Grand Rounds Recap 5.23.18
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Clinical Diagnostics: Venous Blood gas WITH DR. Koehler
Blood Gas Basics
For a primer on venous blood gas (VBG) please see Dr. Koehler's post on VBGs. The purpose of a blood gas is to determine a patient's acid-base status and respiratory status. On every blood gas, bicarb is calculated based using the Henderson-Hesselbach equation and is not measured directly. Bicarb is directly measured on a basic metabolic panel or renal panel. Studies have shown that tourniquet use does not affect values on a blood gas.
VBG = ABG?
Obtaining an arterial blood gas (ABG) is associated with increased pain and risk of bleeding, hematoma formation, and nerve injury. Studies have shown that the pH between and ABG and VBG correlates even in a shock state. There is a wide variation in the correlation of pCO2 between ABGs and VBGs. This is because at higher values, pCO2 does not correlate well between ABGs and VBGs. However, when the pCO2 is less than 45mmHg studies have shown that there is good correlation between ABGs and VBGs.
ABGs are useful and should be used instead of a VBG to evaluate a patient's oxygenation status (PaO2), if there is concern for extreme hypercarbia, if a patient's exact acid-base status needs to be evaluated, or if there is a concern methemoglobinemia and co-oximetry is not available.
Clinical Scenarios
Metabolic Acidosis: In diabetic ketoacidosis or other metabolic acidosis, a VBG is considered standard of care in identifying a metabolic acidosis in the Emergency Department. This is due to the strong correlation between pH between an ABG and VBG.
Respiratory Acidosis: In a COPD exacerbation, clinical improvement with therapeutic interventions along with a decrease in pCO2 should be considered reassuring. This should be associated with an increase in pH as the hypercarbic respiratory acidosis improved. However, if there is concern for extreme hypercarbia, a lack of clinical improvement with appropriate interventions. This is because at higher pCO2 values some studies have shown variation in the correlation of pCO2 between an ABG and VBG.
Methemoglobinemia: The diagnosis of methemoglobinemia on a VBG requires co-oximetry. If co-oximetry is not available and there is clinical concern for methemoglobinemia an ABG, looking at the PaO2 should be used. In this clinical scenario, the patient will present with hypoxia refractory to supplemental oxygen with a normal PaO2 on ABG.
Carbon Monoxide Poisoning: Studies have shown that carboxyhemoglobin levels correlate well between an ABG and VBG if the carboxyhemaglobin level is below 20%. There is insufficient evidence for how well carboxyhemoglobin levels correlate at levels greater than 20%
Clinico-Pathologic Case Conferece: Cardiac Tamponade WITH DRS. Spigner and Bonomo
The Case
An elderly male with metastatic squamous cell carcinoma of the lung presents with cough, upper abdominal pain, nausea and vomiting, and decreased PO intake. On exam he is found to be tachypnic, tachycardic, and to have JVD. Evaluation is notable for significant multiple lab abnormalities including elevated LFTs. On imaging, a new pleural effusion was diagnosed on CXR and new ascites was found on abdominal CT. While in the ICU the patient declines, developing worsening hypotension, after being treated for sepsis with IV fluids and antibiotics.
Test of Choice: POCUS for Pericardial Effusion
Diagnosis: Malignant Pericardial Effusion and Constrictive Pericarditis with Tamponade Physiology
Congratulations to Dr. Bonomo for the correct test of choice and diagnosis! On POC cardiac ultrasound the patient was found to have a large pericardial effusion with tamponade physiology. The patient was taken emergently to the cath lab where more than two liters of fluid was removed with improvement in the patient's hemodynamics.
Cardiac Tamponade on Ultrasound
Studies have shown that Beck's triad has very poor sensitivity for the diagnosis of cardiac tamponade and so the use of adjunct tools, such as POC cardiac ultrasound can be very useful in identify a pericardial effusion with tamponade physiology. For a refresher on POC cardiac ultrasound please check out this post. Collapse of any chamber of the heart, often first being the right atrium, during diastole is concerning for tamponade. This can progress to right ventricular collapse during diastole. Very rarely does left sided heart collapse occur during diastole. This is due to the muscular nature of the left side of the heart. If this is seen, it is often clinically associated with hemodynamic collapse. A plethoric IVC can also be seen in the setting of tamponade physiology.
Congestive Hepatopathy
Congestive hepatopathy, which was responsible for this patient's abdominal pain and new ascites, occurs in patients with right heart failure or other cardiac conditions that cause elevated central venous pressure. Congestion often coexists in the setting of reduced cardiac output and can present with abnormal LFTs, jaundice, upper abdominal pain and ascites. Treatment of congestive hepatopathy is focused on treating the underlying etiology.
Taming the SRU: Naloxone WITH DR. Soria
Naloxone (Narcan)
Naloxone is a short-aciding opioid antagonist that can be given intranasally (IN), intravenously (IV), intramuscularly (IM), or sublingually (SL). IV naloxone is the route of administration with the fastest time of onset. Time of onset for IM or SL dosing of naloxone is thought to be equivalent. Although considered off-label use, a naloxone infusion can be considered in patients who required repeated doses of naloxone. Infusion dosing should be based on the initial dose of naloxone required to achieve a normal respiratory rate. The Infusion can be started at two thirds of the initial dose per hour and should be titrated to respiratory rate while avoiding withdrawal signs or symptoms. Studies have shown that patients who are one hour post single dose of naloxone administration who are a GCS of 15 with normal vitals can be safely discharged from the Emergency Department.
Naloxone was synthesized in the 1961 with the initial goal of treatment for constipation in chronic pain patients. In 1971 it was approved by the FDA for treatment of opiate overdose.
