EMERGENCY MEDICINE AND THE LABORATORY Ehsan Bolvardi(MD) bolvardie@mums.ac.ir
Objective ED and laboratory Inappropriate use of laboratory ABG sampling ABG sampling error
Introduction The laboratory is an extremely important and necessary resource for the emergency department; One of the most crucial requirements for an emergency department is an efficient laboratory; The laboratory must provide reliable, efficient, timely services ;
Stat tests Many tests sent by the emergency department are for stat results. (CCU & ICU ) Emergency departments must decide which tests must be performed on a stat basis and which tests are necessary but do not require immediate analysis.
Examples of tests that are needed on a stat basis: Complete blood cell count with differential, Total creatine phosphokinase, Electrolytes, urine pregnancy, blood urea nitrogen, creatinine, urinalysis, glucose, amylase, calcium, Arterial blood gases; Basic toxicology screens, including aspirin, acetaminophen, phenytoin, and ethanol levels. Digoxin and phenobarbital levels, Creatinine phosphokinase (CPK-MB),Troponin carbon monoxide, and measured oxygen saturation.
Potential benefit to patients, Whether immediate results will change the treatment plan, The cost of the laboratory test in question, Resources available to the emergency department,including personnel, The standard of care in the community, The availability of certain tests or procedures at nearby tertiary care centers Medical-legal considerations.
Equipment Equipment must be fast, reliable, durable, efficient, and cost effective. The newer machines appear to meet these criteria and may well be less costly per test than older more expensive equipment.
INAPPROPRIATE USE OF LABORATORY SERVICES A major problem experienced in teaching hospitals centers around the inappropriate ordering of tests, generally by junior housestaff. There are numerous reasons why emergency department housestaff order unnecessary laboratory tests: a)fear of being embarrassed by senior housestaff or attendings b)fear of malpractice lawsuits. Protocols will aid in prevention of overuse and inappropriate use of the laboratory.
ABG Pre-sampling preparation verify that an order for the arterial blood gas has been written; review medical chart; necessary materials ; Identify artery for sampling ; radial artery is the preferred sampling site owing to three primary factors: a) it is easy to access, b) it is a superficial artery and therefore easier to palpate, stabilize, and puncture, c) it has a collateral blood flow;
Performing the arterial puncture
Withdraw the needle and apply pressure ; Remove the needle from the syringe. Needle should never be recapped, bent, or purposely broken because of the danger of self puncture. Air bubbles are removed from the blood ; Cap the syringe and place the syringe in the bag of ice;
Five types of arterial blood sampling errors Air in the Blood Sample. Venous sampling or admixture. Excessive or improper anticoagulant use. Rate of Metabolism. Temperature disparities between machine and patient.
Air in the Blood Sample Effect on PaO2( Primary parameter affected). Effect on PaCO2 (and ph). Room air has a Po 2, of approximately 150 mm Hg (sea level) and a Pco 2 of essentially zero; Air bubbles that mix and equilibrate with arterial blood will shift the Pao 2 toward 150 mm Hg and will lower the Paco 2.
Venous Sampling or Admixture Most common in femoral punctures or hypotensive patients because of difficulty in Assesment ; 0.5 ml of venous blood with a PO2 of 31 mixed with 4.5 ml,of arterial blood with a PO2 of 86, yields a mixed sample with a PO2 of 56;
Anticoagulant Effects Heparin must be added to the syringe as an anticoagulant; Because the ph of heparin is near 7.0, and the Po 2 and Pco 2 of the heparin solution are near room air values, excess heparin can alter all three ABG measurements; 0.05 to 0.10 ml of a dilute solution (1000 units/ml) will anticoagulate 1 ml of blood without affecting its ph. Pco 2, or Po 2; Sufficient amount usually remains in the dead space of the syringe and needle for anticoagulation without distortion of the ABG determination.
Metabolism continues after sampling Oxygen is consumed and Carbon Dioxide produced; If a delay of more than 10 minutes is anticipated, the specimen must be immersed in an ice bath; Leukocytes and platelets continue to consume oxygen in the sample after it is drawn and can cause a significant fall in Pao 2 over time at room temperature, especially in the setting of leukocytosis or thrombocytosis. ph: Decrease 0.05/hr PaCO2: Increase 5 mmhg/hr PaO2: Decrease by 20 mmhg/hr (150 mmhg/hr if initial PaO2 over 250 mmhg)
After collection Specimens held at room temperature must be analyzed within 10-15 minutes of drawing; iced samples should be analyzed within 1 hour. Container of ice and water (to immerse syringe barrel if specimen will not be analyzed within l5 min); If a delay of greater than 30 minutes is anticipated, a glass syringe should be used; Plastic syringes have been shown to allow for an increase in PaO2 if analysis is delayed more than 30 minutes;
Temperature disparities between machine and patient The electrodes are maintained at 37 C in a thermostatically regulated waterbath; therefore, all ABG measurements are made at 37 C regardless of the patient's temperature; ph, Paco 2, and Pao 2 are all temperature dependent because gas solubilities are a function of temperature; When body temperature is higher than 37 C, the reported Pao 2 and Paco 2 measured at 37 C will be lower than the actual values in the patient;