Blood Gases and Related Tests

1
Blood Gases and Related Tests

• RET 2414
• Pulmonary Function Testing
• Module 6.0

2
Objectives

• Describe how pH and PCO2 are used to assess
  acid-base balance
• Interpret PO2 and oxygen saturation to assess
  oxygenation
• Identify the correct procedure for obtaining an
  arterial blood gas specimen
• List situation in which pulse oximetry can be
  used to evaluate a patients oxygenation

3
Objectives

• Describe the use of capnography to assess changes
  in ventilatory-perfusion patterns of the lung
• Describe at least two limitation of pulse
  oximetry

4
Reasons for Obtaining an ABG

• Assessment of ventilatory status
• Assessment of acid-base balance
• Assessment of arterial oxygenation

5
Acid Base Balance

• The maintenance of cellular function depends on
  an exacting environment.
• One of the most important environmental factors
  is the hydrogen ion concentration (H), commonly
  expressed as pH.

6
Acid Base Balance

• The pH is a function of the relation of HCO3-
  (base) to PCO2 (acid) in the blood in the
  following fashion

pH HCO3- (base) metabolic component
kidney 20 CO2 (acid) respiratory
component lungs 1
7.40
Normal pH
7
Acid Base Balance

• Acidemia an acidic condition of the blood
• pH lt 7.35
• Alkalemia an alkaline condition of the blood
• pH gt7.45

8
Acid Base Balance

• Respiratory Component (PCO2)

Red Blood Cell
Plasma
Tissues
CO2
CO2
dCO2
H2CO3
9
Acid Base Balance

• Excretion of CO2 is one of the lungs main
  functions

PA CO2 40 mmHg
PaCO2
PvCO2
46 mmHg
40 mmHg
Pc CO2
10
Acid Base Balance

• Respiratory Component (PCO2)
• Ventilation PCO2 pH
• Respiratory Acidosis

11
Acid Base Balance

• Respiratory Component (PCO2)
• Ventilation PCO2 pH
• Respiratory Alkalosis

12
Acid Base Balance

• Metabolic Component (HCO3- and BE)
• Bicarbonate (HCO3-) is the primary blood base and
  is regulated by the kidneys and not the lungs.
• Normal HCO3- 24 mEq/L

13
Acid Base Balance

• Metabolic Component (HCO3- and BE)
• Base Excess (BE) is a measure of metabolic
  alkalosis or metabolic acidosis expressed as the
  mEq of strong acid or strong alkali required to
  titrate one liter of blood to a pH of 7.40
• Normal B.E. -2 to 2 mEq/L

14
Acid Base Balance

• Metabolic Component (HCO3- and BE)
• HCO3- or B.E.
• Metabolic Acidosis

15
Acid Base Balance

• Metabolic Component (HCO3- and BE)
• HCO3- or B.E.
• Metabolic Alkalosis

16
Acid Base Balance

• Combined Respiratory / Metabolic
• Respiratory and metabolic component moving
  toward the same acid/base status
• PCO2 HCO3- Acidosis
• PCO2 HCO3- Alkalosis

17
Acid Base Balance

• Compensation
• Abnormal pH is returned toward normal by
  altering the component NOT primarily affected,
  i.e., if PCO2 is high, HCO3- is retained to
  compensate
• PCO2 HCO3-
• HCO3- PCO2

18
Acid Base Balance

• Normal metabolism produces approximately 12,000
  mEq of hydrogen ions per day. Less than 1 is
  excreted by the kidneys, because the normal
  metabolite is CO2 which is excreted by the lungs.

19
Acid Base Balance

• Acid-Base imbalance is not life-threatening for
  several hours to days following renal shutdown
  but becomes critical within minutes following
  cessation of breathing.

WOW!
20
Normal Values

• pH PCO2 (mmHg) HCO3-
  (mEq/L)
• 7.40 40 24

21
Acceptable Ranges (2 SD)

• pH PCO2
  HCO3-
• Normal 7.35 7.45 35 45 22 -
  26
• Acidotic lt7.35 gt45 lt22
• Alkalotic gt7.45 lt35 gt26

22
Arterial Oxygenation

• Tissue hypoxemia exists when cellular oxygen
  tensions are inadequate to meet cellular oxygen
  demands.
• PaO2 has become the primary tool for clinical
  evaluation of the arterial oxygenation status.

23
Arterial Oxygenation

• Hypoxemia an arterial oxygen tension (PaO2)
  below an acceptable range.
• Arterial Oxygen Tensions for Adult and Child
• Normal 97 mm Hg
• Acceptable Range 80 mm Hg (range decreases with
  age)
• Hypoxemia lt80 mm Hg

24
Systematic Interpretation

• Assessment of ventilatory status
• Assessment of acid-base balance
• Assessment of arterial oxygenation

25
Exercise 1

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.26
• 35 - 45 PCO2 56
• 22 - 26 HCO3- 24
• -2 - 2 BE -4
• gt80 PO2 50

Acute ventilatory failure with hypoxemia (Acute
respiratory acidosis with hypoxemia)
26
Exercise 2

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.56
• 35 - 45 PCO2 29
• 22 - 26 HCO3- 24
• -2 - 2 BE 3
• gt80 PO2 90

Acute alveolar hyperventilation without
hypoxemia (Acute respiratory alkalosis without
hypoxemia)
27
Exercise 3

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.56
• 35 - 45 PCO2 44
• 22 - 26 HCO3- 38
• -2 - 2 BE 14
• gt80 PO2 75

Uncompensated metabolic alkalosis with hypoxemia
28
Exercise 4

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.20
• 35 - 45 PCO2 38
• 22 - 26 HCO3- 15
• -2 - 2 BE -13
• gt80 PO2 90

Uncompensated metabolic acidosis without hypoxemia
29
Exercise 5

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.45
• 35 - 45 PCO2 20
• 22 - 26 HCO3- 16
• -2 - 2 BE -7
• gt80 PO2 90

Chronic alveolar hyperventilation without
hypoxemia (Compensated respiratory alkalosis
without hypoxemia)
30
Exercise 6

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.42
• 35 - 45 PCO2 72
• 22 - 26 HCO3- 46
• -2 - 2 BE 18
• gt80 PO2 45

Chronic ventilatory failure with
hypoxemia (Compensated respiratory acidosis with
hypoxemia)
31
Exercise 6

• A 76-year-old man with a long history of
  symptomatic COPD entered the hospital with
  basilar pneumonia. He was alert, oriented, and
  cooperative.
• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.58
• 35 - 45 PCO2 45
• 22 - 26 HCO3- 42
• -2 - 2 BE 17
• gt80 PO2 38

32
Exercise 6

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.58
• 35 - 45 PCO2 45
• 22 - 26 HCO3- 42
• -2 - 2 BE 17
• gt80 PO2 38

Question Is this uncompensated metabolic
alkalosis with severe hypoxemia?
33
Exercise 6

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.58
• 35 - 45 PCO2 45
• 22 - 26 HCO3- 42
• -2 - 2 BE 17
• gt80 PO2 38

Uncompensated metabolic alkalosis with severe
hypoxemia?
34
Exercise 6

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.58
• 35 - 45 PCO2 45
• 22 - 26 HCO3- 42
• -2 - 2 BE 17
• gt80 PO2 38

A metabolic alkalosis with hypoxemia must be
clinically correlated because a disease process
causing metabolic alkalosis would not be expected
to produce severe hypoxemia.
35
Exercise 6

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.58
• 35 - 45 PCO2 45
• 22 - 26 HCO3- 42
• -2 - 2 BE 17
• gt80 PO2 38

Correct Interpretation Acute alveolar
hyperventilation (respiratory alkalosis)
superimposed on chronic hypercapnia (chronic
ventilatory failure) with severe hypoxemia.
36
Exercise 7

• A 67-year-old man admitted to the Emergency
  Department with exacerbated COPD. He was alert,
  oriented, and cooperative.
• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.25
• 35 - 45 PCO2 90
• 22 - 26 HCO3- 38
• -2 - 2 BE 12
• gt80 PO2 34

37
Exercise 7

• Acceptable Range ABG Result
• 7.35 - 7.45 pH 7.25
• 35 - 45 PCO2 90
• 22 - 26 HCO3- 38
• -2 - 2 BE 12
• gt80 PO2 34

Acute ventilatory failure superimposed on chronic
hypercapnia (chronic ventilatory failure) with
severe hypoxemia.
38
Pulse Oximetry

• Pulse oximetry (SpO2) is the noninvasive
  estimation of SaO2

39
Pulse Oximetry

• SaO2

40
Pulse Oximetry

• SaO2

41
Pulse Oximetry

• SaO2

42
Pulse Oximetry

• SaO2

43
Pulse Oximetry

• Pulse oximetry may be used in any setting in
  which a noninvasive measure of oxygenation status
  is sufficient.
• O2 therapy
• Surgery
• Ventilator management
• Diagnostic procedures
• Bronchoscopy
• Sleep studies
• Stress testing
• Pulmonary Rehabilitation

44
Pulse Oximetry

• Pulse oximetry uses light to work out oxygen
  saturation. Light is emitted from light sources
  which goes across the pulse oximeter probe and
  reaches the light detector.

45
Pulse Oximetry

• If a finger is placed in between the light source
  and the light detector, the light will now have
  to pass through the finger to reach the detector.
  Part of the light will be absorbed by the finger
  and the part not absorbed reaches the light
  detector.

46
Pulse Oximetry

• Hemoglobin (Hb) absorbs light. The amount of
  light absorbed is proportional to the
  concentration of Hb in the blood vessel. By
  measuring how much light reaches the light
  detector, the pulse oximeter knows how much light
  has been absorbed. The more Hb in the finger ,
  the more light is absorbed.

47
Pulse Oximetry
48
Pulse Oximetry

• The pulse oximeter uses two lights to analyze
  hemoglobin, red and infared, to detect the amount
  of oxyhemoglobin (O2Hb) and deoxyhemoglobin (rHb)
  

49
Pulse Oximetry

• The pulse oximeter works out the oxygen
  saturation by comparing how much red light and
  infra red light is absorbed by the blood.
  Depending on the amounts of oxy Hb and deoxy Hb
  present, the ratio of the amount of red light
  absorbed compared to the amount of infrared light
  absorbed changes.

50
Pulse Oximetry
51
Pulse Oximetry
52
Pulse Oximetry

• Using this ratio, the pulse oximeter can then
  work out the oxygen saturation.

53
Pulse Oximetry

• Using this ratio, the pulse oximeter can then
  work out the oxygen saturation.

54
Pulse Oximetry

• Pulse oximeters often show the pulsatile change
  in absorbance in a graphical form. This is called
  the "plethysmographic trace " or more
  conveniently, as "pleth".

55
Pulse Oximetry

• If the quality of the pulsatile signal is poor,
  then the calculation of the oxygen saturation may
  be wrong. Always look at pleth before looking at
  oxygen saturation.

56
Pulse Oximetry

• Never look only at oxygen saturation !

57
Pulse Oximetry

• Always look at pleth before looking at oxygen
  saturation!

58
Pulse Oximetry

• Interfering Substances
• COHb
• MetHb
• Intravascular dyes (indocyanine green)
• Nail polish or coverings

59
Pulse Oximetry

• Interfering Factors
• Motion artifact, shivering
• Bright ambient lighting
• Hypotension, low perfusion (sensor site)
• Hypothermia
• Vasoconstriction drugs
• Dark skin pigmentation

60
Pulse Oximetry

• Criteria for Acceptability
• Correlation with measured SaO2
• SpO2 and SaO2 should be within 2 from 85-100
• Elevated levels of COHB (gt3) or MetHb (gt5) may
  invalidate SpO2

61
Pulse Oximetry

• Criteria for Acceptability
• Adequate profusion of the sensor site as seen in
  the plethysmographic tracing and correlation with
  patients heart rate

62
Pulse Oximetry

• Criteria for Acceptability
• Know interfering substances or agents should be
  eliminated, e.g., nail polishes, acrylic nails,
  etc.
• Readings should be consistent with the patients
  clinical history and presentation.

63
Oxygen Saturation

• Oxygen saturation is the ratio of either
  oxygenated Hb (Oxyhemoglobin or O2Hb) to total
  available Hb (reduced Hb or rHb O2Hb) or total
  Hb (O2Hb rHb COHb MetHb), depending on the
  instrumentation used to measure and report values

64
Oxygen Saturation

• Co oximeters actually measures SaO2 using
  spectrophotometry
• Ratio of oxyhemoglobin to total Hb
• SaO2 ___ O2Hb
  X 100
• (O2Hb rHb COHb MetHb)
• Pulse oximeters estimate the SaO2 by using a
  noninvasive probe that measures absorption or red
  and near-infrared light
• Ratio of oxyhemoglobin to available Hb
• SpO2 O2Hb__ X 100
• (O2Hb rHb)

65
Oxygen Saturation
Oxyhemoglobin (O2Hb)
Reduced Hb (rHb)
COHb MetHb
1 2 3 4 5 6 7 8 9 10
11 12 13 14
O2Hb
10 13
Pulse Oximeter
76 SpO2
O2Hb rHb
O2Hb
10 14
CO- Oximeter
71 SaO2
O2Hb rHb COHb MetHb
66
Oxygen Saturation

• SaO2 is calculated by some blood gas analyzers
  based on PaO2 and PH
• Convenient but inaccurate!
• SvO2 can be measured by a reflective
  spectrophotometer in the pulmonary artery
  catheter (Swanz-Ganz)

67
Oxygen Saturation

• Normal Values
• SaO2 97
• SvO2 75
• COHb .5 - 2 of Total Hb
• MetHb 1.5 of Total Hb
• Total Hb 14 16 gm (males)
• 13 15 gm (females)

68
Capnography

• Capnography is the continuous, noninvasive
  monitoring or expired CO2 and analysis of the
  single-breath CO2 waveform

69
Capnography

• Capnography
• Allows trending of changes in alveolar and dead
  space ventilation
• End-tidal PCO2 (PetCO2) is reported in mm Hg

70
Capnography
Normal Arterial ETCO2 Values

• ETCO2
• from Capnograph

• Arterial CO2 (PaCO2)

Normal PaCO2 Values
Normal ETCO2 Values
35 - 45 mmHg
30 - 43 mmHg
71
Capnography

• Arterial - End Tidal CO2 Gradient
• In healthy lungs the normal PaCO2 to ETCO2
  gradient is 2-5 mmHg
• In diseased lungs, the gradient will increase due
  to ventilation/perfusion mismatch

72
Capnography

• 
  .
• Normal (ventilation) is 4 L of air per minute
• Normal (perfusion) is 5L of blood per minute.
• So Normal ratio is 4/5 or 0.8
• When the is higher than 0.8, it means
  ventilation exceeds perfusion
• When the is lt 0.8, there is a mismatch
  caused by poor ventilation.

73
Capnography

• Ventilation-Perfusion Relationships
• Relationship between ventilated alveoli and blood
  flow in the pulmonary capillaries

Deadspace Ventilation Alveoli ventilated but not
perfused
74
Normal V/Q
Capnography
.
.
ETCO2 - PaCO2 Gradient 2 to 5 mmHg
75
Shunt Perfusion Low V/Q
Capnography
.
.

• Mucus plugging
• ET tube in right or left
• main stem bronchus
• Atelectasis
• Pneumonia
• Pulmonary edema
• In short anything that causes the alveoli
  tocollapse or alveolar filling

ETCO2 - PaCO2 Gradient 4 to 10 mmHg
No exchange of O2 or CO2
76
Dead Space Ventilation
Capnography
.
.
ETCO2 - PaCO2 Gradient is large
High V/Q
Ventilation is not the problem!
Perfusion is the problem No exchange of O2 or
CO2occurs
77
Dead Space Ventilation
Capnography
ETCO2 33 mmHg
PaCO2 53 mmHg
Alveoli that do not take part in gas exchange
will still have no CO2 Therefore they will
dilute the CO2 from the alveoli that
were perfused
The result is a widened ETCO2 to PaCO2 Gradient
78
Capnography

• Dead Space Ventilation
• Disease processes that may cause Dead Space
  Ventilation
• Pulmonary embolism
• Hypovolemia
• Cardiac arrest
• Shock
• In short, anything that causes a significant
  drop in pulmonary blood flow

79
Normal Capnogram - Phase I
CO2 mmHg
A
B
80
Anatomical Dead Space

• Anatomical Dead Space
• Internal volume of the upper airways
• Nose
• Pharynx
• Trachea
• Bronchi

Anatomical Deadspace Conducting Airway - No Gas
Exchange
81
Normal Capnogram - Phase II
CO2 mmHg
C
B
Mixed CO2, rapid rise in CO2 concentration
82
Normal Capnogram - Phase III
CO2 mmHg
D
C
Time
83
Normal Capnogram - Phase IV
Inspiration starts, CO2 drops off rapidly
CO2 mmHg
D
E
84
Normal Capnogram
Capnography
Stable trend
85
Capnography

• Hyperventilation - Decrease in ETCO2

• Possible Causes
• Increase in respiratory rate
• Increase in tidal volume
• Decrease in metabolic rate
• Fall in body temperature

86
Capnography

• Hypoventilation - Increase in ETCO2

• Possible Causes
• Decrease in respiratory rate
• Decrease in tidal volume
• Increase in metabolic rate
• Rapid rise in body temperature

87
Capnography

• Rebreathing

• Possible Causes
• Expiratory filter that is saturated or clogged,
  expiratory valve that is sticking
• Inadequate inspiratory flow, or insufficient
  expiratory time
• Anything that causes resistance to expired flow

88
Capnography

• Endotracheal Tube in Esophagus

• Possible Causes
• Missed Intubation - when the ET tube is in the
  esophagus, little or no CO2 is present
• NOTE A normal capnogram is the best evidence
  that the ET tube correctly positioned.

89
Case Study

• A 29 year old male with head injury, and a
  compound fracture of his femur sustained in a
  motorcycle accident
• 2 weeks post trauma on mechanical ventilation
  with the following physiological values
• PaCO2 42 mmHg PaO2 95 mmHg
• ETCO2 38 mmHg Total Rate 14 bpm
• Minute Ventilation 7 L/Min

90
Case Study
Normal capnogram, stable trend ETCO2/PaCO2
gradient 4 mmHg
91
Case Study
Sudden decrease in ETCO2 from 38 mmHg to 18 mmHg
and remains there RR increases to 24 bpm
Minute Volume increases to 12 Lpm
92
Case Study
ABG was drawn with the following
results PaCO2 38 mmHg PaO2 59
mmHgPaCO2/ETCO2 gradient 20 mmHg
93
Case Study

• Ventilation /perfusion lung scan was consistent
  with a pulmonary embolism
• A sudden drop in ETCO2, associated with a large
  increase in the PaCO2/ETCO2 gradient, is often
  associated with pulmonary embolism

94
Blood Gases and Related Tests

• Questions?

Thank You!