Blood gas and acid base evaluation Chapter 20

Step one Normal Ph 7.35 – 7.45 PCO2 35 – 45 mmHg ( 31 – 39 ) PO2 60 – 100 mmHg BE -2 - +2 HCO3 24 AG 8 – 16 Na , K , Cl Ph = pKa + log X HCO3/PCO2 x Solubility A-a gradient N < 20 mmHg O2 – Hb dissociation scale

Step 1 Ph = pKa + log X HCO3/PCO2 x Solubility CO2 + H2O – H2CO3 – H + HCO3 A-a gradient N < 20 mmHg O2 – Hb dissociation scale

Step two Is there an OXYGENATION problem PO2 Saturation Must always be interpreted with FiO2 PF Ratio = PO2/FiO2 Normal > 400 ALI < 300 ARDS < 200

Step 2 Reason for oxygenation problem Work out A-a gradient Decide if it is an intrinsic lung problem

Step 3 Is there a VENTILATION problem Look at PaCO2 <35 hyperventilation >45 hyperventilation Remember that ventilation always interpreted with pH in mind

Step 4 What is the pH Normal Compensated Mixed Alkalosis Acidosis

Step 5 Does the PaCO2 FIT IN with the pH Alkalosis + PaCO2 < 40 mmHg = Respiratory Alkalosis Acidosis + PaCO2 > 40 mmHg = Respiratory acidosis If the PaCO2 does not fit in with the ph , move to next step If the PaCO2 fit in with Ph = PRIMARY ABNORMALITY

Step 6 Look at the HCO3 Does the SBC fit in with the pH Alkalosis + SBC > 24 = Metabolic alkalosis Acidosis + SBC < 24 = Metabolic acidosis This will be the primary abnormality if the PaCO2 did not fit in with pH

But wait , there is more Remember that basic 4 abnormalities are Resp acidosis Metab acidosis Resp alkalosis Metab alkalosis Uncompensated disorders Compensated disorders Mixed disorders Chronic versus acute

Step 7 Determine the degree of COMPENSATION Rules apply ( tables ) Thus the body always tries to compensate for the primary abnormality If compensation as expected = normal compensation If compensation not as expected = mixed disorder

Step 8 Calculate the ANION GAP if metabolic acidosis AG = Cations – Anions = 8-16 mEq/L = unmeasured anions AG = (Sodium + Potasium) – (Choride + Bicarbonate) Acidosis + AG > 16 Mm = High anion gap metabolic acidosis Acidosis + AG normal = Low anion gap metabolic acidosis

Step 9 Look at the Haemotological variables Haematocrit Haemoglobin Abnormal haemoglobins

Step 10 Look at the metabolites and electrolytes Lactate Glucose Calcium Sodium Potassium Magnesium

Intracellular pH 6.8 Neutral at body temp of 37 degrees celcius Hydrogen = Hydroxyl Dissociation of water and other electrlytes

Administration of 8.4% NaHCO3 Rather treat the cause Ph < 7.1 metabolic acidosis Acidific toxins etc to aid renal excretion Hypernatraemia , hyperosmolality , fluid overload

Chloride/Sodium Ratio 0.74-0.78 > in hyperchloraemic metabolic acidosis 0.9% Saline , Voluven , Venofundin

Sodium Hyponatraemia Hypervolaemic hyponatraemia Normovolaemic hyponatraemia Hypovolaemic hyponatraemia < 130 Mm – No elective surgery < 120 Mm – Neurologic derangements Central pontine demyelinization if corrected too fast

Sodium Hypernatraemia Hypervolaemic Normovolaemic Hypovolaemic > 150 no elective surgery Brain oedema if corrected too fast

Potassium Hypokalaemia < 3,5 Causes Symptoms and signs Redistribution, Decreased intake, Increased losses Symptoms and signs Cardiovascular , musculoskeletal