Arterial Blood Gas Analysis L Lester www.3bv.org Bones, Brains & Blood Vessels
Drawn from radial or femoral arteries. Invasive procedure Caution must be taken with patient on anticoagulants
ph: 7.35-7.45 PaO 2 : 10-13kPa (75-100mmHg) PaCO 2 : 4.7-6kPa (35-45mmHg) HCO 3 : 22-26mmol/l SaO 2 : 95-99% Base excess (+2)-(-2)
Indication of oxygenation status Determination of acid-base state Assess ventilation
Respiratory Failure = PaO2 <8kPa Type I PaCO 2 <6.5kPa Type II PaCO 2 >6.5kPa
PaO 2 is the partial pressure of O 2 Measures O 2 dissolved in the plasma. 3% of arterial oxygen content O 2 Saturation measures the amount of Hb saturated with oxygen. 97% of arterial oxygen content.
Shows relationship between PaO 2 and O 2 saturation SaO2% These two values trend in the same direction Curve allows us to estimate PaO 2 based on non-invasive peripheral O 2 saturation. PaO2
SaO2 % Oxyhaemoglobin Dissociation Curve PO2 PaO 2
Potential for delay in detection of acute hypoxaemia as a large decrease in PO2 will not produce a significant fall in SaO2 until the steeper portion of O 2 Hb curve is encountered at a PO2 of approx 8kPa Pulse oximeter may not detect hypoxaemic event until well after it occurred Unable to detect hyperoxia eg. Neonatal O 2 toxicity Does not assess ventilation ie supplemental O 2 may prevent hypoxaemia despite severe hypoventilation
Right shift ( O 2 affinity for Hb) Acidosis Hyperthermia Hypercarbia Left shift ( O 2 affinity for Hb) Alkalosis Hypothermia Hypocarbia Decreased 2,3-DPG O2 release to cells and tissue hypoxia even when sufficient O2 in blood Increased 2,3 DPG
SaO2 % Oxyhaemoglobin Dissociation Curve PO2 PaO 2
This relationship is critical for homeostasis Significant deviations from normal PH ranges are poorly tolerated and may be life threatening Achieved by respiratory and renal systems
Respirator y Metabolic H 2 O + CO 2 D H 2 CO 3 D HCO 3 + H +
PH=6.10 + log ([HCO3] [0.03xPCO2]) The Henderson-Hasselbalch equation shows that PH is determined by the ratio between HCO 3 concentration PCO 2,
Rapid regulation (short-term) Lungs Chronic control (long-term) Kidneys Gut
A process that tends to lower the extracellular fluid ph (which is equivalent to raising the hydrogen concentration). From the Henderson-Hasselbalch equation, this can be induced by a fall in the extracellular (or plasma ) bicarbonate concentration or by an elevation in the PCO2.
A process that tends to raise the extracellular fluid ph (which is equivalent to lowering the hydrogen concentration). From the Henderson-Hasselbalch equation, this can be induced by an elevation in the extracellular (or plasma ) bicarbonate concentration or by a fall in the PCO2.
CO2 is an acid HCO 3 is a base Regulated by the kidneys Regulated by the lungs Too much causes alkalosis Too much causes acidosis
Respiratory Acidosis **********too much CO 2 Respiratory Alkalosis **********too little CO 2
Metabolic Acidosis *********too little HCO3 Metabolic Alkalosis *********too much HCO3
ph 7.30 (7.35-7.45) PaCO 2 5.3kPa (4.7-6kPa) HCO 3 15mmol/l (22-26mmol/l)
Failure of kidney function Blood HCO 3 which results in $ availability of renal tubular HCO 3 for H + excretion ph < 7.35
Fixed acid accumulation and low serum bicarbonate Renal failure Renal/GI losses Lactic Salicylate eg. diarrhoea Ketones Methanol Phosphate Ethylene glycol HCl
Renal/Hepatic failure Excessive diarrhoea Diabetic ketoacidosis Lactic acidosis Methanol, aspirin ingestion Cardiac arrest
Symptoms and signs Depend on rapidity of onset and severity Fatigue, abdominal pain, dyspnoea Kussmaul respirations Effects Negative cardiac contractility Peripheral venous constriction
ph 7.50 (7.35-7.45) PCO 2 5.33kPa (4.7-6kPa) HCO 3 30mmol/l (22-26mmol/l)
# plasma HCO 3 > 26mmol/l ph > 7.45
# loss of acid from 1. stomach (vomiting eg. Pyloric stenosis/ nasogastric suction) 2. kidney (mineralocorticoid excess, loop or thiazide diuretics)
ph 7.50 (7.35-7.45) PaCO 2 4kPa (4.7-6kPa) HCO 3 22mmol/l (22-26mmol/l)
ph > 7.45 PCO 2 < 4.7kPA (35mmHg) Too much CO 2 exhaled (hyperventilation)
Pulmonary causes Pneumonia. Pulmonary edema. Non-pulmonary causes Sepsis. Pain. Liver disease. Salicylate intoxication. Brain lesions. Pulmonary embolus. Interstitial fibrosis. Asthma.
ph 7.30 (7.35-7.45) PaCO 2 8kPa (4.7-6kPa) HCO 3 26mmol/l (22-26mmol/l)
Too much CO2 ph < 7.35 PCO2 > 6kPa (45mmHg)
Any cause of hypoventilation: Neuromuscular disease Head injury Sedatives, narcotics Atelectasis Obstructed airway
ph 7.21, PaCO 2 8.4kPa, Pa0 2 9.3kPa, HC0 3 27mmol/L 1. Evaluate the ph. ph is not in normal range, but is decreased (acidosis). 2. Evaluate the PaC0 2.
Evaluate the ph Normal 7.35-7.45 Below 7.35= acidaemia Above 7.45= alkalosis If a patient has a mixed acid-base disorder, the ph identifies the process in control
Evaluate ventilation Normal PaCO 2 4.7-6kPa (35-45mmHg) greater than 6kPa= ventilatory failure and respiratory acidosis less than 4.7kPa = alveolar hyperventilation and respiratory
Evaluate metabolic process Normal HCO 3 22-26mmol/l HCO 3 < 22 = metabolic acidosis HCO 3 > 26 = metabolic alkalosis
Determine primary and compensating disorder When both PCO 2 and HCO 3 are abnormal, one reflects the primary acid-base disorder and the other reflects the compensating disorder To decide which is which, check the ph
Three states of compensation possible: 1. Noncompensation- alteration of only pco 2 or HCO 3 2. Partial compensation- when both pco 2 and HCO 3 are abnormal and because compensation is incomplete, the ph is also abnormal;
Evaluate oxygenation Normal PO 2 10-13kPA (75-100mmHg) PO 2 8-9.99kPa (60-75mmHg) = mild hypoxemia PO 2 5.3-8.8kPa (40-60mmHg)= moderate hypoxemia
Interpret Final analysis should include Degree of compensation The primary disorder
Respiratory acidosis D metabolic alkalosis Respiratory alkalosis D metabolic acidosis In respiratory conditions, therefore, the kidneys will attempt to compensate and visa versa
Extra base chemicals in patient s blood Includes bicarbonate, sulfates, phosphates Amount of acid needed to bring ph to 7.4 Negative value is a base deficit or acidosis
Anion Gap Na + K + = Cl - Albumin Other Cations Na + - (Cl - +HCO 3- ) = Anion Gap HCO 3- Other Anions
Anion Gap= Na+ - (Cl - +HCO 3- ) Measure of fixed or organic acids eg. Phosphates, ketones, lactate, drugs: salicylates, biguanides, toxins: ethylene glycol Helps determine presence and cause of metabolic acidosis.
Normal= 12 +/-2 <12= Diarrhoea, RTA, CaCl or other acids > 15 = Methanol Uraemia Diabetic ketoacidosis Paraldehyde Alcoholic ketoacidosis
In chronic respiratory acidosis (COPD) the kidneys increase the elimination of H+ and absorb more HCO3. The ABG will show normal ph, #CO2 and #HCO3 Buffers kick in within minutes. Respiratory compensation is rapid and starts within minutes and complete within 24 hours. Kidney compensation takes hours and up to 5 days
PARAMETER COMPENSATION Metabolic Acidosis PCO 2 1kPa per 6 mmol/l Δ HCO 3 Metabolic Alkalosis PCO 2 1kPa per 6 mmol/l Δ HCO 3 Acute Respiratory Acidosis Chronic Respiratory Acidosis HCO 3 HCO 3 0.75mmol/l per 1kPa Δ PCO2 3mmol/l per 1kPa Δ PCO2 Acute Respiratory Alkalosis HCO 3 1.5mmol/l per 1kPa Δ PCO2 Chronic Respiratory Alkalosis HCO 3 3 mmol/l per 1kPa Δ PCO2
Compare to previous ABG Look at patient s history and present situation Determine what is needed to correct situation
Mr. O Brien is a 60 y/o with pneumonia. He is admitted with dyspnoea, fever. ABG: ph 7.28 (7.35-7.45) PCO 2 7.5kPa (4.7-6kPa) HCO 3 25mmol/l (22-26mmol/l) PO 2 9.3kPa (10-13kPa)
Case #1 Respiratory Acidosis with mild hypoxaemia
Ms. O Keeffe is a 24 year old college student. She has a history of Crohn s disease c/o 4 day history of bloody diarrhoea. An ABG is obtained: ph 7.28 (7.35-7.45) PCO 2 5.7kPa (4.7-6kPa) HCO 3 20mmol/l (22-26mmol/l) PO 2 11.7kPa (10-13kPa)
Case #2 Non-compensated Metabolic Acidosis
You find Mr. O Regan in respiratory distress. H/O diabetes and is febrile. ABG: ph 7.00 (7.35-7.45) PCO 2 7.9kPa (4.7-6kPa) HCO 3 14mmol/l (22-26mmol/l) po 2 8.5kPa (10-13kPa)
Case #3 Metabolic and Respiratory acidosis with moderate hypoxemia
Mrs Kinnane is a thin, 61y/o COPD patient. Her ABG: ph 7.37 PCO 2 8.4kPa HCO 3 35mmol/l PO 2 7.7kPa SaO 2 89%
Case #4 Fully compensated respiratory acidosis with moderate hypoxemia
Mrs. Murphy is found pulseless and not breathing. After a couple of minutes she responds with a pulse and is breathing. ABG: ph 6.89 PCO 2 9.34kPa HCO 3 13mmol/l
Case #5 Metabolic & Respiratory Acidosis With severe hypoxemia
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