Rapid Ultrasonography in Shock: A Lifesaving ED Tool

Rapid Ultrasonography in Shock:  Is this really useful?

Caring for patients with undifferentiated hypotension, causes anxiety for most health care providers.  This is natural.  Fear, however, must not lead to hesitation or poor decision-making.  Therapies chosen early in shock disproportionately impact mortality.  For example, giving fluids to a patient in cardiogenic shock may worsen the outcome, but aggressively giving fluids to a patient in septic shock has proven to be lifesaving.  Being able to properly classify a patient’s shock is critical, but patients are often too unstable to wait for confirmatory lab tests or undergo diagnostic imaging such as CT scans.  Point of care ultrasound allows physicians to expand diagnostic capabilities at the bedside, and in real time, collect information that will favorably affect outcomes.

The Rapid Ultrasonography in Shock (RUSH) exam, was developed to give providers a framework for managing hypotensive medical patients.  In brief, the exam focuses on three physiologic parameters, the pump, the tank and the pipes.  Examining the heart (pump) from four basic positions, identifies the presence of pericardial fluid, abnormal left ventricular contractility and acute right ventricular strain.  Measuring the diameter and respiratory change of the inferior vena cava (tank), provides an estimate of volume status-and more importantly volume responsiveness.  Assessment of the abdominal aorta (pipe), identifies dissection or aneurysmal dilatation, and finally compressing the deep veins of the legs (more pipes), detects deep vein thromboses.  See the original article on RUSH by Dr. Perera for details on how these exams are completed.¹

Are we as emergency providers capable of learning the RUSH exam? 

A significant body of evidence supports the proficiency of emergency medicine and hospital medicine providers in using point of care ultrasound to complete the various parts of the RUSH exam.  For example, compared to cardiologists reading formal echocardiograms, emergency physicians with 2 days of training accurately predicted ejection fraction as normal, moderately depressed or severely depressed 84% of the time. ²  Calculation of the IVC distensibility index ((largest IVC diameter-smallest IVC diameter)/smallest IVC diameter) accurately predicts fluid responsiveness in critically ill patients.  A distensibility index greater than 18% predicted a rise in cardiac index of greater than 15% with greater than 90% sensitivity and specificity.³  In a study evaluating accuracy of aortic diameter measurements, 3^rd year emergency medicine residents had near perfect correlation with formal ultrasonography. ⁴ Finally, emergency physicians with one hour of training were able to exclude DVT with 95% negative predictive value using compression venography.  Each physician performed five supervised exams in training prior to participating in the study. ⁵

How will this change what I do in the ED?

Recently, a 58 year-old woman from out of town came to our ER with substernal chest pain.  The EKG showed left bundle branch block.  With no comparison EKGs to review, we performed a limited bedside echo and saw septal hypokinesis.  This information influenced the cardiologist to take the patient for cardiac catheterization despite negative cardiac enzymes.  A 90% lesion of the left anterior descending artery was discovered.  A 73 year-old woman hospitalized a week earlier with a UTI, had syncope during physical therapy.  She was profoundly hypotensive, and with bedside ultrasound had a large right ventricle, a septum bowing to the left, a full IVC and an uncompressible common femoral vein.  She was given thrombolytics and her shock resolved within 30 minutes.  A 48 year-old cirrhotic patient had a cardiac arrest on arrival to the emergency department.  After resuscitation, his bedside ultrasound showed a hyperdynamic heart, a flat IVC and large amount of peritoneal fluid.  On peritoneal aspiration, the fluid was bloody not ascitic.  The patient was rushed to the OR and found to have a bleeding mesenteric varix. 

Rapid Ultrasonography in Shock is a bedside physiologic assessment using point of care ultrasound.  The exam can be performed competently with limited training and practice, and provides invaluable information in managing the sickest patients in the emergency department.

1.  Perera, et al.  The RUSH Exam:  Rapid Ultrasound in Shock in the Evaluation of the Critically Ill, Emerg Med Clin N Am 28 (2010) 29-56.

2.  Moore, MD et al.  Determination of Left Ventricular Function by Emergency Physician Echocardiography of Hypotensive Patients, Acad Emerg Med, March 2002, Vol 9, No 3.

3.  Barbier, et al.  Respiratory changes in inferior vena cava diameter are helpful in predicting fluid responsiveness in ventilated septic patients.  Inten Care Med (2004) 30:1740-1746.

4.  Costantino TG, et al.  Accuracy of Emergency Medicine Ultrasound in the Evaluation of Abdominal Aortic Aneurysm.  J of Emerg Med. Vol. 29, 2005. No 4;455-460.

5.  Frazee, et al.  Emergency Department Compression Ultrasound to Diagnose Deep Vein Thrombosis, J Emerg Med, 2001 Feb:20 (2):107-12.

Ultrasound-Guided Peripheral IVs: An Alternative to Central Venous Catheters?

One of the most common indications for placing a central venous catheter is “no IV access”.  I certainly have placed numerous central lines for this reason.  Unfortunately, this practice is not without consequences.  In addition to patient discomfort and mechanical complications, central line associated blood stream infections (CLABSI) occur in U.S. hospitals approximately 250,000 times per year.  By conservative estimates each CLABSI lengthens hospital stay by two weeks, increases cost by $30,000 and carries a mortality that ranges from 12 to 25%.  CLABSIs can be reduced by adhering to proven practices during insertion and maintenance of central lines.  However, there is also a direct correlation between CLABSI rates and a hospital’s device utilization ratio, defined as the number of central line days divided by the number of patient days in a hospital.  Wouldn’t it be nice if we could just insert fewer central lines?

The use of ultrasound guidance for peripheral IV insertion has been described as early as the 1970s but has increased in the last eight years with the rise of point of care ultrasound.¹  Most patients with difficult venous access lack visible, palpable veins near the surface of the skin.  Edema, obesity, dehydration, IVDA and frequent venipuncture can all cause this problem.  The ultrasound machine can be used to visualize veins deeper beneath the surface of the skin that are often larger, less mobile and have not been previously punctured.  Ultrasound-guided peripheral IVs may be placed by physicians or nurses, can be learned quickly and have high success rates. In an ER setting, Bauman et al saw an 87% success rate among nurses placing ultrasound-guided peripheral IVs for difficult access patients.¹ Gregg et al showed that in a mixed ICU over a 6 month period, 148 requests were received to place ultrasound-guided peripheral IVs after traditional IV access failed, and ultrasound-guided peripheral lines were placed successfully in 147 of those cases.²  While early studies showed low rates of “IV survival” when placed under ultrasound guidance, this problem has largely been eliminated by the use of longer IV catheters, with more of their length within the actual vein.  Infection rates have been shown to be low and comparable to traditional IV insertion using standard aseptic technique.  Finally patient satisfaction with this technique is high.³  Schoenfeld and colleagues surveyed patients who underwent ultrasound-guided peripheral IV insertion and found that the study population, 62% of whom had a central line placed in the past year, had an average satisfaction score of 9.2 out of 10.4⁴

In my practice, I have found an important role for ultrasound-guided IV access.  In the emergency department, the technique has allowed us to avoid placing central venous catheters in patients who otherwise wouldn’t need them.  In the intensive care unit, ultrasound-guided lines are placed so that existing central lines may be removed.  Both doctors and nurses have learned the technique and are inserting lines with high success rates.  To learn more about ultrasound-guided peripheral IV insertion visit us at www.hospitalprocedures.org.

1.  Bauman et al.  Ultrasound-guidance vs. standard technique in difficult vascular access patients by ED technicians.  Am J Emerg Med. 2009 Feb;27(2):135-40.

2.  Gregg et al.  Ultrasound-guided peripheral intravenous access in the intensive care unit.  J Crit Care. 2010 Sep;25(3):514-9.

3.  Adhikari et al.  Comparison of infection rates among ultrasound-guided versus traditionally placed peripheral intravenous lines. J Ultrasound Med. 2010 May;29(5):741-7.

4.  Schoenfeld et al.  Ultrasound-guided peripheral venous access in the emergency department:  A patient centered survey.  Western Journal of Emerg Med.  Nov, 2011:475-77.

Dargin et al.  Ultrasonography-guided peripheral intravenous catheter survival in ED patients with difficult access.  Am J Emerg Med. 2010 Jan;28(1):1-7

Paracentesis: Do I Really Need the Ultrasound?

In residency, I spent a great deal of time at the bedside assessing for shifting dullness in patients who may have had ascites.  Today, when I ask residents about shifting dullness, I often get blank stares.  “Why would I do that when I can use the ultrasound”, they wonder.   Ultrasound guidance for bedside procedures has increased dramatically over the last decade.  Indeed, for some procedures such as central venous catheter placement, ultrasound guidance has become the standard of care.  Its role in paracentesis, however, is less well-defined. 

Blind paracentesis is most often performed in the left lower quadrant at a location that mirrors McBurney’s point after a clinical assessment for ascites.  A study of bedside clinical acumen, however, reveals that the physical exam has a 58% chance of recognizing ascites.¹   Also, blind paracentesis is often unsuccessful for small volumes of fluid.  The success rate when 500 ml of fluid is present is 78%.  This rate drops to 44% when 300 ml of fluid are present.²   Another finding complicates the picture further.  Several studies have shown that ascitic fluid is consistently found in smallest quantities in the left lower quadrant.   Fluid first pools in the perihepatic region or occasionally the perivesicular region but the left pericolic gutter is consistently the last area to fill.^3,4  We can compensate for this by positioning patients so that gravity works with us, and we frequently choose the left because the risk to the spleen, which sits higher in the abdomen is thought to be less than risk to an enlarged liver or potentially distended bladder.  Also, the sigmoid colon is much more mobile than the cecum, theoretically lowering the risk of bowel perforation. But without ultrasound we lack the ability to diagnose ascites consistently, when we do make an accurate diagnosis we frequently end up with a dry tap and we usually choose the location least likely to contain fluid.

There are no randomized trials to assess the utility of bedside ultrasound for paracentesis.  However, a retrospective review of 1297 paracenteses, in which 56% were performed with ultrasound guidance, showed fewer adverse events (1.4 vs 4.7 p=0.01 ), post-paracentesis infections (0.41 vs 2.44 p=0.01) and hematomas (0 vs 0.87 p=0.01) when ultrasound was used.⁵

To be clear, I’m not advocating for real time ultrasound guidance for our Class C cirrhotics who come to the ED every two weeks to have 8 liters of ascites removed.  In fact, I cringe to see residents “checking for fluid” on patients such as these.  But in cases of new or subtle ascites, bedside ultrasound is an invaluable tool to confirm the diagnosis, identify small pockets of ascites and guide us safely to the fluid.

1.     Cattau EL, Benjamin SB, Knuff TE et al: The accuracy of the physical examination in the diagnosis of suspected ascites. JAMA 1982; 247: 1164-1166

2.     Giacobene JW, Siler VE: Evaluation of diagnostic abdomi- nal paracentesis with experimental and clinical studies. Surg Gynecol Obstet 1960; 110: 676-686

3.     Yeh HC, Wolf BS: Ultrasonography in ascites. Radiology 1977; 124: 783-790

4.     99. Proto AV, Lane EJ, Marangola JP: A new concept of ascitic fluid distribution. AJR 1976; 126: 974-980

5.     Patel PA et al:  Evaluation of hospital complication and cost associated with using ultrasound guidance during abdominal paracentesis procedure.  J Medical Economics, 2011;  October 19.

In Defense of FAST: Focused Assessment with Sonography in Trauma

As a third year medical student at a busy trauma center, one of my jobs was to hold a bag of saline connected to the trauma victim’s abdomen and throw the bag to the ground when it emptied.   While I helped with diagnostic peritoneal lavages, radiology residents bumped their way to the bedside with a probe connected to an unwieldy ultrasound machine.  Our trauma surgeons observed with skepticism the radiologists tried to convince us that they could diagnose hemoperitoneum with sound waves.

Since this time, the Focused Assessment with Sonography in Trauma (FAST) has become an important adjunct to the initial evaluation of trauma patients.  It allows providers to quickly detect the presence of intraabdominal free fluid, pericardial fluid and now even the presence of a pneumothorax (Extended FAST or E-FAST).   Ultrasound training has become routine in emergency medicine and the equipment has become more portable without sacrificing image quality.  The American College of Surgeons, Committee on Trauma now endorses FAST in its Advanced Trauma Life Support program. 

Despite this progress, the FAST exam has come under recent scrutiny.  A study by Fox, et al. looking at pediatric blunt trauma found that while FAST had a 96% specificity[e1] , the sensitivity was poor at 52%.¹  Friese, et al. found FAST to have low sensitivity (26%) in blunt pelvic trauma,² and Gaarder, et al. found the FAST exam underperformed in patients with hemodynamic instability (62% sensitivity).³  Based on these and other studies showing poor sensitivity in specific clinical scenarios, some editorialists are calling for the elimination of sonography in the initial assessment of trauma patients. 

To determine the utility of FAST, however, requires a careful look at the data.  In none of the three studies was the training of the ultrasonographers reported.  Accuracy of FAST exam is highly operator dependent.  Also, in the study by Friese, the interval from FAST to laparotomy was over two hours.  Considering that most pelvic bleeding is venous it would be interesting to know what the sensitivity would have been immediately prior to surgery. 

Advantages to FAST include ability of the treating physician to perform the exam quickly at the bedside, avoidance of an invasive procedure and ionizing radiation, ability to perform serial exams, and has a 70-90% overall sensitivity with >95% specificity for detecting hemoperitoneum.⁴  In a retrospective validation study at my institution, 696 FAST examinations were documented over a two year period.  Exams were performed by attending emergency physicians, all of whom had received one hour of didactic training and one hour of hands on scanning using live models.  There were 658 true negative FAST Exams (95%), 6 true positive exams (1%), 38 false negative exams (5%) and 0 false positive exams.  Of the 38 (5%) false negative exams, 20 were due to extraperitoneal injuries and only 6 patients with a false negative exam required laparatomy.⁵

Focused Assessment with Sonography in Trauma remains a useful adjunct to the initial evaluation of the traumatized patient.  A positive FAST exam in a hemodynamically unstable patient requires laparotomy and has been shown to decrease time to the operating room.⁶  Hemodynamically stable patients may undergo careful confirmatory evaluation with diversion to the OR for the development of hypotension. For patients with a negative FAST examination, those with low suspicion of intraabdominal injury may undergo observation with serial examinations, while those with high suspicion of intraabdominal injury require further diagnostic testing.

1.  Fox JC et al. Test characteristics of focused assessment of sonography for trauma for clinically significant abdominal free fluid in pediatric blunt abdominal trauma. Acad Emerg Med 2011 May; 18:477

2.  Friese RS et al. Abdominal ultrasound is an unreliable modality for the detection of hemoperitoneum in patients with pelvic fracture. J Trauma 2007 Jul; 63:97-102.

3.  Gaarder C et al. Ultrasound performed by radiologists — Confirming the truth about FAST in trauma. J Trauma 2009 Aug; 67:323.

4. Stengel D, Bauwens K, Rademacher G, Mutze S, Ekkernkamp A. Association between compliance with methodological standards of diagnostic research and reported test accuracy: meta-analysis of focused assessment of US for trauma. Radiology. 2005;236:102-111.  

5.  Holmes, et al.  FAST ENOUGH?  Validating FAST in a New Trauma Center, Department of Surgery, Ventura County Medical Center, Abstract for Presentation at Southern California American College of Surgeons Annual Assembly

6. Griffin XL, Pullinger R. Are diagnostic peritoneal lavage or focused abdominal sonography for trauma safe screening investigations for hemodynamically stable patients after blunt abdominal trauma? A review of the literature. J Trauma. 2007;62:779-784

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 [e1]For what? For hemoperitoneum?

Ultrasound-guided Central Venous Access: Are Landmarks a Thing of the Past?

Point-of-care ultrasound is one of the most rapidly evolving areas of medicine.  In addition to becoming the “stethoscope of the future” for bedside diagnostic evaluations, ultrasound has improved the safety and efficiency of a wide range of procedures.  The use of ultrasound to assist in central venous cannulation was first studied in 1993 by Uretsky et al.  In this prospective evaluation, 302 patients underwent ultrasound-guided internal jugular cannulation and were compared to 302 patients undergoing landmark based internal jugular cannulation.  The authors showed a significantly increased success rate using the ultrasound.  Further, access times and complication rates were decreased by the ultrasound-guided approach.¹  Since 1993, multiple randomized investigations have confirmed higher success rates using  ultrasound guidance for internal jugular vein cannulation and lower incidences of carotid puncture, carotid hematoma, hemothorax and pneumothorax.^2,3,4,5  In 2001, in its Quality Evidence Report, the Agency for Healthcare Research and Quality (AHRQ) strongly recommended real-time, dynamic guidance for all central venous catheter insertions to prevent complications.⁶

Despite the AHRQ recommedation, ultrasound-guided central venous access has not been universally adopted for a number of reasons.  Access to affordable point-of-care ultrasound machines, has been limited, especially in smaller hospitals.  Training of novice ultrasonographers, while shown to be cost effective, is not widely available.  Also, despite evidence to the contrary, many providers still hold the belief that ultrasound-guided access requires more time and resources.  Novice users, may indeed require more time to prepare the ultrasound machine and probe, and are more likely to require a second proceduralist to hold the ultrasound probe during the cannulation; however, proficiency with the ultrasound-guided technique has been demonstrated after a single one hour training session.⁷ Furthermore, despite improved clinical outcomes, ultrasound guidance has not yet been shown to save money.  The only cost analysis to date, has shown increased cost using ultrasound guidance ($494,820,000 vs $390,780,000).  This cost difference was not mitigated by its reduction in the cost of managing pneumothoraces and hematomas, but still needs to be considered in the context of safety and patient satisfaction.

Evidence supporting ultrasound-guided internal jugular venous catheter insertion continues to grow.  But what about venous cannulation in the femoral and subclavian veins?  Similar to internal jugular lines, femoral venous catheters placed under ultrasound guidance have been placed with higher success rates and with fewer complications.⁸  Some early studies have also demonstrated successful placement of subclavian venous catheters under ultrasound guidance.⁹  Currently, few clinicians perform ultrasound-guided subclavian venous catheters because it is technically challenging.  The subclavian vein, for most of its course, runs beneath the clavicle.  This creates a sonographic shadow, preventing direct visualization of the needle entering the vein.  In the July issue of Critical Care Medicine, Fragou et al  present a study of 463 patients randomized to ultrasound-guided or landmark based subclavian venous cannulation.  Not only did the authors show a decreased time to cannulation and decreased rate of complications, they also achieved a 100% success rate in the ultrasound-guided group.¹⁰  The authors describe the technique used for ultrasound  guidance:  If a portion of the subclavian vein was not directly visible, the operator would slide the ultrasound probe laterally until able to visualize the compressible axillary vein.  The axillary vein was then cannulated under direct visualization.  Next the ultrasound probe was moved medially and tilted in such a way that the subclavian vein could be visualized under the clavicle, and passage of the guidewire into the subclavian vein was verified by ultrasound. 

So do you have to use the ultrasound?  In my practice, I try to perform ultrasound-guided cannulation when possible.  I don’t routinely use the ultrasound for femoral venous lines, but because of increased rates of catheter related blood stream infections and venous thrombosis, I try to only place femoral lines in emergent circumstances, such as codes and severe hemorrhagic shock.  In accordance with AHRQ and IHI guidelines, I do place all internal jugular venous catheters under ultrasound guidance.  For subclavian lines, there is a growing body of evidence, including last month’s CCM article, that ultrasound guidance is achievable, efficient and improves safety.  So don’t forget your anatomy, but consider ultrasound-guided simulation training if you are not currently proficient in this technique.

1.  Denys BG, et al.  Ultrasound-assisted cannulation of the internal jugular vein.  A prospective comparison to the external landmark-guided technique.  Circulation. 1993 May;87(5):1557-62.

2.  Keenan SP, et al  Use of ultrasound to place central lines.  J Crit Care. 2002 Jun;17(2):126-37

3.  Slama M et al.  Improvement of internal jugular vein cannulation using an ultrasound-guided technique.  Intensive Care Medicine, 1997;23:916-919.

4.  Randolph AF, et at.  Ultrasound guidance for placement of central venous catheters:  a meta-analysis of the literature.  Critical Care Med.  1996; 24:2053-2058.

5.  Miller AH et al.  Ultrasound guidance vs landmark technique for central venous catheter placement in the emergency department.  Acad Emerg Med. 2002. Aug 9(8):800-5.

6.  Agency for Healthcare Research and Quality.   Making health care safer:  a critical analysis of patient safety practices.  Evid Rep Technol Assess 2001;(43);i-x; 1-668.

7.  Evans LV, et al.  Simulation training in central venous catheter insertion:  Improved performance in clinical practice.  Acad Med.  2010.  Sep;85(9):1462-9.

8.  Prabhu MV, et al.  Ultrasound-guided femoral dialysis access placement:  a single center randomized trial.  Clin J Am Soc Nephrol.  2010 Feb;5(2):235-9

9.  Gualtieri E, et al. Subclavian venous catheterization: greater success rate for less experienced operators using ultrasound guidance. Crit Care Med. 1995; 23:692–697.

10.  Fragou M et al.  Real time ultrasound-guided subclavian vein cannulation vs the landmark method in critical care patients.  A prospective randomized study. Crit Care Med. 2011 Jul;39(7):1607-1612.

Should Endotracheal Intubation Become a Videogame?

Direct vs Video Laryngoscopy

In 1878 William MacEwan first passed a tube into the trachea of an awake patient using his fingers as a guide.  Now health care providers routinely perform endotracheal intubation as a life saving intervention.  Despite over 100 years of experience, this procedure is still associated with significant risk, especially for novice and low volume intubators.  Many tools have been developed to mitigate the risk of not securing an airway, including intermediate airway devices such as the Combitube and laryngeal mask airway and airway adjuncts such as the Eschmann stylet.  In recent years, video laryngoscopes have made their way into emergency departments and intensive care units.  Do these devices offer a better view of the larynx?  Do they increase the chance of successful intubation in routine cases?  What about difficult airways?  Is one video laryngoscope better than the rest?  A flurry of investigations in the last decade have attempted to sort through these important issues.

In 2009, Brown et al showed that good glottic views improved from 80% to 93% by using a video Macintosh larygoscope in routine emergency room patients [1].  Subsequently, Ayoub et al randomized 42 medical students to Macintosh vs Glidescope facilitated intubation training on manikins and then showed a significant improvement in first pass success rates on actual patients when using the Glidescope [2].  Patients predicted to have difficult airways were excluded from this study.  For such patients, there is a paucity of randomized trials comparing direct laryngoscopy to video laryngoscopy outside of the operating room.  Some data suggests video laryngoscopy, when used for predicted difficult airways, reduces force on maxillary incisors [3] and causes less cervical spine movement in trauma patients [4].  A potential disadvantage of video laryngoscopy is a longer time to tracheal intubation [5], although this has not been shown to be true with all devices and probably improves as intubators gain more experience with indirect laryngoscopy. 

Since their introduction over a decade ago, several video laryngoscopes have been developed, and their manufacturers have engaged in fierce competition to demonstrate superiority.  Several head to head comparisons have evaluated different devices [6] [7] [8] [9].  Among non-industry sponsored studies, no video laryngoscope has risen to the top as clearly superior.  However, each has distinct advantages.  For example, the Glidescope comes in pediatric sizes that can be used in adults with limited interincisor opening.  The Storz V-Mac uses a traditional Macintosh blade that provides a straighter trajectory to the glottis when compared to the Glidescope and McGrath.  These have steeply angulated blades requiring the use of specialized stylets.  The integrated tube channel of the Pentax Airway Scope may facilitate more rapid intubation compared to other devices.

In my own practice in emergency and critical care medicine, I mostly supervise as residents secure the airway.  In these settings, I ask residents to start with direct laryngoscopy.  In response to their complaints, I point out that electronic equipment sometimes fails and that they may someday practice in a facility that does not own a video laryngoscope.  For trauma patients with cervical immobilization, I start with video laryngoscopy to decrease the risk of cervical motion.  In patients with predicted difficult airways, I have the video laryngoscope set up and turned on but start with direct laryngoscopy.  In this setting, the video laryngoscope serves as a backup parachute that is sometimes needed to help land with a secure airway.

[1] Brown CA III et al. Improved glottic exposure with the video Macintosh laryngoscope in adult emergency department tracheal intubations. Ann Emerg Med 2010 Aug; 56:83.

[2] Ayoub CM et al. Tracheal intubation following training with the GlideScope® compared to direct laryngoscopy. Anaesthesia 2010 Jul; 65:674.

[3] Lee RA et al. Forces applied to the maxillary incisors during video-assisted intubation. Anesth Analg 2009 Jan; 108:187.

[4] Robitaille A et al. Cervical spine motion during tracheal intubation with manual in-line stabilization: Direct laryngoscopy versus GlideScope® videolaryngoscopy. Anesth Analg 2008 Mar; 106:935.

[5] Walker L et al. Randomized controlled trial of intubation with the McGrath® Series 5 videolaryngoscope by inexperienced anaesthethists. Br J Anaesth 2009 Sep; 103:440.

[6] Maassen R et al. A comparison of three videolaryngoscopes: The Macintosh laryngoscope blade reduces, but does not replace, routine stylet use for intubation in morbidly obese patients. Anesth Analg 2009 Nov; 109:1560

[7] Liu L et al. Tracheal intubation of a difficult airway using Airway Scope, Airtraq, and Macintosh laryngoscope: A comparative manikin study of inexperienced personnel. Anesth Analg 2010 Apr; 110:1049.

[8] van Zundert A et al. A Macintosh laryngoscope blade for videolaryngoscopy reduces stylet use in patients with normal airways. Anesth Analg 2009 Sep; 109:825

[9] Liu EHC et al. Tracheal intubation with videolaryngoscopes in patients with cervical spine immobilization: A randomized trial of the Airway Scope® and the GlideScope®. Br J Anaesth 2009 Jun 19; [e-pub ahead of print]. (http://dx.doi.org/10.1093/bja/aep164)