arterial blood gases

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  • 1.

2. ARTERIAL BLOOD GASES By Dr Y.N MAURYA M.B.B.SD.C.H Medical Officer Deen Dayal Upadhyay HospitalGovt.ofNCT New Delhi- 64 3. ARTERIAL BLOOD GASES

  • Arterial bloodgassamplingis a medical technique used to check gas levels in theblood . It typically involves using a thin needle andsyringeto puncture anartery , This technique is commonly used on patients whose breathing is controlled by amechanical respiratoror who are having serious difficulties with breathing.

4. A.B.G SAMPLE COLLECTION

  • ULNER,RADIAL BRACHIAL DORSALIS PEDIS FEMORAL ARTERIES CAN BE USED.
  • ARTERIAL LINE ---- FREQUENT SAMPLING
  • INTERMITENT STAB---INFREQ. SAMPLING
  • UNDER FULL ASEPTIC CONDITION
  • IF ULNER/RADIAL ART IS USED--- MODIFIED ALLENS TEST

5. MODIFIED ALLENS TEST

  • 1 .INSTRUCT PATIENT TO CLENCH HIS FIST
  • 2.USING YOUR FINGER APPLY OCCLUSIVE PRESSURE ON BOTH RADIAL & ULNER ARTERY
  • 3. WHILE APPLYING OCCLUSIVE PRESSURETO BOTH ARTERY,HAVE THE PATIENT RELAX HIS HAND.BLANCHING OF PALM &FINGER SHOULD OCCUR

6. MODIFIED ALLENS TEST

  • 4. RELEASE THE OCCLUSIVE PRESSURE ON ULNER ARTERY & NOTICE FLUSHING OF HAND WITHIN 7-10 SEC.THIS DENOTE THAT ULNER ARTERY SUPPLY IS ADEQUATE &ITS SAFE TO PRICK RADIAL ARTERY. IF IT DOEST OCCUR IT MAENS ULNER ARTERY SUPPLY IS NOT SUFFICIENT & RADIAL ARTERY IS NOT SAFE TO PRICK.

7. SAMPLE SIZE&PRECAUTION

  • ABOUT 2 mlBLOOD IN AIR TIGHT HEPARINISED SYRINGE/ VIAL
  • GLASS CAPILLARY CAN ALSO BE USED
  • COTINUOUS AGITATION BY ROLLING
  • FIO2 SHOULD BE WRITTENON SAMPLE

8. CONTRAINDICATIONFORARTERIALPUNCTURE

  • INFECTION AT SITE.
  • ALLENSTESTNEGATIVE.
  • ON ANTICOAGULANT THERAPY.
  • SEVERE PERIPHERAL VASCULAR DISEASE.
  • DISTAL TO SURGICAL SHUNT.

9. SpO 2 andSaO 2

  • SpO 2and SaO 2are often used interchangeably, but theyare not same
  • When O 2saturation is measured by pulse oximeter..... SpO 2
  • When O 2saturation is measured byCO- oximeter..... SaO 2
  • SpO 2is also called functionalarterial O 2saturation and SaO 2 asfractionalarterial O 2saturation
  • Only trueCO-oximeter candetermineanaccuratevalue for SaO 2

10. SpO 2 andSaO 2

  • SpO 2 == HbO 2
  • HbO 2+Hb
  • SaO 2== HbO 2
  • HbO 2 + Hb+COHb+MetHb+SfHb+COSfHb
  • SaO 2== SpO 2 [1-SaCO] ( Nellcor equation )
  • Non functional Hb is2-3 %
  • In heavy smoaker it may be up to15 %

11. O 2Saturation & Pulse oximetry

  • We can know arterial blood gases and arterial oxygen saturation from ABG machine with help ofO 2DISSOCIATION CURVE.

12. O 2DISSOCIATION CURVE 13. O 2Saturation & Pulse oximetry

  • ABG machinecalculateso 2saturation based on pH, PaCO 2, temp,by using normal adult o 2dissociation curve
  • o 2dissociation curve in presence of
  • 1) fetal Hb ,low 2,3 DPGcurve shifts Lt
  • 2)sickle cell, chr. Hypoxia, cyanotic H D, chr.Asthama, high altitude. curve shifts Rt.And ABG machine becomes inaccurate

14. O 2Saturation & Pulse oximetry

  • If we can know arterial blood gases and arterial oxygen saturation with help of ABG machine, thenwhat is need of pulse oximeter?

15. O 2Saturation & Pulse oximetry

  • ADVANTAGE OF PULSE OXIMETRY
  • Noninvasive
  • Portability
  • Continuous monitoring
  • Ease of use (no calibration)
  • Rapidity(warn decr. in saturation before sign and symptom)

16. Principle of Pulse oximetry

  • It is based on different absorption coefficients of oxyHb& deoxyHb fordifferent wavelength of lights ( BEERS LAW )
  • For oxyHb& deoxyHb 805nm isisobestic point.
  • Above 805 nm oxyHb absorbs more light than deoxyHb & below it deoxyHb absorbs more light than oxyHb. Because of this fact by using two wavelength of lights actual o 2 saturatn level can be measured.
  • Pulse oximeter uses two wavelength of light red(660nm) and infrared (950nm).

17. Principle of Pulse oximetry Absorption Spectrum of Hemoglobin 18. PulseOximetry

  • 1)Pulse oximeter emits two wavelength of lights (red& infrared)fromXenon LED. These lights are sensed by photodiodes just opposite to LED or adjacent to LED.
  • 2)Itmeasures %saturation of deoxyHb with O 2( not PaO 2 )based on absorption spectrum of Hb inpulsatile arterial blood .
  • 3) Sensitivity rapidly decreases atSpO 2 >90%
  • 4) Normallyin blood dysfunctional Hb is 2-3% hence all oximeters are standardized against known standard CO-oximeter& adjusted,so that it can display SpO 2 close to SaO 2.

19. PulseOximetry

  • 4)Accuracy depends on
  • a ) % of dysfunctional Hb.
  • b ) age of LED.(Aging of LEDcauses change in peak wavelength).
  • c ) LED from same batch& same manufacturer mayvary by 30nm in peak wavelength.
  • d ) Good red & infrared waveform i.e signal strength (tissues are easily penetrated by infrared but not so easily by red. Thus red waveform is lost first, this is why false reading is given when signal is weak)

20. PulseOximetry

  • There are two types of probes
  • 1 ) Transmission probeFinger type
  • 2 )Reflectanceprobe Rectal type
  • Dot provide accurate reading inCO-poisoning, methemoglobinemia, decreased cut. perfusion, strong vasoconstriction, deep pigmentation, nail polish, intravascular dye, movement, venous pulsation, infrared heating lamp& strong coloredlights.
  • ECG synchronization technique can be used to removemovement artifact.

21. Hypoxia & PulseOximetry

  • Hypoxic hypoxia.
  • Register low saturation
  • Anaemic hypoxia .
  • Normalsaturation
  • Circulatory hypoxia
  • Lowsignalstrength
  • Histotoxic hypoxia
  • Normalsaturation

22. SaO 2 andPulseCO- Oximetry

  • Pulse oximeter dot measure methemoglobin& carboxyHb.So becomes inaccurate in these cases.
  • IL-282 CO-oximeter is gold standard for o 2saturationmeasurement.
  • PulseCO-oximeteremits at leastfourwavelength of light which measures oxyHb, deoxyHb, methemoglobin& carboxyHb
  • Forinfantspulse oximeter is more usefulbecause CO-oximeter show false elevation of CO-Hb .(fetal Hb absorb more than two wavelength of lights)

23. SaO 2 andoxygencontent

  • Tissues need a requisite amount of oxygen molecules for metabolism.Neither the PaO 2nor the SaO 2tells how much oxygen is in the blood. It is provided by the oxygen content, CaO 2(units = ml O 2 /dl). CaO 2is calculated as: CaO 2=quantity O 2bound+quantity O 2dissolved to hemoglobin in plasma CaO 2=(Hb x 1.34 x SaO 2 )+(.003 x PaO 2 )
  • Hb = hemoglobin in gm%; 1.34 = ml O 2that can be bound to each gm of Hb; SaO 2is percent saturation of hemoglobin with oxygen; .003 is solubility coefficient of oxygen in plasma:.003 ml dissolved O 2 /mm Hg PO 2 .

24. Oxygen dissociation curve:SaO 2vs. PaO 2 O 2content 25. The Key to Blood Gas Interpretation :

  • Equation Physiologic Process
  • 1)PaCO 2equationAlveolar ventilation
  • 1)Alveolar gas equation Oxygenation
  • 2)Oxygen content equationOxygenation
  • 3)Henderson-Hasselbalch equationAcid-base balance

26. PaCO 2equation

  • PaCO 2=0.863V CO 2 /V A
  • V CO 2 ,ml of CO2 produced/ min;V A, alveolar ventilation
  • V A isincreased . PaCO 2 decreases
  • V A isdecreased . PaCO 2 increases

27. Alveolar Gas Equation

  • PAO 2= PIO 2- PaCO 2 /R,R= resp.quotient
  • PAO 2= PIO 2- PaCO 2 / O.8
  • PAO 2= PIO 2- 1.2 (PaCO 2 ),
  • PIO 2 = FIO 2(P B 47 mm Hg)
  • PAO 2= FIO 2(P B 47 mm Hg) - 1.2 (PaCO 2 )

28. Alveolar Gas Equation PAO 2= FIO 2(P B 47 mm Hg) - 1.2 (PaCO 2 )

  • Except in a temporary unsteady state, alveolar PO 2(PAO 2 ) is always higher than arterial PO 2(PaO 2 ). Thus,
  • If FIO 2 &P Bare constant ,as PaCO 2increases both PAO 2and PaO 2will decrease(hypercapnia causeshypoxemia).
  • IfP B &PaCO 2are constant , as FIO 2decreases both PAO 2and PaO 2will decrease(suffocation causes hypoxemia).
  • If PaCO 2 &FIO 2are constant , as P Bdecreases (e.g., with altitude), both PAO 2and PaO 2will decrease(mountain climbing causes hypoxemia).

29. P(A-a)O 2

  • P(A-a)O 2is the alveolar-arterial difference in partial pressure of oxygen. itactuallyresults from gravity-related blood flow changes within the lungs (normal ventilation-perfusion imbalance).
  • PAO 2is alwayscalculated , based on FIO 2 , PaCO 2and barometric pressure.
  • PaO 2is alwaysmeasured , in a blood gas machine.
  • Normal P(A-a)O 2ranges from @ 5 to 25 mm Hg at room air (it increases with age).A higherP(A-a)O 2means the lungs are not transferring oxygen properly from alveoli into the pulmonary capillaries. Except for right to left cardiac shunts, an elevated P(A-a)O 2signifies some sort ofproblem within the lungs.
  • If P(A-a)O 2is negative---there is error in calculation/measurement

30. Ventilation-Perfusion imbalance

  • A normal amount of ventilation-perfusion (V-Q) imbalance accounts for the normal P(A-a)O 2 .
  • Most common cause of low PaO 2is anabnormal degree of ventilation-perfusionimbalance Virtually all lung disease lowers PaO 2via V-Q imbalance, e.g., asthma, pneumonia, atelectasis, pulmonary edema, COPD.
  • Diffusion barrier isseldoma major cause of low PaO 2(it can lead to a low PaO 2during exercise).

31. Henderson - Hasselbalch equation

  • Weak acid ionizes as,
  • HA=H + + A -
  • pH = pK+log[A - ]/[HA]

32. Causes of low PaO 2

    • NON-RESPIRATORY P(A-a)O 2 Cardiac right to left shunt Increased
    • Decreased PIO 2 Normal Low mixed venous oxygen content* Increased
    • RESPIRATORY Pulmonary right to left shunt Increased Ventilation-perfusion imbalance Increased Diffusion barrier Increased Hypoventilation (increased PaCO 2 ) Normal
    • *Unlikely to be clinically significant unless there is right to left shunting or ventilation-perfusion imbalance

33. NORMAL A.B.G VALUES

  • HB--12-15gm/dl
  • HCT--35-55
  • FIO2-----
  • PaO280(60)-100mmHg
  • PaCO2 35-45mmHg
  • pH7.35-7.45
  • K+--3.5-4.5 mmol/L
  • Na+--135-145 mmol/L
  • HCO3 -- 22-26 mmol/L
  • BE--2 mmol
  • SaO 2-->90%

34. ABG Interpretation

  • First, does the patient have an acidosis or an alkalosis.
  • Second, what is the primary problem metabolic or respiratory.
  • Third, is there any compensation by the patient respiratory compensation is immediate while renal compensation takes time.

35. ABG Interpretation

  • It would be extremely unusual for either the respiratory or renal system to overcompensate
  • The pH determines the primary problem
  • After determining the primary and compensatory acid/base balance, evaluate the effectiveness of oxygenation

36. Abnormal Values

  • pH < 7.35
  • Acidosis (metabolic and/or respiratory)
  • pH > 7.45
  • Alkalosis (metabolic and/or respiratory)
  • paCO 2> 45 mm Hg
  • Respiratory acidosis (alveolar hypoventilation)
  • paCO 2< 35 mm Hg
  • Respiratory alkalosis (alveolar hyperventilation)
  • HCO 3< 22 meq/L
  • Metabolic acidosis
  • HCO 3> 26 meq/L
  • Metabolic alkalosis

37. Putting It Together -Respiratory

  • So,
  • paCO 2> 45 with a pH < 7.35 represents arespiratory acidosis
  • paCO 2< 35 with a pH > 7.45 represents arespiratory alkalosis
  • For a primary respiratory problem,pHandpaCO 2move in theoppositedirection
    • For each deviation in paCO 2of 10 mm Hg in either direction, 0. 08 pH units change in the opposite direction

38. Putting It Together -Metabolic

  • And
  • HCO 3< 22 with a pH < 7.35 represents ametabolic acidosis
  • HCO 3> 26 with a pH > 7.45 represents ametabolic alkalosis
  • For a primary metabolic problem,pHandHCO 3are in thesamedirection, andpaCO 2 is also in thesamedirection

39. Compensation

  • The bodys attempt to return the acid/base status to normal (i.e. pH closer to 7.4)
  • Primary Problem Compensation
  • respiratory acidosis metabolic alkalosis
  • respiratory alkalosis metabolic acidosis
  • metabolic acidosis respiratory alkalosis
  • metabolic alkalosis respiratory acidosis

40. Compensation Decrease4 mmol/L 0.17 Chr. Resp. Alk. (10 Decr. in PaCO2) Decrease2mmol/L 0.08 Ac. Resp. Alk. (10 Decr. in PaCO2) Increase3.5mmol/L 0.03 Chr. Resp. Acid. (10 Incr. in PaCO2) Increase1mmol/L 0.08 Ac. Resp. Acid. (10 Incr. in PaCO2) CHANGE IN HCO3 DECR.IN pH DISORDER Inc. 0.5-1/ 1 Inc in HCO3 PaCO2=0.7(HCO3)+201.5 Met.Alkalosis Dec. 1-1.5/1Dec in HCO3 PaCO2=1.5(HCO3)+82 Met.Acidosis Change inPaCO2/ Change in HCO3 PaCO2 DISORDER 41. HOW KNOW THE DISORDER > 26 > 45 ~ Normal Compensated > 26 > 45 > 7.45 Partly Compensated > 26 Normal > 7.45 Acute Metabolic Alkalosis < 22 < 35 ~ Normal Compensated < 22 < 35 < 7.35 Partly Compensated < 22 Normal < 7.35 Acute Metabolic Acidosis < 22 < 35 ~ Normal Compensated < 22 < 35 > 7.45 Partly Compensated Normal < 35 > 7.45 Acute Respiratory Alkalosis > 26 > 45 ~ Normal Compensated > 26 > 45 < 7.35 Partly Compensated Normal > 45 < 7.35 Acute Respiratory Acidosis HCO 3 PaCO 2 pH 42. 43. Effectiveness of Oxygenation

  • Further evaluation of the arterial blood gas requires assessment of the effectiveness of oxygenation of the blood
  • Hypoxemia decreased oxygen content of blood - paO 2less than 60 mm Hg and the saturation is less than 90%
  • Hypoxia inadequate amount of oxygen available to or used by tissues for metabolic needs

44. Causes of Hypoxemia

  • Inadequate inspiratory partial pressure of oxygen
  • Hypoventilation
  • Right to left shunt
  • Ventilation-perfusion mismatch
  • Incomplete diffusion equilibrium

45. Assessment of Gas Exchange

  • Alveolar-arterial O 2tension difference
    • A-a gradient
    • PAO 2 -PaO 2
    • PAO 2= FIO 2 (PB - PH 2 O) - PaCO 2 /RQ*
  • arterial-inspired O 2ratio
    • PaO 2 /FIO 2
    • P/F ratio
  • *RQ=respiratory quotient= 0.8

46. THANK YOU