hypoxemia and hypercapnia

Question 1

Hypoxemia vs hypoxia, anoxia

Answer 1

hypoxemia: reduced oxygen tension (partial tension) below that normally experienced by that organism, low pO2,

Hypoxia: when low oxygen levels cause end organ/tissue dysfunction, reduction of oxygen supply

Anoxia- total lack of oxygen at the tissue level which causes end organ dysfunction

measure hypoxemia via pulse ox or ABG
A-alveolar, a-arterial
Torr-mmHG,
FIO2- fraction of inspired O2

Question 2

Classification of hypoxemia

Answer 2

Normal PaO2: 80-100 mmHg

Mild hypoxemia: 60-80, moderate hypoxemia: 40-60, severe hypoxemia <40

Question 3

4 types of hypoxia

Answer 3

Hypoxemic, anemic, stagnant, histotoxic

Question 4

4 causes of hypoxemia

Answer 4

• Decreased partial pressure of oxygen on atmosphere
• Decreased diffusion across the alveolar or capillary membrane
• slow or absent blood flow in the capillaries that affects ability of oxygen to travel into the blood
• metabolic derangements such as acidosis or fever

Question 5

Mechanisms of hypoxemia

Answer 5

alterations in partial pressures
Ventilation/perfusion (V/Q) Mismatch, Increased/decreased
Systemic shunt
Decreased diffusion

Question 6

Daltons law of gases

Answer 6

Daltons law: each of the gases present in a space contribute to the total pressure in proportion to it’s relative abundance

760 mmHg is the atmospheric pressure at sea level
760 mmHg= PO2 + PCO2 + PN2

760 mmHg x .21 (FIO2) ~150 mmHg

Question 7

henry’s law of gases

Answer 7

at a constant temperature, the amount of gas that dissolves in a certain type/amount of liquid is directly proportional to the partial pressure of that gas when in equilibrium with that liquid

Question 8

Oxygen hemoglobin dissociation curve

Answer 8

Left shift, lowers oxygen unloading to tissues, Hb binds more tightly to it: increase pH, decrease 23BPG, decrease temp, things that stabilize protein

Right shift, increases oxygen unloading to tissuew, Hb releases O2 to tissues: decreased pH, increase 23BPG, increase temp, things that denature proteins

Question 9

Hypoxemia: the A-a gradient

The ALveolar- arterial gradient

Answer 9

PAO2-PaO2

Used to help define the mechanism hypoxemia, assesses the integrity of the alveolar-capillary unit,
alveolar oxygen partial pressure: PAO2=FIO2 (Pb-Ph20)-PACO2[FIO2 + (1-FIO2/R0]

Simplified on room air at sea level with 100% water vapor in alveolus becomes
A-a gradient=150-5/4(PaCO2)-PaO2

PACO2=150- .8(PaCO2)

Question 10

Causes of elevated A-a Gradient

Answer 10

Normal in a young nonsmoker is 5-10 mmHg, A-a gradient increases with age

Diffusion defects, V/Q mismatch, systemic shunts (right to left shunt)

Question 11

Hypoxemia: diffusion of Oxygen

Answer 11

Ficks 1st law: flux of gas is related to the concentration present at steady state

gas moves from regions of high concentration to those of low concentration with a magnitude directly proportional to the concentration gradient

Ficks 2nd law: predicts how diffusion causes the concentration to change over time

Question 12

Clinical causes of pulmonary diffusion defects

Answer 12

result from thickening or destruction of the interstitial space, alveolar wall, or capillary

Both acute and chronic conditions apply

Pulmonary edema, interstitial ling disease (Sarcoid, idiopathic, pulmonary fibrosis, ARDS)
Emphysema

Question 13

Hypoxemia V/Q Mismatching

Answer 13

V= ventilation--air that reaches the alveoli
Q= perfusion--blood that reaches capillary

Ideal V/Q ratio is 1, where ventilation and perfusion are perfectly matched

Area of perfusion with low ventilation=Pulmonary shunt
Area of ventilation with low perfusion= dead space

Question 14

Causes of V/Q ratio

Answer 14

Decreased V/Q ratio: ventilation decreased or perfusion increased: Asthma, COPD, pulmonary edema, pleural effusions, mucous plugging, hepatopulmonary syndrome, anatomic shunt

INCREASED V/Q ratio: perfusion decreased or ventilation increased: PE, hyperventilation, dead space

Question 15

Decreased V/Q ratio

Answer 15

Hepatopulmonary syndrome: Normal V, increased Q, due to arteriovenous malformations (AVMs)

Alveoli are perfused but no ventilation occurs: Alveoli are perfused but no ventilation occurs, Normal Q decreased V, Intrapulmonary shunt

Bronchial arteries and coronary arteries return blood to circulation without passing by the alveoli to participate in gas exchance : Normal Q, decreased V, Anatomic shunt, usually accounts for less than 3 % of total circulation

Question 16

Increased V/Q ratio

Answer 16

Occurs when there is normal ventilation but low/no perfusion: Low Q, normal V, PE, physiologic dead space

Increased ventilation with normal perfusion: Increased V normal Q, hyperventilation which is usually not pathologic

Question 17

physiologic dead space

Answer 17

Anatomic Dead space: areas of the respiratory system that don’t participate in gas exchange, mouth pharynx, larynx, trachea, conduting airways, typically 150 ml

Alveolar dead space: not all alveolar units are as efficient in exchanging gas as they should be, in a normal healthy person, alveolar dead space is minimal

Question 18

Hypoxemia: systemic shunts

Answer 18

Causes of systemic shunts:generally consider either intracardiac or extracardiac

Genetic or acquired,

Genetic contidions: Hereditiary hemmorhagic telangiectasias, VonHippel lindaue VHL syndrome

Right to left shunt, pulmonary AVM, systemic AVM (liver, kidney, spleen, GIT, Cerebral spinal cord, iris, spermatic cord, intercostal space

Question 19

Diagnosing systemic shunts

Answer 19

Diagnosed by intra and extracardiac shunt studies,

Intacardaic bubble study, extracardiac shunt study

Based on berggren shunt equation
Qs/Qt= CcO2- CaO2/CcO2-CvO2
CC- end capillary, Ca- arterial, Cv venous

Question 20

Oxygen carrying capacity

Answer 20

Oxygen content of blood, looks at total oxygen carrying ability, based on hemoglobin, dissolved oxygen, and saturation

CaO2= (Hgb x 1.34) x (sPO2) + (PaO2 x .003)

Normal value is 20 gm/dl
Hb carries the greates weight and therefore the greates impact in how much O2 is carried

DaO2= CaO2 x CO