a b s t r a c t

Background: Intraosseous (IO) access can be complicated by obesity. successful placement of a 25 mm IO needle is unlikely when soft tissue depth exceeds 20 mm.

Objectives: The authors examined the relationship between body mass index (BMI), the ability to palpate the tib- ial tuberosity (TT), and soft tissue depth at recommended IO insertion sites.

Methods: Obese emergency department patients were assessed for a palpable TT and received ultrasound mea- surement of the soft tissue depth at recommended IO insertion sites. Linear and logistic regression were used to determine cut-off BMI values predicting soft tissue depth N 20 mm.

Results: Seventy-five patients were enrolled with a mean BMI of 47.2. The mean soft tissue depth at the proximal humerus, proximal tibial, and distal tibial were 29.6 [95% CI 27.5-31.7] mm, 11.0 [8.9-13.0] mm, and 10.7 [9.4- 12.1] mm, respectively. In 5 patients without a palpable TT the soft tissue depth exceeded 20 mm at all three an- atomic sites. A BMI >=43 and BMI >=60 predicted a soft tissue depth N 20 mm at the proximal tibia and distal tibia, respectively, while no reliable BMI cut-off was identified at the proximal humerus.

Conclusions: In obese adults with a palpable TT or BMI <=43 a 25 mm IO needle is likely adequate at the proximal

and distal tibial insertion sites. Empiric use of an extended 45 mm IO needle is advisable at the proximal humeral insertion site in obese patients.

(C) 2016

Introduction

IO access has demonstrated its utility for adult patients in multiple care arenas, including prehospital and emergency department (ED) set- tings [1,2]. Both peripheral and central Intravenous access can be difficult in critically ill adult patients for a variety of reasons and IO ac- cess can facilitate rapid vascular access [3-6]; however, obesity can present anatomic barriers to successful placement.

An estimated 30% of the adult US population is classified as obese, defined as a body mass index (BMI) N 30 [7]. Emergent intravenous ac- cess in these patients can prove technically difficult due to body habitus [8]. While IO access is a viable option, the distance between the skin and cortical bone is a critical factor in successful IO placement. Obesity has also been cited as an obstacle to adequate IO access [9]. IO needles

? Support: NONE. No conflicts of interest or financial disclosures.

?? Presentations: American College of Emergency Physicians Scientific Assembly, San Francisco, CA, October 2011.

* Corresponding author at: York Hospital Department of Emergency Medicine, 1001 S. George St., York, PA 17405. Tel.: +1 717 851 5067.

E-mail address: [email protected] (T. Kehrl).

require at least 5 mm of excess length for successful placement, which represents a maximum soft tissue depth of 20 mm for 25 mm needles and 40 mm for extended 45 mm needles [10].

Prediction of IO failure is also important for clinical reasons. Unsuc- cessful or futile attempts at IO access can delay care, contribute to med- ical waste and generate complications. An incorrectly positioned IO needle and errant fluid administration into the overlying soft tissue can lead to compartment syndrome [11-13].

In this study, we examined the relationship between body mass index (BMI), the ability to palpate the tibial tuberosity (TT), and soft tissue depth at recommended IO insertion sites. We hypothesized that elevated BMI predicts increased cortical bone depth at standard IO insertions sites and, more specifically, that cut-off BMI values exist above which the cor- tical bone depth likely exceeds that accessible with a 25 mm IO needle.

Methods

Study setting and population

This was a prospective, observational study examining a conve- nience sample of obese patients presenting to an academic, urban,

http://dx.doi.org/10.1016/j.ajem.2016.06.055

0735-6757/(C) 2016

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Table

Mean soft tissue depth from skin to cortical bone at typical IO insertion sites.

	All patients BMI N 30 (mm, [95% CI])	TT non-palpable (mm, [95% CI])
Proximal tibia	11.0 [8.9-13.0]	29.7 [12.7-46.8]
Distal tibia	10.7 [9.4-12.1]	20.8 [8.7-33.0]
Proximal humerus	29.6 [27.5-31.7]	35.8 [28.5-43.1]

Abbreviations: BMI (body mass index), mm (millimeter), CI (confidence interval), IO (intraosseous), TT (tibial tuberosity).

tertiary care center ED with an annual volume of approximately 90,000 patients. The study was approved by the institutional review board.

Study protocol

Adult ED patients were screened for potential enrollment between January 2009 and June 2010 by review of their BMI in the electronic medical record. BMI was calculated using the current height and weight as entered at the time of presentation to the hospital or as entered by primary care providers within one year of screening.

Patients were eligible for inclusion if they were at least 18 years of age and their BMI exceeded 30. Planned exclusion criteria included any existing condition that might preclude unabated IO access, such as overlying infection, trauma or previous amputation; however, none of these conditions were encountered during the study. Written consent was obtained. Enrolled patients received Ultrasound measurement of the soft tissue depth at the three most commonly recommended IO in- sertion sites: Proximal humerus, proximal tibia and distal medial tibia. The sonographer also evaluated the tibial tuberosity (TT) and classified it as palpable or non-palpable. All screening, consent and sonography was performed by one of two emergency ultrasound fellows actively training during the study period. Data was directly entered into a dedi- Ultrasound technique“>cated study spreadsheet (Microsoft Excel; Microsoft, Redwood, WA).

Ultrasound technique

Patients were placed in the standard position for IO needle insertion. A 12 to 13 MHz linear ultrasound transducer (Philips, Bothell, WA) was placed orthogonal to the patient’s skin at each of the three IO insertion sites and the beam was directed along the intended IO needle path. A transverse cross section of the soft tissue was visualized and ultrasound calipers were used to measure the sonographic depth from skin to cor- tical bone.

Fig. 1. Observed and mean observed proximal tibial depth by body mass index.

Fig. 2. Observed and mean observed distal tibial depth by body mass index.

Outcome measures

The primary outcome measure was the soft tissue depth at each of the three IO insertion sites, defined as the distance from the skin surface to the outer bony cortex. This was recorded both as a continuous vari- able (depth in millimeters (mm)) and as a dichotomous variable (<= 20 mm or N 20 mm). This dichotomization served as a surrogate for depth-related IO placement failure.

As a secondary outcome measure, the tibial tuberosity (TT) was assessed in each patient and recorded as either palpable or non- palpable.

Data analysis

In addition to basic descriptive statistics for the study sample, simple linear regression was used to model the mean soft tissue depth as a function of BMI at each of the three insertion sites. Based on visual in- spection of the data set and the results of model diagnostic testing, a weighted least squares variance structure was used to estimate the 95% confidence interval for the linear regression sample mean. Logistic regression was subsequently employed to model the probability of soft tissue depth exceeding 20 mm as a function of BMI. Appropriate standard model diagnostics were performed to assess the fit and predic- tive capacity of each model. Prior to undertaking this analysis, the

Fig. 3. Observed and mean observed proximal humeral depth by body mass index.

T. Kehrl et al. / American Journal of Emergency Medicine 34 (2016) 1831-1834 1833

Fig. 4. Predicted probability of proximal humerus depth exceeding 20 mm by body mass index.

research team made an a priori assumption, based on their opinion, that a “reliable” IO insertion site would have a depth-related predicted fail- ure rate of less than 10%. All statistical analysis was performed using STATA 11.2 (College Station, TX).

Results

Seventy-five patients were enrolled. The mean BMI was 47.2 (SD 10.5) with a median of 45.8 (interquartile range: 39.7-50.4). The TT was palpable in 70 patients with an average BMI of 45.7. The TT was non-palpable in 5 patients with an average BMI of 67.2. The soft tissue depth exceeded 20 mm at all three anatomic IO sites in all patients with a non-palpable TT. Mean depths including 95% confidence inter- vals are shown in Table.

The linear regression model results are shown in Figs. 1-3. Higher BMI was found to be moderately predictive of increased soft tissue depth at both the proximal and distal tibial sites with F-test P b .05 (model adjusted R² of 0.48 and 0.46, respectively). Conversely, BMI was not found to be predictive of soft tissue depth among obese patients at the proximal humeral site (F test P= .08, R²= 0.028).

The logistic regression model results are shown in Figs. 4-6. Among the obese patients in our sample, no BMI threshold reliably predicted a soft tissue depth <=20 mm at the humeral site (Fig. 4). Assuming an ac- ceptable IO failure rate of 10%, a BMI of approximately <= 43 predicts a

Fig. 5. Predicted probability of proximal tibial depth exceeding 20 mm by body mass index.

Fig. 6. Predicted probability of distal tibial depth exceeding 20 mm by body mass index.

soft tissue depth <= 20 mm at the proximal tibial site (Fig. 5). At the distal tibial site a BMI <= 60 predicts a soft tissue depth <=20 mm (Fig. 6).

Discussion

Vascular access is frequently challenging in patients with high BMI. IO access offers a potentially life-saving option, but is limited by body habitus in obese patients. Unsuccessful IO needle placement due to in- adequate needle length creates medical waste and can delay critical medical interventions.

Our results suggest a standard 25 mm IO needle is likely adequate for IO access at the proximal tibia if patient BMI <= 43 and at the distal tibia if BMI <= 60. Otherwise an extended 45 mm IO needle should be used.

For the proximal humerus, our data did not identify a BMI cut-off that might reliably predict an insertion depth <= 20 mm. This supports manufacturer recommendations that standard 25 mm IO needles should not be used at the proximal humeral site in obese adult patients. The proximal humeral site should only be considered in the obese pa- tient if a 45 mm needle is available.

While BMI providesa convenient guide for appropriate IO needle length, its calculation requires more than simple arithmetic and precise measure- ments are often not available in the emergent setting. For this reason, we de- veloped an algorithm for IO placement utilizing the TT (Fig. 7). Our results suggest that a standard 25 mm IO needle should be adequate at the proximal or distal tibia insertion sites in obese patients with a palpable TT.

Fig. 7. Algorithm for IO access location in obese adult.

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The choice of correct anatomic location for IO access is an important consideration during resuscitation. The literature suggests that flow through the proximal humeral site in adults is greater than flow through tibial insertion sites [14,15] and proximal tibial flow is greater than distal tibial flow [16]; however, proximal tibial IO insertions have been found to be 30% to 40% more successful than proximal humeral IO insertions [17,18]. The results of this study, coupled with knowledge of the advantages and disadvantages of the various IO sites, can assist the emergency pro- vider in optimizing IO placement in critically ill obese patients. Given the relative advantages of the humeral and proximal tibial sites and the relative unreliability of the standard 25 mm IO needle in accessing the former site, EMS systems and emergency departments should con-

sider maintaining a stock of extended length 45 mm IO needles.

Limitations

The study involved a small, convenience sample at a single center. In particular, only five patients without a palpable tibial tuberosity were included. The proposed algorithm based on palpation of the tibial tuber- osity requires further validation to confirm clinical accuracy. Ultrasound was used as a noninvasive surrogate for actual measurement. The study did not employ an invasive method to directly measure soft tissue depth, nor was IO access actually obtained to confirm or refute adequate needle length. Only two clinicians performed sonographic measure- ments and inter-rater reliability was not assessed. We did not take into consideration factors such as age, gender, or underlying medical conditions that might predispose patients to peripheral edema or other confounding variables; however, BMI provided an objective mea- surement by which included patients were evaluated.

Conclusion

In obese adults, increased BMI correlates to increased soft tissue depth to cortical bone at standard IO insertion sites. Insertion of a stan- dard 25 mm IO needle is feasible at the proximal tibial and distal tibia for patients with BMI <=43 and BMI <= 60, respectively, and at either of these sites if the TT is palpable. Only 45 mm IO needles should be used at the proximal humerus insertion site and should be accessible to emergency care providers at all times.

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