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Arterial Blood Gases

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  1. Arterial Blood Gases Dr James F Peerless March 2013

  2. Objectives • Indications • Acid-Base Physiology • Procedure • Interpretation • Case Studies • Other Useful Information

  3. What is an ABG? • Blood test which measures the acid-base status and oxygen levels in the blood • Calculated values • pH • PaO2 • PaCO2 • Other values now calculated or derived • Commonly used in: • Acute care • Critical care • Pulmonary medicine

  4. Indications Clinical indications • Any unexpected deterioration in a patient • Acute exacerbation of a chronic disease • Impaired consciousness • Impaired respiratory effort To • Determine the cause of the illness • Determine the severity of a condition • Monitor the progress of a patient

  5. Acid-Base Balance • Tight regulation required to ensure • Enzyme function • Ion distribution • Protein structure • Body pH maintained by several buffer systems • Bicarbonate/ carbonic acid • Phosphate • Hb and plasma proteins

  6. Acid-Base Relationships • The equation below shows the relationship between protons (H+), bicarbonate (HCO3-), water, and CO2 H++ HCO3- H2CO3H2O + CO2 • In any given system, the reaction will tend towards an equilibrium, and the ratio of the reagents can be determined by knowing the dissociation constant and the pH.

  7. Henderson–Hasselbalch Equation • In any given system, the reaction will tend towards an equilibrium, and the ratio of the reagents can be determined by knowing the dissociation constant and the pH.

  8. Acidaemia

  9. Alkalaemia

  10. Acid-base Regulation • 4 components • Initially • Bicarbonate/ carbonic acid buffer system (MINUTES) • Respiratory compensation • Hyper/hypoventilation (MINUTES) • Renal compensation • Changes to H+ and HCO3- secretion/retention (HOURS) • Hepatic • Ureagenesis(HOURS) • AA metabolism  HCO3- + NH4+ • 2HCO3- + 2NH4+ NH2CONH2 + CO2 + 3H2O

  11. Blood Gas Analysis

  12. Acid-base Physiology • Blood has a normal pH of 7.40 • The normal range is between 7.35 and 7.45 • Any pH that is lower than 7.35 is considered acidotic • Acidosis: a state of being acidotic • Acidaemia: a condition of having acidic blood • Any pH that is higher than 7.45 is considered alkalotic • Alkalosis: a state of being alkalotic • Alkalaemia: a condition of having alkaline blood

  13. Normal Values • pH 7.35 – 7.45 • PaCO2 4.7 – 6.0 kPa • PaO2>10 kPa • HCO3- 22 – 26 mmol L-1 • Base excess ± 2 mmol L-1

  14. Primary Disturbance • When determining the cause of acid-base disturbances, look at what process is the primary component • pH 7.28 / PaCO2 5.0 / HCO3- 18 • pH 7.55 / PaCO2 5.0 / HCO3- 38 • pH 7.28 / PaCO2 10.0 / HCO3- 24 • pH 7.55 / PaCO2 2.0 / HCO3- 24 Metabolic Acidosis Metabolic Alkalosis Respiratory Acidosis Respiratory Alkalosis

  15. Compensatory/Secondary Mechanism • When one abnormal mechanism starts to push the pH into the abnormal range of either acidosis or alkalosis, a second process will try to push the pH back toward a normal value. • Important features of compensation: • Compensation is always in the opposite direction • if the primary disturbance is respiratory, the secondary compensatory mechanism must be metabolic • if the primary disturbance is metabolic, the secondary compensatory mechanism must be respiratory • The compensation process never over-corrects the primary disturbance. • If the pH appears to be over-corrected, there is an additional mixed primary disturbance.

  16. Compensation • Respiratory compensation starts within 30 minutes and is maximal within 12 hours. • Metabolic compensation takes about 3-5 days for maximal compensation. • Kidneys are slower than lungs to make changes.

  17. The Davenport Diagram • Displays the relationship between pH, PaCO2 and HCO3- • Explains the compensatory mechanisms that occur in acid-base balance.

  18. The Davenport Diagram

  19. Procedure • Explanation to pt. • Equipment • Sterile gloves • Chlorhexidine • Heparinised syringe • Allen’s Test • Withdraw 1-2 mls • Remove air, and cap off; pressure over puncture • Get it to the machine within 10 minutes (iced samples: 1-2 hrs)

  20. Interpretation – 8 Steps • Assess the patient • Identify the source • Arterial, venous, (or mixed venous) • Check the pH • Normal, acidaemia, alkalaemia • Assess the respiratory part • pCO2 (4.5 – 6 kPa) • Assess the metabolic part • HCO3- (std) (22-26 mmol/L)

  21. Interpretation – 8 Steps • Check the base excess • the difference between the patient’s standard bicarbonate level and 24 • normal range +/- 2 mmol/L • Check for compensation • Complete or partial • Check the pO2 • <10 kPa = hypoxia

  22. Case 1 An 18-year-old insulin-dependent diabetic is admitted to A&E with a 48h H/O vomitting and diarrhoea. As he has was unable to eat, he has taken no insulin. On arrival: • Breathing spontaneously RR 35 min-1, oxygen 4 L min-1 (Hudson mask), SpO2 98% • P 130 min-1, BP 90/65 mmHg, GCS 12 (E3, M5, V4) Arterial blood gas analysis (FiO2 0.3): • pH 6.89 • PaO217.0 kPa • PaCO22.48 kPa • HCO3- 4.7 mmol L-1 • BE -29.2 mmol L-1 http://www.resus.org.uk/pages/alsabgGd.pdf

  23. Case 1 • Assess the patient • Identify the source • Check the pH • Assess the respiratory component • Assess the metabolic component • Check the base excess • Check for compensation • Check the pO2 • Unwell; drowsy • ABG • Life-threatening acidaemia • Low PaCO2 (R. Al.) • V. low HCO3- (M. Ac.) • V. low B.E. • Partial • Well oxygenated “This patient has a partially compensated metabolic acidosis.”

  24. Case 2 • 57-year old patient on the surgical ward with 3-day history of vomitting • Pale, clammy • P 110 BP 100/50 • RR 9 Arterial blood gas analysis: • Inspired oxygen 21% (FiO2 0.21) • pH 7.50 • PaO2 10.0 kPa • PaCO2 7.4 kPa • HCO3- 30 mmol L-1 • BE +4 mmol L-1

  25. Case 2 • Assess the patient • Identify the source • Check the pH • Assess the respiratory component • Assess the metabolic component • Check the base excess • Check for compensation • Check the pO2 • Unwell • ABG • Alkalaemia • High PaCO2 (R. Ac.) • Raised HCO3- (M. Al.) • Positive B.E. • Partial • Mildly hypoxic “This patient has a partially compensated metabolic alkalosis.”

  26. Case 3 A 21-year-old woman is thrown from her horse. On the way to hospital she has become increasingly drowsy and the paramedics have inserted an oropharyngeal airway and given high flow oxygen via a face-mask. Arterial blood gas analysis reveals: • Inspired oxygen 40% (FiO2 0.4) • PaO2 18.8 kPa • pH 7.19 • PaCO2 10.2 kPa • Bicarbonate 23.6 mmol L-1 • Base excess -2.4 mmol L-1

  27. Case 3 • Assess the patient • Identify the source • Check the pH • Assess the respiratory component • Assess the metabolic component • Check the base excess • Check for compensation • Check the pO2 • Drowsy • ABG • Acidaemia • Raised PaCO2 (R. Ac.) • Normal/low HCO3- • Slightly reduced B.E. • Nil • Well oxygenated “This patient has a uncompensated respiratory acidosis.”

  28. Case 4 A 75-year-old woman is admitted to A&E following a VF cardiac arrest. Spontaneous circulation is restored after 2 shocks; the paramedics intubated her trachea and ventilated her with an automatic ventilator. On arrival: • Tube placement confirmed in trachea, tidal volume of 900 ml, rate of 18 breaths min-1, 100% oxygen • P 100 min-1, BP 90/54 mmHg, GCS 3 Arterial blood gas analysis reveals: • Inspired oxygen 100% (FiO2 1.0) • PaO2 25.4 kPa • pH 7.62 • PaCO22.65 kPa • HCO3- 20 mmol L-1 • BE -4 mmol L-1

  29. Case 4 • Assess the patient • Identify the source • Check the pH • Assess the respiratory component • Assess the metabolic component • Check the base excess • Check for compensation • Check the pO2 • Unwell; comatose • ABG • Significant alkalaemia • Low PaCO2 (R. Al.) • Reduced HCO3- (M. Ac.) • Reduced B.E. • Partial • Well oxygenated “This patient has a partially compensated respiratory alkalosis.”

  30. What else can we learn from the ABG? Measured Values • Na+, K+, Cl-, Ca2+ • Glucose • Lactate • Hb Derived Values • Anion Gap • A-a gradient • P/F Ratio

  31. Anion Gap • The anion gap is the difference in the measured cations and the measured anions in plasma. • This difference in the blood is calculated to identify the cause of metabolic acidosis. AG = ([Na+] + [K+]) − ([Cl−] + [HCO3−]) = 3–11 mEq/L

  32. Anion Gap HIGH – “MUDPILES” NORMAL – “FUSED CARS” Fistulae Uretogastric conduits Saline administration Endocrine (hyperparathyroid) Diarrhoea CA Inhibitors Ammonium chloride Renal tubular necrosis Spironolactone • Methanol • Uraemia • DKA • Propylene glycol • Isoniazid • Lactic acidosis • Ethylene glycol (antifreeze) • Salicylates

  33. A-a gradient • The Alveolar–arterial gradient (A–a gradient), is a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. • It is used in diagnosing the source of hypoxemia • Pulmonary (increased) – diffusion or shunt • Extrapulmonary (normal)

  34. P/F Ratio • The P/F ratio is the ratio of arterial oxygen concentration to the fraction of inspired oxygen. • It demonstrates how well the lungs absorb oxygen from the inspired air

  35. Summary • Keep analysis simple, and be methodical • Analysis takes practice • Always relate numbers back to the patient • Check all the numbers • Repeat a gas after an intervention has been made