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Dr James F Peerless March 2013

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1 Dr James F Peerless March 2013
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 Any unexpected deterioration in a pt Acute exxacerbation of a chronic disease Impaired consciousness Impraied respiratory effort Determine the severity of a condition What is causing the pt to be unwell – i.e. resp or metabolic Monitor the progress

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 Decreased hco will drive the eqn to the left, as more co2 and water dissociate to replace hco, and create more h Increase co2 will drive eqn to the left and cause increased h

9 Alkalaemia Alkalosis:
increasing the bicarb concentration will generate alkalosis. When this happens, we call the alkalosis a metabolic alkalosis. decreasing the carbon dioxide will also generate alkalosis. When this happens, we call the alkalosis a respiratory alkalosis.

10 Acid-base Regulation 4 components Initially Respiratory compensation
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
HCO – 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 / PaCO / 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 Dotted lines are buffer lines
Solid lines are of equal PaCO2 Bad is the normal buffer line Abc resp acid Ade resp alk Agc met alk Afe met acid

19 Procedure Explanation to pt. Equipment Allen’s Test Withdraw 1-2 mls
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

21 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)

22 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

23 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 PaO kPa PaCO kPa HCO mmol L-1 BE mmol L-1

24 Case 1 “This patient has a partially compensated metabolic acidosis.”
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.”

25 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 PaO kPa PaCO2 7.4 kPa HCO3- 30 mmol L-1 BE +4 mmol L-1

26 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.”

27 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) PaO kPa pH 7.19 PaCO kPa Bicarbonate 23.6 mmol L-1 Base excess -2.4 mmol L-1

28 Case 3 “This patient has a uncompensated respiratory acidosis.”
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.”

29 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) PaO kPa pH 7.62 PaCO kPa HCO3- 20 mmol L-1 BE -4 mmol L-1

30 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.”

31 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

32 AG = ([Na+] + [K+]) − ([Cl−] + [HCO3−]) = 3–11 mEq/L
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

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

34 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)

35 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

36 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


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