Oxygen Therapy

1
Oxygen Therapy

• Oxygenation
• Oxygen Therapy
• Monitoring Oxygen Therapy
• Arterial blood gases
• Pulse Oximetry
• Oxygen Analyzers
• Hyperbaric Oxygen

2
Oxygen (O2)

• Discovered 1774
• Joseph Priestly
• Dephlogisticated air
• Lavoisier named it oxygen

3
Oxygenation

• Cellular use of O2 to produce energy (ATP)

4
OXYGENATION
inhaled
O2
diffuses out of alveoli into blood
carried by
BLOOD
tissues
Glucose
consumed by cells
to produce
5
3 Stages of Oxygenation

• External respiration
• Oxygen Transport
• Internal respiration

6
External Respiration

• Movement of O2 from atmosphere to pulmonary
  capillary blood

7
External Respiration
inhaled
O2
diffuses out of alveoli into blood
carried by
BLOOD
tissues
Glucose
consumed by cells
to produce
8
External Respiration

• Ventilation - down to terminal bronchioles
  generations 16-19 that are 0.5 mm in diameter
• Diffusion respiratory bronchioles, alveolar
  ducts, alveolar sacs

p 114 Wyka
9
Impairment of external respiration will cause
hypoxemia.

• Hypoxemia - lt normal oxygen tension in
  arterial blood.
• Normal PaO2 80 - 100 torr at sea level on room
  air.

10
Five Commonly Cited Causes of Hypoxemia

• Low PIO2 (PIO2 FIO2(PBAR PH2O)
• Hypoventilation
• V/Q mismatch
• Shunt (anatomical or physiological)
• Diffusion defect (interstitial lung disease)

11
But Only Two Physiologic Causes of Hypoxemia

• alveolar oxygen tension
• pulmonary shunt

12
Alveolar PO2 if

• barometric pressure
• PaCO2
• Mixed venous oxygen content

13
Barometric pressure less than water vapor
tissue fluids will boil or vaporize
Denver 5280 ft
Mt. Everest 29,028 ft
14
Pulmonary Shunt

• True shunt
• Shunt effect

15
Shunt

• Shunt Effect

16
What a shunt really looks like

• Movie

17
Uneven distribution of ventilation causes shunt
effect

• Resistance
• Compliance

18
Raw
19
C
20
Q
V/Q
3.3
VA
.6
Apex

• Base

21
45
105
40
100
PaO2 ?
22
45

• 105

40
100
?
23
Know this

• PaO2
• 60
• 50
• 40
• 30

• SaO2
• 90
• 80
• 70 (75)
• 60

24
2nd Stage of Oxygenation O2 Transport

• Delivery of oxygen to the tissues

25
O2 TRANSPORT
inhaled
O2
diffuses out of alveoli into blood
carried by
BLOOD
tissues
Glucose
consumed by cells
to produce
26
O2 Transport C.O. x O2 content

• O2 content CaO2 O2 combined with Hb plus
• dissolved O2

27
CaO2 SaO2 ( Hb x 1.34)
PaO2(.003)
To calculate O2 content (CaO2)
28
SaO2 Actual Hb bound with O2

• Total Hb

29
Hb x 1.34

• Each gram Hb carries 1.34 ml O2

30
Multiply PaO2 x .003 to find dissolved O2

• .003 solubility coefficient O2

31
Normally each 100 ml blood carries 20.4 ml O2

• 100(15 x 1.34) 20.1
• 100 x .003 .3

Plus
20.40
32
45
105
CaO2 ?
CaO2 ?
Pa O2 ?
15 20 35/2 17.5 17.5/20 87.5 or SaO2 88
which is a Pao2 of 55 mmHg
33
Know this

• PaO2
• 60
• 50
• 40
• 30

• SaO2
• 90
• 80
• 70 (75)
• 60

34
Normal C.O. 5 L/min

• 5000/100 50
• 50 x 20.4 1020 ml of O2 delivered to the
  tissues per minute

35
Normal O2 consumption by tissues at rest

• 250 ml/min
• Tissues use 25 of delivered O2 at rest

36
Most of O2 carried in blood is combined with Hgb

• Without Hgb, C.O. would have to be 83 L/min to
  provide 250 ml O2/min to tissues

37

• heart

38
Factors Affecting O2 Content

• Hb Saturation
• Hbs affinity for O2

39
Hb Saturation

• Depends on PaO2

Plasma
Alveolus
O2
RBC
Hb
40
Oxyhemoglobin Dissociation Curve
SaO2
100
80
Hb acts as a sink for O2 until equilibrium is
attained. In this case, the PaO2 equilibrates at
around 20 torr if one site is taken up by O2.
60
40

• 20

40
60
80
100
20
PaO2
41
Oxyhemoglobin Dissociation Curve
42
Hb Saturation

• Depends on PaO2

Plasma
Alveolus
O2
RBC
Hb
43
Oxyhemoglobin Dissociation Curve
SaO2
100
80
As one site is taken, it becomes harder for O2 to
bind to the remaining sites. This accounts for
the sigmoid shape.
60
40

• 20

20
40
60
80
100
PaO2
44
Hb Saturation

• Depends on PaO2

Plasma
Alveolus
O2
RBC
Hb
45
Oxyhemoglobin Dissociation Curve
SaO2
100
80
The shape provides adequate O2 despite a drop in
pressure along the top flat part. It quickly
unloads O2 along the steep portion without a
large change in pressure.
60
40

• 20

20
40
60
80
100
PaO2
46
Hb Saturation

• Depends on PaO2

Plasma
Alveolus
O2
RBC
Hb
47
Oxyhemoglobin Dissociation Curve
SaO2
100
80
60
The partial pressure of O2 indicates the Hb
Saturation.
40

• 20

40
60
80
100
20
PaO2
48
Oxyhemoglobin Dissociation Curve
SaO2
100
80
60
40

• 20

20
40
60
80
100
PaO2
49
Hbs Affinity for O2

• pH
• PaCO2
• 2,3 DPG (diphosphoglycerate - synthesized in the
  RBC when hypoxia present to facilitate O2
  unloading.)
• Temperature

50
Affinity for O2

• pH
• PaCO2
• 2,3 DPG
• Temperature

51
Decreased Affinity
Hb

• 2

Get Away
52
Decreased Affinity
Hb

• 2

Get Away
53
Decreased Affinity
Hb

• 2

Get Away
54
Oxyhemoglobin Dissociation Curve
SaO2
100
80
60
Right Shift
40

• 20

20
40
60
80
100
PaO2
55
Oxyhemoglobin Dissociation Curve
SaO2
100
80
60
Left Shift
40

• 20

20
40
60
80
100
PaO2
56
Affinity for O2

• pH
• PaCO2
• 2,3 DPG
• Temperature

57
Oxyhemoglobin Dissociation Curve Shifts

• To the right facilitates Hb unloading oxygen to
  the tissues where the CO2 is higher.
• To the left facilitates loading O2 at the lung
  where CO2 is low.
• CO2s effect on O2 loading is the Bohr effect.
• O2s effect on CO2 loading is the Haldane effect.

58
3rd Stage Internal Respiration

• O2 diffuses out of capillaries into tissue cells
• Finally into mitochondria
• Used as terminal electron acceptor in respiratory
  chain

59
Internal Respiration
inhaled
O2
diffuses out of alveoli into blood
carried by
BLOOD
tissues
Glucose
consumed by cells
to produce
60
Allows oxidative phosphorylation to continue

• cytochrome oxidase gives oxygen two electrons
• which then picks up two hydrogens
• to form water

61
Final outcome of oxygenation

• Production of ATP (adenosine triphosphate)
• Water and carbon dioxide as by-products

62
Inadequate oxygen for metabolism (anaerobic
metabolism)

• Lactic acid is by-product

63
Autoregulation of oxygen through tissue still
unknown
64
Summary

• Must consider all three stages when assessing
  patients oxygenation status and need for oxygen
  therapy.
• Must not just look at PaO2.
• Cardiac output and oxygen content determine O2
  transport.
• Even if adequate levels of oxygen reach the
  tissues, cellular respiration may be impaired.