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Introduction

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Recently point-of-care ultrasonography (POCUS) has risen in prominence within acute care medicine.  It has been shown to provide immediate and critical information about a variety of conditions ranging from nephrolithiasis to pulmonary edema.  However, POCUS is not without its critics, who point out weaknesses including detection of incidental findings (i.e., small post-traumatic pneumothorax or pleural effusions leading to unnecessary procedures).  There has been much debate about which test is best for any given condition, for example chest X-ray (CXR) versus POCUS for the diagnosis of pneumonia.
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When approaching a crashing patient with unclear diagnosis, these considerations are largely moot.  First, it should be recognized that most crashing patients are suffering from some form of cardiopulmonary failure.  Tamponade may manifest initially with dyspnea and tachypnea.  Pulmonary embolism may cause hypotension without causing hypoxemia.  Tension pneumothorax can present with simultaneous pulmonary and cardiac failure.  Regardless, most crashing patients are suffering from some form of cardiac failure, pulmonary failure, or both.  Until a definite diagnosis has been reached it may be best to conceptualize these patients as having cardiopulmonary failure.  Prematurely focusing on the wrong organ is a common pitfall. 
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The Triple-Barreled Shotgun

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At the bedside of a crashing patient with cardiopulmonary failure, there are three tests which are fast, widely available, and yield immediate information: CXR, EKG, and POCUS.  Each of these has its own strengths and weaknesses.  For example, CXR yields a complete survey of the thorax, whereas chest ultrasonography provides detailed information about exactly what is going on underneath the ultrasound probe.  Bedside echocardiography often yields a wealth of information, but most patients don't have a prior echocardiogram to compare this with.  However, patients are more likely to have a recent EKG or CXR on file, so comparison of current versus prior EKG or CXR may help sort out acute vs. chronic pathology. 
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Integration of results from different tests and the clinical scenario is crucial.  Research studies may show excellent performance of a specific test (e.g. POCUS for pneumonia diagnosis), but with a crashing patient we will be rushed and our performance will not match a research study.  Therefore, combining POCUS with CXR may help us confirm our findings with ultrasonography and avoid missing anything.  Redundancy promotes safety.  A finding on one test may also help us clarify or detect an abnormality on another test.  For example, electrocardiographic features of right ventricular strain would prompt us to search more carefully for evidence of pulmonary embolism with POCUS.
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What is the role of arterial blood gas (ABG) in a patient with undifferentiated cardiopulmonary failure?

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ABG is immediately available, but is often not helpful.  Most patients have an adequate pulse oximetry tracing and a known bicarbonate level, so ABG only adds information about ventilation (PaCO2).  Unlike EKG, CXR, and POCUS, an ABG cannot yield a specific diagnosis. 
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ABG is sometimes used as a trigger for intubation or transfer to the ICU.  However, these decisions should be based primarily on clinical information including the overall picture, diagnosis, and expected disease course (rather than any specific ABG value).  It is not uncommon to encounter a patient in extremiswho obviously requires intubation despite having a “normal” ABG (1).  Alternatively, low PaO2values on ABG may provoke unnecessary anxiety.  For example, a chronically hypoxemic patient with saturation of 85% may have a PaO2 of 50 mm.  The PaO2of 50 mm seems scarier than the saturation of 85% because it is a lower number, despite that these two pieces of data are equivalent:
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The interpretation of an ABG depends largely on the diagnosis that the patient has.  For example, a normal ABG in an asthmatic patient would be highly concerning.  Attempting to interpret an ABG in a patient with unknown diagnosis is difficult.  When accepting patients in transfer from outside hospitals, I sometimes receive calls where the transferring clinician will quickly launch into a recitation of a series of ABGs.  Without context this is inscrutable.  On occasion I have requested to video-conference with the patient via i-phone or Skype, which is invariably more helpful. 
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In undifferentiated cardiopulmonary failure with a known oxygen saturation and serum bicarbonate level, the ABG is most likely to alter management if it reveals ventilatory failure (i.e., hypercapnia).  For a patient with clinical signs of hypercapnia (e.g. somnolence, multifocal myoclonus) or a history suggestive of hypercapnia (e.g. COPD or obesity hypoventilation syndrome), obtaining an ABG may be helpful.  For patients without signs or risk factors for hypercapnia (e.g. a young patient with acute hypoxemic respiratory failure and normal mental status), hypercapnia will occur only at the point of respiratory exhaustion, at which point it should be clinically obvious that the patient requires intubation.  If there is an intermediate level of concern for hypercapnia, peripheral venous blood gas (VBG) correlates very well with ABG and can easily be obtained along with other labs.  If the VBG doesn't reveal hypercapnia, clinically significant hypercapnia is very unlikely (Kelly 2014).
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There is no evidence to support the utility of ABG for diagnosing patients with undifferentiated respiratory failure.  The only study evaluating this is Burri 2011, which performed a post-hoc analysis of patients presenting to the emergency department with dyspnea.  The only disorder for which ABG had any diagnostic utility was anxiety-induced hyperventilation (figure below).  Thus, ABG may be useful in specific patients (particularly if hyperventilation is a possibility).  However, broadly applying ABG to the entire population of patients with respiratory failure appears unhelpful.  For more on this, see a refreshingly honest transcription of a journal club discussion from Indiana University (2). 
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Unfortunately, the concept that we should order an ABG on a crashing patient remains dogmatically engrained in much of medical culture.  There is usually one member of the ICU team who will indignantly demand to know the ABG values.  There seems to be a belief that the ABG reveals some sort of divine truth about the patient, if we could only be smart enough to decrypt its meaning.  
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In summary, there are certainly many specific questions that an ABG may help answer (e.g. Is tachypnea due to anxiety or respiratory dysfunction? Is somnolence due to hypercapnia or a neurologic problem?).  However, an ABG ordered in undifferentiated cardiopulmonary failure without any specific question in mind is unlikely to add anything to thoughtful evaluation with the triple-barreled shotgun.  Sometimes it may be a distraction from the clinical condition of the patient.
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Conclusions

Notes
(1) It should be noted that there is a difference between a euboxic ABG (i.e. an ABG where all the labs are within the “normal” range) and what may be called a “reassuring” or “clinically appropriate” ABG.  For a patient with significant tachypnea, it is physiologically appropriate and reassuring to see a respiratory alkalosis.  On the contrary, if a patient with tachypnea and increased work of breathing has a PaCO2 of 40mm, this indicates significant pulmonary dysfunction which is forcing them to work very hard to achieve a “normal” PaCO2 level.  Thus, a “normal” PaCO2level in a patient with dyspnea and tachypnea is actually quite concerning.
(2) If you do read this, would add one comment to the discussion.  I disagree that a pure asthma/COPD flare would be expected to have a normal A-a gradient.  Many COPD patients are on chronic supplemental oxygen with a permanentA-a gradient, even on a good day.  Ventilation-perfusion mismatch and mucus plugging can cause an abnormal A-a gradient in either disease.  I completely agree that asthmatics and COPD patients rarely have severe or refractory hypoxemia, so this would be a sign to look for an additional or alternative diagnosis.
Image credits:
Shotgun: https://en.wikipedia.org/wiki/Volley_gun#/media/File:Pistolet-trois-coups-p1030505.jpg
Panic Button:   http://www.themainewire.com/2013/03/homeland-security-funds-panic-buttons-maine-schools/
Venn Diargam:  http://www.smartdraw.com/examples/view/3+circle+venn+diagram/
Hemoglobin Saturation Curve: Licensed under CC BY-SA 3.0 via Wikimedia Commons -http://commons.wikimedia.org/wiki/File:Hb_saturation_curve.png#mediaviewer/File:Hb_saturation_curve.png

 
Josh Farkas
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