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ARTERIAL BLOOD GAS ANALYSIS

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1 ARTERIAL BLOOD GAS ANALYSIS
Andy Jackson

2 Why do we need blood gas analysis?
Pulse oximetry is an extremely useful bedside tool for monitoring capillary saturation of haemaglobin by oxygen (SpO2). However, there are limits to this usefulness:

3 Anaemia Reduction in red blood cells means that less oxygen can be transported. Oximetry may therefore create a false sense of security in the face of undetected anaemia.

4 Carbon monoxide Carboxyhaemaglobin increases infra-red light absorption, giving falsely high SpO2 readings. Inhaled CO affects readings for up to 4 hours after a cigarette. Dobson, F. (1993) shedding light on pulse oximetry. Nursing Standard 7 (46): 4-11

5 Hypercapnia Pulse oximetry does not measure carbon dioxide levels or pH.

6 Poor peripheral perfusion
Pulse oximetry relies on perfusion and pulse - poor perfusion, for what ever reason will cause a poor signal.

7 Nail varnish Absorbs light, so can cause significant under-reading.
(Wahr, J.A. and Tremper, K.K. (1996) Oxygen measurement and monitoring techniques, in c. Prys-Roberts and B.R. Brown Jr. (Eds) International Practice of Anaesthesia, Oxford: Butterworth Heinemann:2/159/1-19)

8 Discomfort Prolonged wearing of a pulse oximeter probe can cause significant discomfort. (Woodrow P. (2000) Intensive Care Nursing: A framework for practice Routledge

9 Therefore, a more accurate assessment of the patient’s respiratory / metabolic status is required.
ABG analysis remains the gold standard.

10 What does arterial blood gas analysis tell us?
Oxygenation (PaO2), Ability to eliminate carbon dioxide (PCO2), pH / acid-base balance.

11 Oxygenation Oxygen delivery is dependant upon 3 factors:
cardiac output Hb SaO2

12 Oxygenation 97% of oxygen is transported by Hb.
Hb composed of haem, a pigment, which contains 4 iron atoms and globin which is a protein. O2 + Hb combine to form oxyhaemoglobin.

13 Oxygenation 3% of oxygen is dissolved in plasma - this is represented on an ABG sample as the PO2. PO2 determines how much O2 combines with haemoglobin.

14 Oxygenation When PO2 is high, Hb binds with a large amount of oxygen and may become fully / near fully saturated. When PO2 is low, Hb is only partially saturated. Pulmonary capillaries, PO2 is high and ++O2 combines with Hb. Tissue capillaries, PO2 low, O2 released from Hb to diffuse into cells.

15 Oxygenation This relationship is illustrated by the oxygen-haemoglobin dissociation curve.

16 Oxygen dissociation curve

17 Oxygen dissociation curve
At the top “flat” part of the curve, increasing PO2 results in negligible changes in SaO2. On the “steep” part of the curve, small changes in PO2 result in large changes in SaO2.

18 Oxygen dissociation curve
Curve can move to left or right. Move to right decreases affinity of Hb for O2. Move to left increases affinity of Hb for O2.

19 Oxygen dissociation curve shifts: causative factors
RIGHT pyrexia acidosis high 2,3 DPG LEFT hypothermia alkalosis Low 2,3,DPG

20 Carbon dioxide transport
Carbon dioxide is transported in 3 ways: dissolved in plasma (7%), combined with Hb as carbaminohaemoglobin (23%), as bicarbonate in plasma (70%).

21 Carbon dioxide transport as bicarbonate.
This can be illustrated by the following reversible equation, which occurs within red blood cells: PCO2 + H2O H2CO3 (carbonic anhydrase) H2CO H+ + HCO3

22 Carbon dioxide transport
H+ combine mainly with Hb. HCO3 leaves RBC and enters plasma. Cl - diffuse from plasma into RBC (chloride shift) and combines with K+ to form KCl. HCO3 combines with Na to form NaHCO3. Process is reversed in pulmonary capillaries.

23 Carbon dioxide transport
Increase in PCO2 causes increase in HCO3 and H+ ions. This has important effects on respiratory stimulation and acid -base balance.

24 pH / acid - base balance pH is a measure of amount of free hydrogen ions. Increased free hydrogen ions = acidosis. Decreased free hydrogen ions = alkalosis. pH of blood needs to be maintained within a fairly narrow range in order for enzyme reactions to occur.

25 pH / acid - base balance Normal range for pH is 7.35 - 7.45
pH < 7.35 = acidosis pH > 7.45 = alkalosis pH maintained by 3 main mechanisms: The buffer systems, the respiratory system, and the kidneys.

26 pH / acid - base balance When an abnormality in pH occurs, these systems compensate for the imbalance. Compensation mechanisms will only (try to) restore pH to normal - overcompensation will not occur acidosis will not become alkalosis).

27 Buffer system Main buffering system is the carbonic acid / bicarbonate mechanism. carbonic acid = weak acid bicarbonate = weak base Normal body physiological processes tend to produce more acid - consequently, the body maintains more bicarbonate ions compared to carbonic acid.

28 Other buffering mechanisms are:
Hb phosphates proteins

29 Respiratory system Function of respiratory system is to maintain normal levels of CO2 and O2. CO2 is the most important chemical stimulus for regulating respiration. Combines with water to form carbonic acid which dissociates into bicarbonate and free hydrogen. Hypoxia also important respiratory stimulus, but only below PO2 of 8.8kpa.

30 Respiratory system Hydrogen ions stimulate chemosensitive area in medulla. Increased CO2 stimulates increased rate and depth of respiration until CO2 normal. Decreased CO2 decreases rate and depth of respiration until CO2 normal.

31 Respiratory system Respiratory response to acidosis is rapid (3 mins) and is maximised in hours.

32 The kidneys The kidneys regulate pH by: excreting hydrogen ions
producing and reabsorbing filtered bicarbonate and phosphate (chemical buffers). Process is slow, may take hours to show effect and days to maximise.

33 ACIDOSIS Clinical effects include:
Central Nervous System depression leading to coma myocardial depression arrythmias MAY BE RESPIRATORY OR METABOLIC IN ORIGIN.

34 Respiratory acidosis - causes:
Lung disease (COPD, CF, pneumonia) Opiates Impaired neuromuscular function (Guillan -Barre, spinal cord injury) Severe head injury

35 Respiratory acidosis Compensation by kidneys (increase HCO3).
Treatment = NIPPV, IPPV, physiotherapy

36 Metabolic acidosis - causes:
Inadequate elimination of H+, loss of HCO3 or excessive accumulation of acids other than H2CO3. Due to: renal failure diarrohea DKA Lactic acidosis

37 Metabolic acidosis Compensation by the respiratory system to eliminate CO2. Treatment = HD, CVVH insulin, fluids inotropes.

38 ALKALOSIS Clinical effects include: muscle spasm / convulsions
Hypokalaemia MAY BE RESPIRATORY OR METABOLIC IN ORIGIN.

39 Respiratory alkalosis - causes:
Hyperventilation (anxiety, PE) oxygen deficiency (high altitiude, PE) CVE or severe head injury

40 Respiratory alkalosis
Compensation is by the kidneys, which remove bicarbonate and reabsorb / reduce secretion of hydrogen ions. Oxygen, rebreath CO2.

41 Metabolic alkalosis - causes:
Excessive bicarbonate ions, due to: loss of hydrogen ions (vomiting, Gastro Intestinal fistula) over-administration of bicarbonate diuretics.

42 Metabolic alkalosis Compensation is by respiratory system - hypoventilation (not very effective). Treat the cause (e.g. anti-emetics) IT IS HARDER TO TREAT A METABOLIC ALKALOSIS THAN AN ACIDOSIS.

43 Interpretation of blood gases
Look at the PO2 - is it normal? Look at pH - < 7.35, > 7.45? Look at base excess : <-3 = acidosis, > +3 = alkalosis.

44 Base excess Definition: the number of mols of acid required to restore 1 litre of blood to a pH of 7.4, while the PCO2 is held constant. Can be used to calculate the dose (in mmols) of NaHCO3 to correct a metabolic acidosis: BE x 0.2 x weight (kg) = mmols NaHCO3

45 Interpretation of blood gases
PCO2 - is it normal? (respiratory problem) HCO3 - is it normal?(metabolic problem)

46 Scenario 1 pH = 7.48 PO2 = 12.6 PCO2 = 3.7 HCO3 = 22 BE = +5

47 Scenario 2 pH = 7.2 PO2 = 18 PCO2 = 14.6 HCO3 = 30 BE = +6

48 Scenario 3 pH = 7.1 PO2 = 8.6 PCO2 = 7.3 HCO3 = 10.5 BE = -13

49 Scenario 4 pH = 7.53 PO2 = 14 PCO2 = 5.5 HCO3 = 33 BE = +7

50 Scenario 5 pH = 7.37 PO2 = 8.8 PCO2 =7.8 HCO3 = 35 BE = +7

51 Scenario 6 pH = 7.35 PO2 = 12 PCO2 = 3.1 HCO3 = 15 BE = -9

52 Scenario 7 pH = 7.28 PO2 = 10 PCO2 = 4.5 HCO3 = 12 BE = -8

53 Further reading: Woodrow, P. (2000) Intensive Care Nursing: A framework for practice Routledge. T.E. Oh, A.D. Bersten and N. Soni (Eds) (2003) Oh’s Intensive Care Manual 5th Edition Butterworth Heinemann

54 Abbreviations CO2 CO pH – a measure of acidity or alkalinity of a solution ABG – Arterial blood gas Hb – Heamoglobin SaO2 Saturation of oxygen in arterial blood

55 SpO2 – Saturation of oxygen in capillary blood
O2 - Oxygen PO2 – Partial pressure of oxygen H2CO3 - Carbonic Acid HCO3 - Bicarbonate H+ - Hydrogen PCO2 – Partial Pressure of Carbon Dioxide H2O – Water CL – Choloride RBC – Red Blood Cells K – Potassium KCL – Potassium Chloride

56 Na – Sodium NaHCO3 – Sodium Bicarbonate CNS – Central Nervous System NiPPV – Non invasive Positive Pressure Ventilation IPPV – Invasive Positive Pressure Ventilation DKA – Diabetic Ketoacidosis HD – Heamodialysis CVVH Insulin – Insulia via continuous veno-venous heamo-filtration PE- Pulmonary Embolism CVE – Cerebrovascular event BE – Base excess


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