Lawton Luke - Own the Arterial Blood Gas

A 26 year old asthmatic with a history of ICU admissions presents to your department in acute respiratory distress. She has been treated by her GP for 2 days with bronchodilators and glucocorticoids. Her vital signs on arrival are: HR : /min BP : 110/70 mmHg RR : /min Sats : 92% 40% O T : 37.2 o C Arterial blood gas analysis is undertaken in the resuscitation area. Descr ibe and interpret the results. (100%) 1. Acid base status Mild acidaemia Mild hypercapnoea : r espiratory acidosis Expect HCO - for CO Acute = 24 + 1 = 25mmHg Chronic = 24 + 4 = 28mmHg Mild decrease in HCO - : m etabolic acidosis E xpect CO = 8 + (1.5 x 18) = 8 + 27 = 35 mmHg Anion Gap = 142 18 105 = 142 = 19 RAGMA Delta Ratio = (19 12) /( 24 18) = 7/6 = 1 Implies isolated RAGMA Therefore diagnose dual disturbance: primary respiratory acidosis and metabolic acidosis.

No compensation. 2. Oxygenation Critical hypoxia on supplemental oxygen. A-a gradient in mmHg for FiO 40% = 300 (1.25 x 48) 65 = 300 60 65 = 300 = 175 Grossly elevated ! (expect for 26yo female = 26/4 + 4 = ~10) Gross V/Q mismatch Implies critical/life threatening asthma Also consider : ARDS (SIRS/aspiration) , i nterstitial lung disease , o ther respiratory membrane disease (eg LRTI) , PE A-a gradient in mmHg for FiO 40% = 40 kPa (1.25 x 6.4) 8.7 = 40 8 8.7 = 23.3 kPa 3. Electrolytes Normonatraemia Mild hypokalaemia Possible intracellular shift if using salbutamol Expect K + for pH = 5 + 0.5 x 5 = 5.25 mmol/L Therefore significantly K + deficient on correction Replace K + and monitor ECG Normochloraemia Moderate hyperglycaemia Secondary to glucocorticoid administration Possible diabetes check medical Hx Moderate hyperlactataemia Likely due to salbutamol administration (B ) Type A less likely with normal haemodynamics Type B less likely with N temperature, but evaluate for sepsis 4. Interpret **This is a critically ill patient*** Asthmatic, and ABG shows the dual processes of r espiratory acidosis and RAGMA Respiratory acidosis VERY concerning given Hx of asthma Indicates critical/life threatening disease Hypercapnoeic asthma often precedes respiratory arrest Needs urgent treatment High dose bronchodilators, MgSO , CPAP, possible IPPV Involve intensive care service RAGMA , h yperlactataemic Salbutamol Sepsis/shock less likely Check to exclude other drug causes (Iron, isoniazid, paraldehyde, toxic alcohols, metformin etc) Also consider other causes Diabetic ketoacidosis (h yperglycemic ) : check med Hx and urine/blood ketones Uraemia : c heck renal function

A 26 year old asthmatic with a history of ICU admissions presents to your department in acute respiratory distress.

She has been treated by her GP for 2 days with bronchodilators and glucocorticoids . The stem again provides valuable information. This is a high risk patient , and candidates will need to add perspective to their answer. Usually asthmatics present with hyperventilatory hypocapnoea. Here the presence of hypercarbia indicates severe asthma, and a PEFR <25% of the patients norm . Respiratory collapse is imminent.

This is underlined by a comment during the evaluation of the A-a gradient to show the candidates understanding of severity. Several minor issues require comment in this blood gas, including the presence of a significant K + deficit, hyperglycaemia (presumably related to steroids) and hyperlactataemia (salbutamol therapy). Again these are included in the main body of working, so that the interpretation phase can be focused on the major issues. Critically, in the interpretation phase the candidate must highlight the importance of the presentation. Phrases such as this is a critically ill patient are useful in this instance. An overview of relevant treatment is indicated.

A full differential for the RAGMA can also be included should time allow.

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Dhuper S, Maggiore D, Chung V and Shim C. Profile of near-fatal asthma in an inner-city hospital. Chest 2003; 124 (5): 1880-84 Martin TG, Elenbaas RM and Pingleton SH. Use of peak expiratory flow rates to eliminate unnecessary arterial blood gases in acute asthma. Ann Emerg Med 1982 11(2):70