penetration. This occurred even when the angle of penetration was such that the bevel on the plastic housing of the introducer was parallel with the surface of the neck (Figure 8c), as directed by the instructions. Neither Pertrach needles nor airway/ dilators caused posterior tracheal wall damage. In one cadaver insertion, the small Pertrach airway/dilator tore a large flap of tissue from the cricothyroid region of the anterior tracheal wall (Figure 9a-c).

DISCUSSION

Greater

/ Lesser Diameter

Flgure 6. Ratlo of lubrkated to nonlubrlceted force in dogs as a function of the fmctlonal Increase In dlam&er at the discontlnulty. The &stoiW# (*) indleato devices for whkh there were slgntflcant (P c 0.05 by one-way AIWVA) dlff&rences between the nonlubrlcated and the lubricated forces.

We tested the cadavers as soon as possible postmortem. Because of postmortem tissue changes and the fact that the cadaver tests were done at room temperature, the absolute force values measured in the fresh cadavers might be different from the values that would be obtained in living subjects. However, it is reasonable to expect that the relative magnitude of

423

Cannula Force and Complications Table 3. Complications Observed during Cannula Penetration and Airway Insertion in Cadavers and Canines (Each value represents the number of complications in 5 subjects) Cannuia Complication Posterior wall penetration cadaver canine Cricothyroid membrane tear cadaver canine Paratracheal deflection cadaver canine

Abelson

Nu-Trake

Small Pertrach

Large Pertrach

3 3

1 0

0 0

0 0

0 0

0 0

1 0

0 0

0 2

0 0

0 0

0 0

forces obtained for the different cannulas would be similar in cadavers and living subjects. This expectation is confirmed by the fact that the trend of force measurements in the cadaver experiments paralleled the results obtained in the anesthetized dog (Figure 4). Our data indicate that the dog is an adequate model for comparing cannula insertion forces when cadavers are not available. There were no significant differences between the forces measured in the cadaver and in the dog without lubricant during either initial penetration or airway insertion. This finding is consistent with the report by Ruhe and colleagues

Figure 7. (a) Bronchoscopic scopic view of the resulting

view showing the tip of the Abelson wound in the posterior wail mucosa.

(9) that the canine cricothyroid membrane, cricothyroid muscles, and cricothyroid area in toto are similar to those of the human. The tracheal diameters in the adult human and the 25-kg dog are comparable (10). We tested insertion of the cannulas with and without lubrication. Lubrication had a large effect in reducing the peak insertion forces for the Abelson and no effect on the forces for the large Pertrach. This result was unexpected, because a larger diameter device would have more tissue contact and thus might be expected to exhibit more force reduction due to lubrication. On the other hand, lubrication would be

trocar embedded

in the posterior

tracheal wail. (b) Broncho-

424

P. H. Abbrecht,

(4

Figure 8. (a) Nu-Trake mucosa. (c) Introducer orientation Increases

PI. R. Kyle, W. H. Reams, J. Brunette

(4

Introducer/stylet embedded in the posterlor tracheal wall. (b) Resulting wound In the posterior oriented so that the beveled surface of the ptastlc houring Is par&e1 to the plane of the neck. the tendency for the lntroducerlstylet to penetrate the posterior tmChO8l wall.

expected to have little effect on the forces required to overcome discontinuity in cannula diameter, because tissue tearing rather than friction is the major resistance at points of discontinuity. Therefore, we plotted the reduction in force due to lubrication against the fractional increase in diameter at the discontinuity for each device (Figure 6). The figure shows that the amount of reduction in force due to lubrication did vary inversely with the relative size of the discontinuity in diameter. Analysis of the incidence of complication rates for the different cannulas provides insight into cannula design features that would tend to reduce complications. The highest incidence of complications was

seen with the Abelson cannula. consisted of posterior tracheal misplacement of the cannula in The high complication rate due to several factors:

wall This

These complications wall laceration and paratracheal tissues. with the Abelson is

1) Cricothyroid membrane penetration and airway insertion are done in a single step with the Abelson so that the Abelson does not provide a separate guidance mechanism for insertion of the large airway. In contrast, the other cannulas that were tested accomplish airway placement with a two-step process. With the Pertrach cannulas, the initial penetration is done with a small, straight needle, and with the Nu-Trake,

425

Cannula Force and Complications

(4

Figure 9a-c. Bronchoscopic roid membrane.

views during insertion

of small Pertrach airway/dilator

initial penetration is done with a straight introducer/ stylet. These introducers provide guidance for the insertion of the larger airways, thus reducing the possibilities of airway misplacement or laceration of the posterior tracheal wall.

showing

progressive

tearing of cricothy-

2) The marked curvature of the Abelson makes it difficult for the operator to ascertain correct alignment with the tracheal axis and predisposes to malpositioning and perforation. The lack of curvature of the Nu-Trake and Pertrach introducers makes it rela-

426

P. H. Abbrecht,

tively easy for the operator to align the devices with the tracheal axis and thus avoid posterior wall perforation or lateral deviation of the cannula. 3) The Abelson is the only cannula for which the operator must exert a torque (mean 15.9 inch pounds) during initial penetration. Requirement for this torque exertion reduces the controllability of the penetration process. 4) Another factor that may affect controllability and thereby complication rate is the amount of force that the operator must exert during the initial penetration of the cricothyroid membrane. There is an “explosive giving-way” (9) of the cricothyroid membrane at the moment of maximal applied force. The tendency for the cannula to impact and lacerate the posterior wall would be expected to be related to the maximal amount of force at the time of breakthrough. No complications occurred during penetration with the small Pertrach needles. There were complications with the larger Abelson and Nu-Trake cannulas. The penetration forces for the Pertrach needles are very low. The resultant good control of the penetration process was probably another reason that there were no complications during the initial penetration. While the penetration forces for the Nu-Trake and the Abelson cannulas were similar, the complication rate was higher for the Abelson. As noted above, the curvature of the Abelson cannula makes alignment of the tracheal axis more difficult and thus increases the risk of malplacement and laceration. These results relating penetration forces and oc-

R. R. Kyle, W. H. Reams, J. Brunette

currence of complications suggest that reducing penetration force is one factor that will reduce complications during penetration. Since penetration force tends to increase with device diameter, using smaller diameter introducers should help minimize complications. 5) The fact that the penetration length of the Abelson is almost 60% longer than that of the Nu-Trake may also contribute to the higher complication rate experienced with the Abelson. On the basis of the above analysis, several design features may reduce the complications encountered during CT cannula placement. These features include 1) provision of an introducer mechanism separate from the airway to facilitate positioning and reduce laceration, 2) use of straight introducer to eliminate torque and permit easy visual alignment with the tracheal axis, 3) use of small diameter introducers to decrease the force required for penetration, 4) restriction of the initial introducer length to only that necessary to enter the tracheal lumen, and 5) provision of adequate grip area for improved controllability, especially for devices that require large insertion forces. Lubrication is most effective in reducing insertion force for devices that do not have large diameter discontinuities.

Acknowfedgment- We thank Mr. George S. Holborow of the Uniformed ServicesUniversity Multidiscipline Laboratories for making available the cadavers.

REFERENCES 1. King HK, Wang LF, Khan AK, Wooten DJ. Translaryngeal guided intubation for difficult intubation. Crit Care Med. 1987;15:869-71. 2. Clinton JE, Ruiz E. Emergency airway management: methods to meet the challenge. Top Emerg Med. 1988;10:31-41. 3. Toye FJ, Weinstein JD. Clinical experience with percutaneous tracheostomy and cricothyroidotomy in 100 patients. J Trauma. 1986;26:1034-40. 4. Weiss S. A new emergency cricothyrotomy instrument. J Trauma. 1983;23:155-8. 5. Claffey LP, Phelan DM. A complication of cricothyroid “minitracheostomy’‘-oesophageal perforation. Intensive Care Med. 1989;15:140-1. 6. Randell T, Kalli I, Lindgren L. Minitracheotomy: complica-

tions and follow-up with fiberoptic tracheoscopy. Anaesthesia. 1990;45:875-9. 7. Towler MA, McGregor W, Rodeheaver GT, et al. Influence of cutting edge configuration on surgical needle penetration forces. J Emerg Med. 1988;6:475-81. 8. Glantz AS, Shnker BK. Primer of applied regression and analysis of variance. New York: McGraw-Hill; 1990:24-33. 9. Ruhe DS. Williams GV. Proud Go. Emergency airway by cricothyroid puncture or.tracheotomy. TransAmkr Acad-Opl thalmol Otolaryngol. 1960#4:182-203. 10. Altman PL, Dittmer DS, eds. Respiration and circulation. Bethesda, Maryland: Federation of American Societies for Experimental Biology, 1971:112.