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

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%

4. Blood Gas ReportBlood Gas Report
(mixed/central venous)(mixed/central venous)
• pH = 7.32-7.42
• PvCO2 = 40 - 50 (mm Hg)
• PvO2 = 36 - 42 (mm Hg)
• Oxygen Saturation > 70%
• Base Excess = -2 to +2

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

9. Alveolar HypoventilationAlveolar Hypoventilation
• No increase in P(A-a)O2 gradient
• Muscle weakness
• Neuromuscular Junction Disease
• Reduced Respiratory Drive (drugs)
• Chest wall elastic loads
• Brainstem injury

10. Approach To HypoxemiaApproach To Hypoxemia
PaO2 A-a Do2 pCO2
N
FIO2
N
Alv. Hypo.
100% O2 Corrects V/Q Mis.
No Correction Shunt
Diffusion

11. Changes in P(A-a)O2 with FIO2

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

14. Pulse OximetryPulse Oximetry
Whole Blood
PaO2
SaO2
SpO2:
Pulse Oximetry

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

22. Respiratory Acid-Base Status
• Respiratory Disturbances
– CO2+H20 H2CO3 H+
+ HCO3
– Acute changes:
• Delta 10 mm Hg PaCO2, pH changes by 0.08
• Chronic change: 40 + B.E
– Alveolar Ventilation
• VA CO2 pH
• Respiratory Acidosis pCO2 > 45
• Respiratory Alkalosis pCO2 < 35

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

33. PaO2 vs PvO2 in Cardiogenic Shock

34. Paradoxical Respiratory Acidosis of Cardiopulmonary Arrest

35. Four Determinants of Central Venous Oximetry
ScvOScvO22 ≅≅ SvOSvO22 = SaO= SaO22 - VO- VO22 / C.O. x Hgb x 1.36/ C.O. x Hgb x 1.36
ScvO2 = Central venous saturation (%)
SvO2 = Mixed venous saturation (%)
SaO2 = Arterial oxygen saturation (%)
VO2 = Oxygen consumption mL (O2/min)
Hgb = Hemoglobin concentration (g/dL)
Cardiac Output (C.O.) = dL/min

36. Effect of changes in PaO2 on SvO2
600
200
100
80
60
40
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600 700
PaO2 (mm Hg)
SvO2
SvO2

37. Effect of changes in Hgn on SvO2
13
10
7.5
5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8 10 12 14
Hemoglobin (g/dL)
SvO2
SvO2

38. The Effects of Cardiac Output on SvO2
0.87
0.83
0.73
0.66
0.55
0.31
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10 12
Cardiac Output (L/min)
SvO2
SvO2

39. Effect of Oxygen Consumption (VO2) on SvO2
0.85
0.79
0.74
0.68
0.57
0.46
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 100 200 300 400 500 600
Oxygen Consumption (VO2)
SvO2
SvO2

40. Venous Arterial CO2 Difference
• Circulatory Failure
– Associated with Tissue Hypercarbic Acidosis
– Hypovolemia, sepsis, shock …
• Cardiac Index = e (1.787–0.151(v-aCO2))
– Endpoint of Resuscitation
• PvO2
– Enpoint of Resuscitation

41. SvO2 vs SaO2 response in Septic Shock "Golden Hour"
27
51
68
73
100 100 98
91
0
10
20
30
40
50
60
70
80
90
100
1 2 3 4
Days
Percentage
SvO2
SaO2

42. REFERENCES
• Current Opinion Critical Care 2001; 7: 204-211
• NEJM 2001; 345: 1368-1377
• Critical Care Medicine 2002; 30: 1686-1692
• Circulation 1969; 40: 165
• Thorax 2002; 57: 170-177
• Academic Emer Med 1999; 6: 421