Examines the utility of both arterial and venous blood gas analysis in critical illness
Edward Omron MD MPH FCCP
Pulmonary, Critical Care, and Internal Medicine
Morgan Hill, CA 95037
www.docomron.com
Arterial and Venous Blood Gas Analysis Edward Omron MD, MPH
1. Arterial And Venous BloodArterial And Venous Blood
Gas AnalysisGas Analysis
ByBy
Edward Omron MD MPH FCCPEdward Omron MD MPH FCCP
Pulmonary Critical Care MedicinePulmonary Critical Care Medicine
Morgan Hill, CAMorgan Hill, CA
www.docomron.comwww.docomron.com
2. INDICATIONSINDICATIONS
• ABG
– Oxygenation
– Ventilation
– Acid-Base Status
• VBG
– Ventilation and Acid-Base Status
– Cardiac Output (venous arterial PCO2 difference)
– Endpoint of resuscitation (PvO2 and ∆PCO2)
3. Blood Gas Report(Blood Gas Report(ArterialArterial))
• pH (No Units) 7.35-7.45
• PaCO2 (mm Hg) 35-45
• PaO2 (mm Hg) 110 - 0.5(age)
• HCO3
-
(mmol/L): calc. 22-26
• B.E. (mmol/L) -2 to 2
• O2 saturation: calc. >90%
6. ANALYSIS OF OXYGENATIONANALYSIS OF OXYGENATION
• Alveolar Gas Equation
– PAO2 = FIO2(PB - 47) - 1.2(PaCO2)
– PAO2 defines upper limit of PaO2
– FIO2 is 21% at all altitudes
– Factor 1.2 determined by RQ
– Water vapor pressure = 47 mm Hg
– PAO2= 150 - 1.2(PaCO2) at room air
– PAO2 = 102
7. Alveolar-Arterial Oxygen DifferenceAlveolar-Arterial Oxygen Difference
• A-aDo2 = PAO2-PaO2(from ABG)
• Insight in the patients state of gas exchange
– If elevated, defect in gas exchange
– Proper interpretation of the PaO2
8. Causes of a low PaOCauses of a low PaO22 and A-a Doand A-a Do22
• P(B): Altitude
• Alveolar Hypoventilation(Nl A-a Do2)
• V/Q mismatch
• Shunt
• Diffusion Impairment
• Decreased mixed / central venous O2
content
12. Acute Lung Injury
• PaO2/FIO2 ratio
– Normal = 90 mm Hg/0.2 = 450
– Acute lung injury < 300
– Acute Respiratory Distress syndrome
<200
13. OXIMETRYOXIMETRY
• Binding sites for O2 are heme groups
• OXYGEN SATURATION
– % of all heme sites saturated with O2
• Measures the difference in the light absorbance
characeteristics between Oxy Hb and Deoxy Hb
• SpO2 = Oxy Hb x 100
Oxy Hb + Deoxy Hb
• ABG SaO2 is a calculated value from PaO2
15. OximetryOximetry
• 54 yo WM with headaches, dyspnea and
a Kerosene heater at home
– ABG: PaO2 = 89, PaCO2 = 38, pH = 7.43
– SaO2 = 98%
– Whats the problem?
16. OximetryOximetry
• Carboxyhemoglobin: Hb +CO
– Does not affect PaO2 only SaO2
– Pulse oximetry reads CO-HB as OxyHb
• Follow Up:
– PaO2 = 79, PaCO2 = 31, SpO2 = 53%, pH =7.36
– CO-Hb46%
17. Correlation of symptoms and signs with carbon monoxide level
Percent of CO in inspired
air
Percent of HbCO in blood Signs and symptoms
0.007 10
Common in cigarette smokers; dyspnea
during vigorousexertion; occasional
tightness in forehead; dilation of cutaneous
blood vessels
0.012 20
Dyspnea during moderate exertion;
occasional throbbing headache in temples
0.022 30
Severe headache; irritability; easy
fatigability; disturbed judgment; possible
dizziness and possible dimness of vision
0.035 40+ Headache; confusion; fainting on exertion
0.080 60+
Unconsciousness; intermittent convulsions;
respiratory failure; death if exposure
prolonged
0.195 80 Fatal
Modified from Winter, P.M., and Miller, J.N.: JAMA 236:1503, 1976, Copyright 1976, American Medical
Association.
18. ProblemProblem
• 42 yo HIV pt with fevers,chills, SOB,cough
– Taking Dapsone for PCP prophylaxis
– ABG: PaO2 82.5, PaCO2 35.2, pH 7.43, SaO2
89%
– PCP Pneumonia, started onPrimaquine, Clinda,
and Prednisone
– ABG: PaO2 378, PaCO2 of 35, pH 7.42, SaO2
80%
– Whats Happening?
19. MethemoglobinMethemoglobin
• Oxidation of Fe++ to Fe+++ state
• Unlike CO-Hb, Met-Hb does depress the
SpO2 reading
• Both Dapsone and Primaquine are
oxidants
• Met-Hb depresses the SpO2 to 80’s
– Further increaeses in Met-Hb do not depress
SpO2
• Methylene Blue administration is Rx
20. Some drugs implicated in causing methemoglobinemia
Generic name Use
Dapsone Skin protectant
Benzocaine Local anesthetic
Metoclopramide Gastric stasis
Nitroglycerin Angina
Phenazopyridine Urinary tract analgesic
Prilocaine Local anesthetic
Primaquine Malaria prophylaxis and treatment
Trimethoprim Urinary antibacterial
Amyl nitrite
Rarely used clinically; often used by drug
abusers
21. ANALYSIS OF VENTILATONANALYSIS OF VENTILATON
• PaCO2 = VCO2 x K
VA
Hypercapnea > 45 mm Hg (Hypoventilation)
Respiratory Acidosis
Hypocapnea < 35 mm Hg (Hyperventilation)
Respiratory Alkalosis
23. BASE EXCESS (B.E.)BASE EXCESS (B.E.)
• Base Excess
• Quantity of acid or base needed to restore plasma
pH to 7.4, at a PCO2 equillibrated to 40 mm Hg
• Positive value, excess base, metabolic alkalosis
• Negative value, excess acid, metabolic acidosis
• Metabolic component of acid-base status
• PCO2 independent
• Estimated by BE = (Total CO2 – 24)
24. Problem Solving
1. LOOK AT THE pH
– Whatever side of pH 7.4 is the primary disorder
2. Look at pH, PCO2direction
– Both decrease or increase, then metabolic
– If move in opposite directions, respiratory
3. Respiration: acute or chronic?
– Acute: 10 mm Hg / 0.08 change in pH
– Chronic: 40+Base Excess
25. • Calculate the Adjusted Anion Gap
– High vs normal ANG differential
– 2.8 mmol of acid /gram serum albumin
– Law of Electrical Neutrality
• Positive charges = negative charges or
• Positive charges - negative charges = 0
– [Na+
] - [Cl-
] -[HCO3
-
] - [Albumin-
] = 0
– [Na+
] - [Cl-
] -[HCO3
-
] = [Albumin-
]
– 140 - 104 -24 = 4.4 gm/dL* 2.8 ≅ 12
– Normal ANG = 12 = 2.8 * [Albumin-
]
– Adjusted ANG = ANG + 2.8(4.4 -Albumin)
26. Arterial Draw:
• pH = 7.28, PaCO2 = 34, HCO3 = 16
• Na = 153 Cl = 106 Total CO2 = 17
• Alb = 3 g/dL
• Primary Acid-Base Disturbance?
• ANG and Adjusted ANG
• Metabolic Acid-Base Status
74 yo male found unresponsive and pulseless
27. • Primary Disorder
– Acidosis and acidemia (pH < 7.4)
• pH and PCO2direction
– Both down: Metabolic Acidosis
• Base Excess
– 16 – 24 = -8 mmols/L
• Adjusted Anion Gap
– ANG + 2.8*(4.4 - 3) or 30 + 4 = 34
– Anion Gap Acidosis
• Compensation?
– 40 + BE or 32 mm Hg
28. Venous Draw
• pH = 7.08, pCO2 = 75, HCO3 = 21
• Na = 145, Cl = 103, Total CO2 =22
• Alb = 3 g/dL
• Primary Acid-Base Disorder?
• ANG and expected ANG?
• Metabolic Acid-Base Status?
29. • Primary Disorder
– pH < 7.4, acidosis and acidemia
• pH and PCO2 direction
– Opposite therefore RESPIRATORY acidosis
• Base Excess
– 22 – 24 = -2 mmol/L
• Adjusted Anion Gap
– ANG + 2.8(4.4 -3) = 20+4 =24
– Anion Gap Acidosis
• 40 + BE rule ≠ Comp in VBG
30. 74 yo male found unresponsive and pulseless
• Why a metabolic acidosis in arterial bed
and respiratory acidosis in venous bed?
– Venous arterial PCO2 difference?
– PaCO2 = 34 and PvCO2 = 75
– PvCO2 – PaCO2 ∝ 1 / cardiac index
• Venous vs Arterial saturation difference?
– PaO2 = 50 mm Hg, saturation = 84%
– PvO2 =18, Venous Saturation = 20%
– Increased oxygen extraction from circulatory
failure
31. Central Venous Oxygen Saturation
ScvO2
• Easily measured with venous blood gas
• Surrogate measurement of mixed venous oxygen sat.
– 5-18% higher
– A low ScvO2 always means a low SvO2!
• Normal ScvO2 ≅ 68-76%
– 25% extraction coefficient of normal physiology
– Peripheral venous gas as a screen for further investigation
