UMEM Educational Pearls - Critical Care

This week's pearl is courtesy of Dr. Evie Marcolini.  Thanks Evie!

Abdominal Compartment Syndrome in Burn Patients

• Patients who receive > 250 ml/kg of fluid in the the 24 hours after burn injury will most likely require abdominal decompression.
• In light of this, bladder pressure monitoring should be part of your practice in resuscitation of the patient with >30% TBSA burns.
• The simple act of placing the bladder probe will increase awareness of the possibility of ACS and prompt measurement of abdominal compartment pressures. 
• ACS can be treated with decompressive laparotomy, or in some cases, percutaneous abdominal decompression.

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Extracorporeal Membrane Oxygenation

• In last week's pearl pertaining to critically ill patients with H1N1, I mentioned the use of ECMO as a potentially life-sustaining treatment for refractory respiratory failure.
• Essentially, ECMO removes blood from the patient and circulates it through an artificial lung with a pump.  For patients with respiratory failure, this is usually accomplished via cannulation of the femoral and internal jugular veins.
• General guidelines to consider ECMO in severe, refractory respiratory failure include:
  • P_aO₂ / F_iO₂ ratio < 100 on 100% F_iO₂ or A-a gradient > 600 mm Hg
  • Age < 65 years
  • No known contraindication to anticoagulation
  • Lack of significant co-morbidities (due to prolonged recovery after weaning from ECMO)

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Critically Ill Patients with H1N1

• Three recent reports published online in the Journal of the Americal Medical Association (JAMA) detail the potential problems of H1N1 infection in the critically ill.
• The three studies (Mexico, Canada, Australia/New Zealand) seem to have recurring themes:
  • shock and multisystem organ failure were common
  • many were healthy, young adults who developed rapid respiratory failure
  • hypoxemia was prolonged and often refractory to conventional modes of mechanical ventilation
• Newer modes of ventilation and therapies were required to treat refractory hypoxemia.  These included high frequency oscillatory ventilation, prone positioning, neuromuscular blockade, nitric oxide, and extracorporeal membrane oxygenation.
• Take Home Point: Involve your intensivist early in the management of ED patients with respiratory failure and suspected H1N1 infection, as non-conventional methods of ventilation may be needed to treat hypoxemia.

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Damage Control Resuscitation

• "Damage control resucitation" is a term that is used to describe the resuscitation strategy of damage control surgical techniques and the tolerance of moderate hypotension, prevention of hypothermia, temporization of acidosis, and the correction of coagulopathy in the severly injured trauma patient.
• In terms of the "lethal triad", it is important to avoid interventions that may cause, or worsen, acidosis.
• A preventable and easily correctable cause of acidosis is hypoventilation.
• In the intubated trauma patient, pay close attention to the minute ventilation to avoid hypoventilation and the accumulation of CO₂.

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Mechanically Ventilated ED Patients and Secretion Mobilization

• As more of our intubated ED patients remain in the ED for longer periods of time, some may develop problems with secretion management (thick/copious amounts of sputum).
• The preferred method of secretion mobilization is heated humidification.
• If you anticipate the duration of intubation to be at least 96 hours, have your respiratory therapist set up a heated humidifier.
• Commonly, clinicians and nurses will instill 5-10 ml of isotonic saline to thin secretions.
• The use of saline to thin secretions is unsupported by the literature and carries a small risk of dislodging the bacterial laden biofilm that covers the endotracheal tube.

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Complications of Resuscitation

• CPR, defibrillation, endotracheal intubation, and cannulation of peripheral and central veins are common procedures during resuscitation of cardiac arrest patients
• Although not obvious immediately, complications from these procedures can develop and manifest several hours after successful return of spontaneous circulation
• Not surprisingly, the most common complications are rib and sternal fractures
• Additional complications to recall include:
  • tracheal mucosal lesions (almost 20%)
  • retropharyngeal bleeding
  • liver/spleen injuries
  • rhabdomyolysis (post-defibrillation)
  • air embolism (central venous access)
  • gastric rupture (very rare; due to continuous air insufflation into the stomach)

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The Supraclavicular Subclavian Central Venous Cathetherization

• Central venous catheters (CVCs) are routinely placed in critically ill ED patients.
• The literature has clearly demonstrated that CVCs placed in the subclavian vein have lower risks of infection and thrombosis when compared to the femoral and internal jugular vein routes.
• Although we routinely teach the infraclavicular approach, don't forget the subclavian vein can also be cannulated via the supraclavicular approach.
• Some pearls on the supraclavicular approach:
  • Identify the clavisternomastoid angle: formed by the lateral head of the sternocleidomastoid muscle (SCM) and the clavicle
  • Insert the needle 1 cm lateral to the lateral head of the SCM and 1 cm posterior to the clavicle
  • Direct the needle at a 45-degree angle aimed at the contralateral nipple
  • The right side is preferred due to a more direct route to the SVC and a lower pleural dome (decreasing the incidence of pneumothorax)
  • Place the patient in Trendelenburg position and aim the bevel of the needle downward

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Pulse Pressure Variation and Volume Responsiveness

• Assessing volume status in the critically ill is extremely challenging, as up to 50% of patients do not respond to a fluid challenge (i.e. increase their stroke volume/cardiac output with additional IVFs).
• As highlighted in previous pearls, traditional measurements such as blood pressure, heart rate, and urine output are extremely variable and inaccurate in determining volume status.
• Pulse pressure variation is an emerging method of volume assessment that, to date, seems even better than ultrasound measurements of the IVC.
• To calculate PPV, print out a tracing from an arterial line that captures both inspiration and expiration use the following formula:
  • ΔPP = 100 × (PPmax - PPmin)/[(PPmax + PPmin)/2]
• Values > 13% indicate that the patient is likely on the ascending portion of their Starling Curve and will augment their cardiac output with additional IVFs.
• Note that arrhythmias and spontaneous breathing can affect measurements, thus patients should be mechanically ventilated and well sedated when measuring PPV.

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High Frequency Oscillatory Ventilation (HFOV)

• Although traditionally used in neonates, HFOV is becoming increasingly popular for select adult patients with ALI/ARDS.
• Benefits of HFOV include:
  • use of smaller tidal volumes than conventional ventilation
  • maintains alveoli open at a relatively constant airway pressure thereby preventing atelectrauma
  • improves ventilation/perfusion
• Indications for use of HFOV are when:
  • conventional ventilator settings require an F_iO₂ > 70% and PEEP > 14 cm H₂O OR
  • pH < 7.25 despite higher tidal volumes and plateau pressures > 30 cm H₂O
• Key variables, along with suggested initial settings, for HFOV include:
  • Frequency: 4 - 7 Hertz
  • Amplitude: 70 - 90 cm H₂O
  • Mean airway pressure: 5 cm H₂O greater than last plateau pressure measured on conventional setting
  • Bias flow: 40 L/min
  • Inspiratory time: 33%
  • F_iO₂: 100%

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Airway Pressure Release Ventilation (APRV)

• As emergency physicians manage mechanically ventilated patients for longer periods of time, it is important to be familiar with newer, alternative modes of ventilation
• APRV is an open-lung ventilation strategy designed to provide oxygenation benefits while augmenting ventilation for patients with low compliance lung disease
• APRV has been described as CPAP with brief, regular, intermittent releases in airway pressure - essentially cycling between two CPAP levels
• The degree of ventilatory support is determined by the duration at each of the 2 CPAP levels and the distending pressure
• The 5 major parameters of APRV, along with suggested initial settings include:
  • P_high (high pressure): set at desired plateau pressure
  • T_high (time spent at the high pressure): 4-6 seconds
  • P_low (low pressure): 0 cm H₂O
  • T_low (time spent at the low pressure): 0.6-0.8 seconds
  • F_iO₂: 100%
• The pressure gradient between P_high and P_low, T_low, and the patient's spontaneous minute ventilation are the primary determinants of alveolar ventilation
• When using APRV, be sure to optimize intravascular volume to offset the decrease in venous return that results from prolonged positive intrathoracic pressure

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Antibiotic Dosing in the Critically Ill Septic Patient

• Current international guidelines recommend that intravenous antibiotics begin within one hour for those with severe sepsis and septic shock.
• Equally as important as choosing the right antimicrobial is choosing the correct dose at the right dosing schedule.
• In fact, there is evidence to suggest improved outcomes in patients given continous antimicrobial infusions (over hours) rather than intermittent bolus dosing (over minutes).
• An important cause of underdosing in critically ill patients, especially those with sepsis, is hypoalbuminemia.
• It is believed that by increasing the unbound fraction, hypoalbuminemia promotes more extensive distribution and greater renal clearance, thereby increasing the risk of underdosing.
• Take Home Point: Critically ill septic patients with hypoalbuminemia require higher dosages, or alternative regimens, to ensure appropriate antimicrobial coverage.

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Internal Jugular CVC Placement and Posterior Wall Penetration

• For a variety of reasons, many critically ill ED patients require central venous access.
• Ultrasound guidance, especially with catheters placed in the internal jugular (IJ), has become standard practice in many EDs.
• Ultrasound guidance is associated with higher success rates, reduced insertion attempts, and reduced placement failures.
• Importantly, ultrasound allows you to visualize the carotid artery which often either partially overlies or even sits direclty under the IJ.
• Recent literature, however, suggests that posterior wall penetration of the IJ, even with ultrasound guidance, may be much more common than previously thought.
• Take Home Point: Even when using ultrasound, maintain strict visualization of the needle in the IJ lumen and recognize that posterior wall penetration (into the carotid) can easily occur.

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Dexmedetomidine and the Critically Ill

• Dexmedetomidine is a newer sedative agent that is being used with increasing frequency in the critically ill
• A few pieces of information regarding dexmedetomidine:
  • highly selective alpha-2 agonist
  • produces dose-dependent sedation and anxiolysis while maintaining arousability at deep levels of sedation (hypercapnic arousal is preserved)
  • onset of action is approximately 15 minutes with peak concentration achieved in about 1 hour
  • metabolized via the liver
  • no known active or toxic metabolites
  • loading dose of 1 mcg/kg over 10 minutes followed by 0.2 - 0.7 mcg/kg/hr
• Primary side effect is bradycardia at excessive doses
• Cost is an issue when compared to propofol and midazolam

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Lorazepam Infusions

• There is some literature that propofol may be better for sedation in the mechanically ventilated patient, yet many emergency physicians still do not have access to the medication
• Lorazepam infusions are frequently used in many EDs for sedation of the mechanically ventilated patient
• Patients receiving continuous infusions of lorazepam are at risk for propylene glycol toxicity
• Propylene glycol toxicity primarily causes a metabolic acidosis and acute tubular necrosis
• Critically ill patients with renal or hepatic dysfunction are at increased risk of toxicity
• Monitoring propylene glycol levels are impractical
• Rather, check the osmol gap: a gap > 10 - 15 reflects significany propylene glycol accumulation
• Hemodialysis effectively removes propylene glycol

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The Cuff Leak Test

• As many of us have undoubtedly experienced, we are now extubating patients in the ED due to prolonged lengths of stay
• Critical to extubation is determining whether laryngeal edema may be present
• Laryngeal edema, resulting in airway obstruction, is one of the most common causes of respiratory distress following extubation
• Although shown to have moderate accuracy, many use the 'cuff leak test' to determine the iikelihood of laryngeal edema
• In most studies, performance of the cuff leak test is as follows:
  • take the average of 6 serial measurements of expired tidal volume with the ETT cuff inflated
  • take the average of 6 serial measurements of expired tidal volume with the ETT cuff deflated
  • a difference of < 110 ml between averages strongly suggests the presence of laryngeal edema
• Take Home Point: patients with a cuff leak test < 110 ml are likely to have laryngeal edema and are at high risk of airway obstruction post-extubation.  It is best not to extubate these patients in the ED.

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Coagulopathy from Acute Liver Failure

• ALF is defined as
  • absence of chronic liver disease
  • acute elevation in AST/ALT accompanied by INR > 1.5
  • any degree of mental status change (encephalopathy)
  • illness less than 26 weeks duration
• The most common cause is acetaminophen toxicity
• Regarding the coagulopathy that develops with ALF:
  • FFP transfusion is not encouraged, as the volume may exacerbate cerebral edema and it has been shown to be ineffective for improving INR elevations
  • The prophylactic transfusion of platelets for extreme thrombocytopenia is also not recommended for similar reasons

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The Maintenance Phase of Therapeutic Hypothermia

Therapeutic hypothermia (TH) has become standard in the care of patients with return of spontaneous circulation from cardiac arrest.  Although the optimal duration of TH is unknown, current literature supports 12-24 hours of cooling to 32-34^oC.  As many of our critically ill patients remain in the ED for seemingly endless lengths of stay, it is likely that most emergency physicians will be managing patients with TH during the maintenance phase of cooling.  Some pearls regarding the maintenance phase:

• Metabolic and hemodynamic homeostasis is critical
• Target volume-cycled mechanical ventilation to maintain a normal pH
• Maintain a MAP > 65 mm Hg
• Maintain blood glucose between 120 - 160 mg/dL
• Frequently check and aggressively replete potassium, magnesium, and phospate

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Acute Hyponatremia and the Critically Ill

• I just left a busy ED shift during which we had a patient with altered mental status and a serum Na of 115 mmol/L.
• Recall that severe hyponatremia may present with lethargy, disorientation, agitation, nausea/vomiting, altered mental status, abnormal respirations, and seizures.
• For severe, symptomatic hyponatremia, the treatment of choice is 3% hypertonic saline
• At a rate of 100 ml/hr, the serum Na should rise approximately 2 mmol/L per hour.
• In general, the duration of treatment with hypertonic saline is based upon sign and sypmtom improvement.
• For those with more longstanding hyponatremia, serum Na should not be increased by more than 12 mmol in the first 24 hours.

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Transient Hypotension and Mortality in Sepsis

• Not surprisingly, septic ED patients with persistent hypotension despite fluid resuscitation have increased mortality.
• What about the more common scenario of septic ED patients who have a transient drop in their BP?
• Recent evidence suggests that ED patients with sepsis who have non-sustained decrease in their BP (SBP < 100 mm Hg) have a 3-fold increased risk of in-hospital mortality compared with those who maintain arterial pressure.
• Take Home Point: Any drop in BP in a septic patient, even if it responds to fluids, portends a higher mortality.  Be vigilant and aggressively resuscitate these patients.

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Heparin for Maintaining Arteral Catheter Patency ?

• Arterial catheter placement is common in many critically ill ED patients.
• Typically, a heparin solution is used in arterial catheters based on the belief that it helps to maintain catheter patency.
• In one of the most recent studies (referenced below), the use of a heparinized solution did not improve the functionality, or increase the duration of patency, of arterial catheters when compared to a saline solution.
• As the incidence of heparin-induced thrombocytopenia (HIT) continues to increase, it is worth noting that the routine use of heparin to maintain arterial catheter patency is not well supported by the literature.

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