Presentation on theme: "Dr James F Peerless March 2013"— Presentation transcript:
1Dr James F Peerless March 2013 Arterial Blood GasesDr James F PeerlessMarch 2013
2Objectives Indications Acid-Base Physiology Procedure Interpretation Case StudiesOther Useful Information
3What is an ABG?Blood test which measures the acid-base status and oxygen levels in the bloodCalculated valuespHPaO2PaCO2Other values now calculated or derivedCommonly used in:Acute careCritical carePulmonary medicineAny unexpected deterioration in a ptAcute exxacerbation of a chronic diseaseImpaired consciousnessImpraied respiratory effortDetermine the severity of a conditionWhat is causing the pt to be unwell – i.e. resp or metabolicMonitor the progress
4Indications Clinical indications Any unexpected deterioration in a patientAcute exacerbation of a chronic diseaseImpaired consciousnessImpaired respiratory effortToDetermine the cause of the illnessDetermine the severity of a conditionMonitor the progress of a patient
5Acid-Base Balance Tight regulation required to ensure Enzyme functionIon distributionProtein structureBody pH maintained by several buffer systemsBicarbonate/ carbonic acidPhosphateHb and plasma proteins
6Acid-Base Relationships The equation below shows the relationship between protons (H+), bicarbonate (HCO3-), water, and CO2H+ + HCO3- H2CO3 H2O + CO2In 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.
7Henderson–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.
8AcidaemiaDecreased hco will drive the eqn to the left, as more co2 and water dissociate to replace hco, and create more hIncrease co2 will drive eqn to the left and cause increased h
9Alkalaemia Alkalosis: increasing the bicarb concentration will generate alkalosis. When thishappens, we call the alkalosis a metabolic alkalosis.decreasing the carbon dioxide will also generate alkalosis. When thishappens, we call the alkalosis a respiratory alkalosis.
10Acid-base Regulation 4 components Initially Respiratory compensation Bicarbonate/ carbonic acid buffer system (MINUTES)Respiratory compensationHyper/hypoventilation (MINUTES)Renal compensationChanges to H+ and HCO3- secretion/retention (HOURS)HepaticUreagenesis (HOURS)AA metabolism HCO3- + NH4+2HCO3- + 2NH4+ NH2CONH2 + CO2 + 3H2O
12Acid-base Physiology Blood has a normal pH of 7.40 The normal range is between 7.35 and 7.45Any pH that is lower than 7.35 is considered acidoticAcidosis: a state of being acidoticAcidaemia: a condition of having acidic bloodAny pH that is higher than 7.45 is considered alkaloticAlkalosis: a state of being alkaloticAlkalaemia: a condition of having alkaline blood
14Primary DisturbanceWhen determining the cause of acid-base disturbances, look at what process is the primary componentpH 7.28 / PaCO2 5.0 / HCO3- 18pH 7.55 / PaCO2 5.0 / HCO3- 38pH 7.28 / PaCO / HCO3- 24pH 7.55 / PaCO2 2.0 / HCO3- 24Metabolic AcidosisMetabolic AlkalosisRespiratory AcidosisRespiratory Alkalosis
15Compensatory/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 directionif the primary disturbance is respiratory, the secondary compensatory mechanism must be metabolicif the primary disturbance is metabolic, the secondary compensatory mechanism must be respiratoryThe compensation process never over-corrects the primary disturbance.If the pH appears to be over-corrected, there is an additional mixed primary disturbance.
16CompensationRespiratory 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.
17The Davenport DiagramDisplays the relationship between pH, PaCO2 and HCO3-Explains the compensatory mechanisms that occur in acid-base balance.
18The Davenport Diagram Dotted lines are buffer lines Solid lines are of equal PaCO2Bad is the normal buffer lineAbc resp acidAde resp alkAgc met alkAfe met acid
19Procedure Explanation to pt. Equipment Allen’s Test Withdraw 1-2 mls Sterile glovesChlorhexidineHeparinised syringeAllen’s TestWithdraw 1-2 mlsRemove air, and cap off; pressure over punctureGet it to the machine within 10 minutes (iced samples: 1-2 hrs)
21Interpretation – 8 Steps Assess the patientIdentify the sourceArterial, venous, (or mixed venous)Check the pHNormal, acidaemia, alkalaemiaAssess the respiratory partpCO2 (4.5 – 6 kPa)Assess the metabolic partHCO3- (std) (22-26 mmol/L)
22Interpretation – 8 Steps Check the base excessthe difference between the patient’s standard bicarbonate level and 24normal range +/- 2 mmol/LCheck for compensationComplete or partialCheck the pO2<10 kPa = hypoxia
23Case 1An 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 6.89PaO kPaPaCO kPaHCO mmol L-1BE mmol L-1
24Case 1 “This patient has a partially compensated metabolic acidosis.” Assess the patientIdentify the sourceCheck the pHAssess the respiratory componentAssess the metabolic componentCheck the base excessCheck for compensationCheck the pO2Unwell; drowsyABGLife-threatening acidaemiaLow PaCO2 (R. Al.)V. low HCO3- (M. Ac.)V. low B.E.PartialWell oxygenated“This patient has a partially compensated metabolic acidosis.”
25Case 257-year old patient on the surgical ward with 3-day history of vomittingPale, clammyP 110 BP 100/50RR 9Arterial blood gas analysis:Inspired oxygen 21% (FiO2 0.21)pH 7.50PaO kPaPaCO2 7.4 kPaHCO3- 30 mmol L-1BE +4 mmol L-1
26Case 2 Assess the patient Identify the source Check the pH Assess the respiratory componentAssess the metabolic componentCheck the base excessCheck for compensationCheck the pO2UnwellABGAlkalaemiaHigh PaCO2 (R. Ac.)Raised HCO3- (M. Al.)Positive B.E.PartialMildly hypoxic“This patient has a partially compensated metabolic alkalosis.”
27Case 3A 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 kPapH 7.19PaCO kPaBicarbonate 23.6 mmol L-1Base excess -2.4 mmol L-1
28Case 3 “This patient has a uncompensated respiratory acidosis.” Assess the patientIdentify the sourceCheck the pHAssess the respiratory componentAssess the metabolic componentCheck the base excessCheck for compensationCheck the pO2DrowsyABGAcidaemiaRaised PaCO2 (R. Ac.)Normal/low HCO3-Slightly reduced B.E.NilWell oxygenated“This patient has a uncompensated respiratory acidosis.”
29Case 4A 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% oxygenP 100 min-1, BP 90/54 mmHg, GCS 3Arterial blood gas analysis reveals:Inspired oxygen 100% (FiO2 1.0)PaO kPapH 7.62PaCO kPaHCO3- 20 mmol L-1BE -4 mmol L-1
30Case 4 Assess the patient Identify the source Check the pH Assess the respiratory componentAssess the metabolic componentCheck the base excessCheck for compensationCheck the pO2Unwell; comatoseABGSignificant alkalaemiaLow PaCO2 (R. Al.)Reduced HCO3- (M. Ac.)Reduced B.E.PartialWell oxygenated“This patient has a partially compensated respiratory alkalosis.”
31What else can we learn from the ABG? Measured ValuesNa+, K+, Cl-, Ca2+GlucoseLactateHbDerived ValuesAnion GapA-a gradientP/F Ratio
32AG = ([Na+] + [K+]) − ([Cl−] + [HCO3−]) = 3–11 mEq/L Anion GapThe 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
33Anion Gap HIGH – “MUDPILES” NORMAL – “FUSED CARS” Methanol Uraemia DKA Propylene glycolIsoniazidLactic acidosisEthylene glycol (antifreeze)SalicylatesFistulaeUretogastric conduitsSaline administrationEndocrine (hyperparathyroid)DiarrhoeaCA InhibitorsAmmonium chlorideRenal tubular necrosisSpironolactone
34A-a gradientThe 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 hypoxemiaPulmonary (increased) – diffusion or shuntExtrapulmonary (normal)
35P/F RatioThe 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
36Summary Keep analysis simple, and be methodical Analysis takes practiceAlways relate numbers back to the patientCheck all the numbersRepeat a gas after an intervention has been made