Arterial Blood Gases and Improving Gas Exchange

Question 1

PaO2 (PO2) definition

Answer 1

partial pressure of arterial oxygen

amount of O2 in blood (both attached to Hgb and dissolved in plasma)

Question 2

SaO2 definition

Answer 2

amount of O2 bound to Hgb
related to PaO2
if Hgb is low, SaO2 will be fine but PaO2 decreases

Question 3

alveolar ventilation

• represented by…
• normal range
• hypoventilation
• hyperventilation

Answer 3

represented by PCO2
normal range 35-45 mmHg
hypoventilation - retains CO2 (CO2 > 40)
hyperventilation - blow off CO2 (CO2 < 40)

Question 4

ventilation in normal individuals without lung pathology

Answer 4

in hypoventilate, CO2 eventually causes respiratory center to stimulate increased ventilation
if hyperventilate, CO2 signals respiratory center to decrease ventilation

Question 5

normal values

• PO2
• PCO2
• pH

Answer 5

PO2
-80
PCO2
-40
pH
-7.4

Question 6

ranges

• pH
• PCO2
• PO2

Answer 6

pH
-7.35-7.45
PCO2
-35-45 mmHg
PO2
-80-100 mmHg

Question 7

PO2 values for…

• hypoxemia
• mild hypoxemia
• moderate hypoxemia
• severe hypoxemia

Answer 7

<80
60-80
40-60
<40

Question 8

goal of ventilation

Answer 8

maintain appropriate [O2], [CO2], and [H+]
control mechanisms are responsive to concentrations of these molecules
-[CO2] and [H+]
-[O2]

Question 9

central control of ventilation

-respond to…

Answer 9

central chemoreceptors (medulla)
-respond to increased PCO2 and pH of CSF

Question 10

peripheral chemoreceptors

-respond to…

Answer 10

carotid bodies (CN IX)
-respond to decreased PO2 or decreased pH
aortic bodies (CN X)
-respond to decreased PO2 or decreased pH

Question 11

effect of decreased PO2 on respiration

Answer 11

little effect until drops 40% or below 60 mmHg

if drops to 40 mmHg, then respiration increases only 50-60%

Question 12

effect of increased PCO2 on respiration

Answer 12

if increases 10j%
-respiration increases by 2x
-even if PO2 is normal
CO2 regulates respiratory activity under normal conditions
subject to adaptation

Question 13

pulmonary disease effect on ventilation

Answer 13

retention CO2 can occur
-difficult to exhale; get pockets of air retained in alveoli
-ex. emphysema
PCO2 chronically elevated
-system adpats to elevated PCO2
therefore, reduced PO2 acts on peripheral chemoreceptors and provided main stimulus for respiration = hypoxic drive

Question 14

blood gas analysis

• provides info on…
• reflects…

Answer 14

information
-blood O2 levels through gas exchange
-blood CO2 levels
-pH - acid-base balance or imbalance
--noted in extra-cellular fluid
reflects oxygen status and acid-base balance needed to sustain life

Question 15

lethal pH levels

-why?

Answer 15

<6.8
-force of cardiac contraction lessens
>7.8
-impaired neuromuscular function

Question 16

3 processes in body to regulate acid-base

Answer 16

chemical buffers
-substances combining with acids or bases
respiratory system
-regulates CO2 in the blood
-central chemoreceptors note change in pH - regulate rate and depth of breathing
renal system
-slower system
-retain or excrete acids and bases to maintain equilibrium

Question 17

blood gas interpretation

• PCO
• pH
• PO2

Answer 17

look at PCO2 to determine alveolar ventilation
look at pH to see if normal acid-base balance and whether acute or chronic
look at PO2 to determine oxygenation or hypoxemia and to what degree

Question 18

pH values for acidosis vs. alkalosis

Answer 18

acidosis: below 7.4

alkalosis; above 7.4

Question 19

respiratory vs. metabolic acidosis/alkalosis causes

Answer 19

respiratory
-abnormal respiratory function
metabolic
-non-respiratory cause

Question 20

metabolic elevator

Answer 20

check pH
examine HCO3-
if metabolic, HCO3- will go in same direction as pH
low pH and low HCO3- = metabolic acidosis

Question 21

respiratory see-saw

Answer 21

check pH
CO2 will go in opposite direction as pH if respiratory imbalance
respiratory acidosis low pH and high CO2

Question 22

respiratory failure

-define

Answer 22

inability to meet needs of body
-“the arterial oxygen, CO2 or both cannot be kept at normal levels. a drop in the oxygen carried in blood is known as hypoxemia; a rise in arterial CO2 levels is called hypercapnia

Question 23

respiratory failure important levels

Answer 23

pH below 7.3

PCO2 above 50; usually also have hypoxemia

Question 24

respiratory failure

-Tx includes

Answer 24

O2 therapy
positive airway pressure
-respiratory ventilation - airflow to help keep airways open (used with sleep apnea and with critically ill patients (ex. with end-stage COPD)

Question 25

COPD and supplemental O2

Answer 25

in emphysema, CO2 is retained in alveoli, equilibrates and is high in blood (long-term compensation
pH can be in normal range, due to increased bicarbonate
set point for central chemoreceptors is higher than 40 mmHg (limited drive to breathe)
low PO2 (hypoxia) allows for respiratory responses, EXCEPT
-if supplemetal oxygen relieves hypoxia, then neither central or peripheral chemoreceptors are sensitive enough to improve respiration
additional supplemental oxygen can lead to respiratory failure

Question 26

Pt. w/ COPD with PO2 75/PCO2 70/pH 7.37

-what is the problem with adding oxygen?

Answer 26

if you relieve their hypoxia and bring PO2 to 90, then patient has no stimulus to increase respiration