Blood Gases: O2 & CO2

Question 1

How can you estimate PA02 from % inspired 02? Estimate PA02 when % inspired 02 is 50%. 100%.

Answer 1

PA02 can be estimated by multiplying % inspired 02 by 6 (PA02 = % inspired O2 x 6). When FiO2 = 50%, PA02 = 50 x 6 = 300 mmHg. When Fi02 = 100%, PA02 = 100 x 6 = 600 mmHg.

Question 2

How can you estimate Pa02 from % inspired 02? Estimate the Pa02 if the patient is breathing 50% 02. 100% 02.

Answer 2

Pa02 can be estimated by multiplying % inspired 02 by 5 (PaO2 = % inspired 02 x 5). When Fi02 is 50%, PaO2 = 50 x 5 = 250 mmHg. When Fi02 is 100%, Pa02 = 100 x 5 = 500 mmHg.

Question 3

Estimate the PA02-Pa02 gradient if the normal, healthy patient has an FiO2 of 0.4.

Answer 3

PA02-Pa02 = 40 x 6 - 40 x 5 = 240 - 200 = 40 mmHg.

Question 4

What is the maximal Pa02 achievable in a young healthy adult breathing room air?

Answer 4

The maximal Pa02 for a young person breathing room air is 104 mm Hg (Guyton). The best achievable Pa02 is 100-105 mm Hg.

Question 5

What is normal Pa02 in the adult breathing room air?

Answer 5

Normal Pa02 ranges from about 78 mmHg in elderly persons to 105 mmHg in young individuals (Pa02 = 102- Age/3).

Question 6

In a young, normal healthy adult, what is the difference between Pa02 and Pv02: 20,40,60,80, or 100 mm Hg?

Answer 6

The difference between Pa02 and PvO2 is 60 mm Hg. The partial pressure of oxygen in mixed venous blood (PvO2) is 40 mm Hg. Pa02 ranges from 60 mm Hg (lower limit of normal) to about 100 mm Hg. The typical PaO2 shown in textbooks is 95 mm Hg. Hence, the difference between PaO2 and PvO2 is 95 40 = 55 mm Hg, which is close to 60 mm Hg.

Question 7

What is the normal Sa02?

Answer 7

Normal SaO2 is 90-97%.

Question 8

If the oxygen saturation is 90%, what will the P02 be? Where is blood with this P02 found?

Answer 8

When oxygen saturation is 90%, PO2 is 60 mmHg. This is arterial blood.

Question 9

What is the P02 if the 02 saturation of hemoglobin is 70%?

Where in the circulation is blood with this PO2 found?

Answer 9

When oxygen saturation is 70%, PO2 is 40 mmHg. This is mixed venous blood.

Question 10

What is the hemoglobin oxygen saturation when the PO2 is 60 mmHg? When the PO2 is 40 mm Hg?

Answer 10

When the P02 is 60 mm Hg, oxygen saturation is 90%. When the PO2 is 40 mm Hg, oxygen saturation is 70%.

Question 11

What is the normal arteriovenous oxygen content difference (CaO2 - CvO2)?

Answer 11

5 ml O2/100 ml blood. This says that 5 ml O2 are extracted from each 100 ml of blood by tissues as blood flows from the arterial to the venous side of the systemic circulation.

Question 12

What two changes can cause SaO2 to remain normal and Sv02 to decrease?

Answer 12

decrease in SvO2 can occur if there is: (1) a decrease in oxygen delivery (decreased cardiac output, decreased hemoglobin concentration, abnormal hemoglobin) with a resulting increased extraction of O2 from the blood, or (2) an increase in 02 consumption (fever, shivering, malignant hyperthermia, thyroid storm).

Question 13

How can you calculate how much oxygen is dissolved in the blood? What law applies?

Answer 13

Multiply PO2 x 0.003, and your answer will be the amount of oxygen dissolved in blood. The units are ml O2/100 ml blood. Henry’s law permits this calculation to be made.

Question 14

When the Pa02 is 200 mmHg at normal body temperature, how many ml of oxygen will dissolve in 100 ml of blood plasma?

Answer 14

0.003x200= 0.6 ml 02/l00 ml blood.

Question 15

Calculate how much oxygen is dissolved in blood when the patient is breathing 50% 02. When the patient is breathing 100% 02.

Answer 15

PaO2s while breathing 50% O2 and 100% O2 are estimated to be 250 (50 x 5) and 500 (100 x 5) mmHg, respectively. The amounts of O2 dissolved are 0.75 (250 x 0.003) and 1.5 (500 x 0.003) ml O2/I00 ml blood when breathing 50% O2 and 100% O2, respectively.

Question 16

If the PaO2 increases from 100 to 400 mmHg, how much

does the amount of dissolved oxygen increase? Explain how you arrived at this answer.

Answer 16

The amount of dissolved O2 increases 0.9 ml O2/100 ml blood. When PO2 is 100 mmHg, dissolved O2 = 0.003 x 100 mmHg = 0.3 ml O2/IOO ml blood. When PO2 is 400 mmHg, dissolved O2 = 0.003 x 400 mmHg = 1.2 ml O2/l00 ml blood. Therefore, the dissolved oxygen increased by 0.9 ml O2/I00 ml blood (1.2 - 0.3 = 0.9).

Question 17

What two factors determine the amount of oxygen carried by hemoglobin?

Answer 17

PO2 and the amount of hemoglobin are the two factors that determine the amount of oxygen carried by hemoglobin.

Question 18

How much 02 is carried by each gram of hemoglobin when saturated?

Answer 18

1.34 ml of O2 is carried by each gram of saturated hemoglobin.

Question 19

What is the maximum oxygen carrying capacity (100% saturation) of blood in a patient with 15 gm Hgb/100 ml blood? Hint: You need to calculate both hemoglobin- bound O2 and dissolved O2. Assume Pa02 = 100 mm Hg.

Answer 19

Hemoglobin (Hgb)-bound 02 = (1.34 ml 02/gm Hgb) (15 gm Hgb/100 ml blood) = 20.1 ml O2/100 ml blood. Dissolved O2 = (100 mmHg) (.003) = 0.3 ml O2/100 ml blood. Total O2 = Hgb-bound O2 + dissolved O2 = 20.1 + 0.3 = 20.4 ml O2/100 ml blood.

Question 20

What is the significance of the flat portion of the oxyhemoglobin dissociation curve?

Answer 20

The flat portion of the oxyhemoglobin dissociation curve facilitates the loading of oxygen by the blood because, in the flat portion of this curve, large changes in the partial pressure of oxygen in arterial blood (Pa02) produce only small changes in oxygen saturation (SaO2).

Question 21

What is the significance of the steep portion of the oxyhemoglobin dissociation curve?

Answer 21

The steep portion of the oxyhemoglobin dissociation curve facilitates unloading of oxygen at tissues because large amounts of oxygen are unloaded from hemoglobin (large decrease in oxygen saturation) in response to a small change in the partial pressure of oxygen.

Question 22

Below what Pa02 are there substantial reductions in arterial blood O2 saturation (SaO2) for a small decrease in PaO2?

Answer 22

When Pa02 falls below 60 mmHg, large reductions in Sa02 occur with small decreases in PaO2.

Question 23

Define P50. The normal P50 is how many mmHg?

Answer 23

The P50 is the 02 partial pressure at which hemoglobin (Hgb) is 50% saturated. The normal P50 = 26-27 mmHg.

Question 24

**What happens to the P50 when the oxyhemoglobin

dissociation curve shifts rightward? Leftward?

Answer 24

The P50 increases when the oxyhemoglobin dissociation curve shifts to the right and decreases when the oxyhemoglobin dissociation curve shifts to the left.

Question 25

List five conditions that cause the oxyhemoglobin dissociation curve to shift rightward.

Answer 25

(1) Increased temperature, (2) increased H+ concentration (decreased pH), (3) increased partial pressure of CO2, (4) increased 2,3-DPG, and (5) sickle cell disease.

Question 26

List seven conditions that shift the oxyhemoglobin dissociation curve to the left.

Answer 26

(1) Decreased temperature, (2) decreased H+ concentration (increased pH), (3) decreased partial pressure of C02, (4) decreased 2,3-DPG, (5) presence of fetal hemoglobin, (6) presence of carboxyhemoglobin, and (7) presence of methemoglobin.

Question 27

With an increase in C02, does the oxyhemoglobin
dissociation curve shift to the right or to the left? Where in the circulation does this shift normally occur? Why is this important?

Answer 27

The oxyhemoglobin curve shifts to the right when PCO2 increases; this rightward shift, which occurs as blood flows through capillaries of the tissues, is important because more 02 is released from Hgb; unloading of 02 is favored with a rightward shift in the oxyhemoglobin dissociation curve, so the tissues get more 02.

Question 28

With a decrease in C02, does the oxyhemoglobin
dissociation curve shift to the left or to the right? Where in the circulation does this shift normally occur? Why is it important?

Answer 28

The oxyhemoglobin curve shifts to the left when PC02 decreases; this leftward shift occurs in pulmonary capillaries as C02 is blown off; 02 loading by hemoglobin is favored in pulmonary capillaries when the oxyhemoglobin dissociation curve shifts left.

Question 29

What is the Bohr effect?

Answer 29

The shift in the oxyhemoglobin dissociation curve caused by carbon dioxide entering or leaving the blood is the Bohr effect. As you know, the increase in PC02 in systemic capillaries is partly responsible for shifting the oxyhemoglobin dissociation curve rightward, which facilitates the unloading of oxygen from hemoglobin. The decrease in PC02 in pulmonary capillaries, on the other hand, helps shift the oxyhemoglobin curve to the left, which facilitates the loading of oxygen onto hemoglobin.

Question 30

Iron is in what state in methemoglobinemia? What is the significance of this?

Answer 30

Normal hemoglobin (Hgb) has iron in the ferrous (Fe2+) state. Oxygen carriage by normal Hgb is excellent. Met- Hgb has iron in the ferric (Fe3+) state. The oxygen carrying capacity in patients with methemoglobinemia is poor.

Question 31

How many grams of Hgb must be in reduced form to produce cyanosis?

Answer 31

5 grams of Hgb per 100 ml blood must be in reduced form (without 02) for cyanosis to develop.

Question 32

Which patient will most easily become cyanotic, the anemic or the polycythemic? Why?

Answer 32

The polycythemic patient will most easily become cyanotic. Cyanosis develops when there is 5 g/100 ml of reduced Hgb. The polycythemic patient, not the anemic patient, is most likely to have this much reduced Hgb.

Question 33

Will cyanosis occur if total Hgb is below 8-9 gm/100 ml blood? Why or why not?

Answer 33

The anemic patient is not likely to become cyanotic. With low levels of Hgb, it is difficult to reduce enough Hgb to turn the patient cyanotic. Note that it is much easier for the polycythemic patient to become cyanotic.

Question 34

In which direction might inhalational agents or IV general anesthetics shift the oxyhemoglobin dissociation curve? Why?

Answer 34

Administration of inhalational or IV anesthetics may cause the oxyhemoglobin dissociation curve to shift to the right because respiratory depression permits PaCO2 to increase.

Question 35

What is the total quantity of 02 delivered to, and used by, the tissues each minute?

Answer 35

250 ml/min of O2 is normally delivered to, and used by, the tissues. This is 3-4 ml O2/kg/min. Note: Oxygen delivery matches oxygen consumption.

Question 36

What is normal 02 consumption in ml 02/min? In ml 02/kg/min? In ml/100g/min?

Answer 36

Oxygen consumption is normally 250 ml 02/min. This is a value you should have memorized. To get your answer in ml 02/kg/min, assume the average person weighs 70 kg, and divide 250 ml 02/min by 70 kg as follows: [250ml O2/min]/[70 kg] = 250/70 ml 02/kg/min = 3.57 ml O2/kg/min. To get the answer in ml O2/100g/min, convert kg to 1000 g; 3.57 ml 02/kg/min = 3.57 ml 02/1000 g/min = 0.357 ml O2/100g/min. Note: You may expect to be asked to convert a normal value to a per kg or per 100 g basis.

Question 37

During monitoring of mixed venous blood oxygen saturation of hemoglobin, readings drop from 74% to 40%. What is the most likely cause? The second most likely cause?

Answer 37

The most probable cause of a decrease in Sv02 is a decrease in cardiac output. An increase in oxygen consumption could also be a likely cause.

Question 38

What determines mixed venous oxygen content (Mv02)?

Answer 38

Mixed venous oxygen content (Mv02) is determined by: (1) oxygen delivery to the tissues (cardiac output, hemoglobin concentration, abnormal hemoglobin), and (2) oxygen consumption (malignant hyperthermia, thyroid storm, fever, shivering). For example, a decreased cardiac output, or anemia, or increased oxygen consumption will decrease MvO2. Note: The question could ask, “What determines mixed venous oxygen saturation (Sv02)”, and the answer would be the same.

Question 39

**What is the best assessment of the adequacy of cardiac

output (i.e., not absolute cardiac output)?

Answer 39

The mixed venous oxygen tension (or saturation) is the best measurement for determining adequacy of cardiac output. A decrease in mixed venous oxygen saturation in response to increased demand usually reflects inadequate tissue perfusion. Thus, in the absence of hypoxia or severe anemia, the mixed venous oxygen tension (or saturation) is the best measurement for detecting adequacy of cardiac output.

Question 40

Venous blood oxygen saturation provides what information?

Answer 40

Venous blood oxygen saturation monitoring provides information on the relationship between oxygen delivery and oxygen consumption.

Question 41

Why can venous blood oxygen saturation appear to increase in a patient with carbon monoxide toxicity?

Answer 41

Venous blood oxygen saturation monitoring uses fiberoptic technology similar to pulse oximetry. Carboxyhemoglobin absorbs red and infrared light in a fashion similar to oxyhemoglobin. When carboxyhemoglobin is present, oxygen saturation readings will be falsely high.

Question 42

What is responsible for continuous oxygenation of the blood during a brief period of apnea, or between breaths?

Answer 42

The functional residual capacity (FRC) serves as a reservoir for 02. The 02 in the FRC diffuses into the blood during a brief period of apnea.

Question 43

**What is the carbon dioxide content (in vol %) in room air? What is the partial pressure of C02 in room air (assume standard pressure)?

Answer 43

The carbon dioxide content of room air is 0.03%. According to Daltons law, the partial pressure of C02 in room air is 0.23 mm Hg.

Question 44

How can you calculate the amount of C02 dissolved in solution?

Answer 44

Multiply PC02 by 0.067 and the result is the ml of C02 dissolved in each 100 ml of solution.

Question 45

Calculate the quantity of C02 dissolved in arterial blood (PaC02 = 40 mm Hg). Calculate the quantity of CO2 dissolved in venous blood (PvC02 = 46 mm Hg).

Answer 45

The amount of C02 that dissolves in solution is 0.067 x PC02. 2.68 ml C02 (0.067 x 40) dissolves in each 100 ml arterial blood, and 3.08 ml C02 (0.067 x 46) dissolves in each 100 ml of venous blood.

Question 46

Compare the solubilities of 02 and C02 in blood.

Answer 46

C02 is approximately 20 times more soluble than 02. The solubility coefficients for C02 and 02 are 0.067 ml CO2/100 ml blood/mmHg and 0.003 ml O2/100 ml blood/mmHg, respectively.

Question 47

How much carbon dioxide normally is produced and eliminated per minute? How much C02 is produced and eliminated in ml/kg/min?

Answer 47

Carbon dioxide is produced and eliminated at a rate of 200 ml/min or 2.4 - 3.2 ml/kg/min. Note: (200 ml/min)/70 kg = 2.9 ml/kg/min.

Question 48

How many ml of C02 is expired from the lungs per 100 ml blood?

Answer 48

Normally, C02 excretion is 200 ml/min. Since cardiac output is 5 liters/min, or 5,000 ml/min, the C02 excretion per 100 ml blood is 200 ml C02/5000 ml blood = 4 ml CO2/100 ml blood.

Question 49

What is the rate of CO2 (in ml/CO2/min) accumulation in a person at rest holding his/her breath?

Answer 49

CO2 initially accumulates at a rate of 200 ml/min.

Question 50

When PO2 decreases, does the blood CO2 dissociation curve shift to the left or the right? Where in the circulation does this shift occur? Why is this important?

Answer 50

When P02 decreases, the CO2 dissociation curve shifts to the left, so more C02 is carried by the blood. This leftward shift occurs as oxygen diffuses out of capillaries of systemic tissues. This left shift of the CO2 dissociation curve facilitates the loading of CO2 into the blood.

Question 51

When P02 increases in the blood, does the blood CO2
dissociation curve shift to the right or the left? Where in the circulation does this shift occur? Why is this important?

Answer 51

When P02 increases, the C02 dissociation curve shifts to the right, so less C02 is carried by the blood. This rightward shift occurs as blood flows through pulmonary capillaries in the lungs. This right shift of the C02 dissociation curve is important because it facilitates the unloading of CO2 from the pulmonary capillaries.

Question 52

What is the Haldane effect?

Answer 52

The Haldane effect describes how changes in P02 in the blood alter the amount of carbon dioxide carried by the blood. In the lungs, the increase in P02 in the pulmonary capillaries causes the carbon dioxide blood dissociation curve to shift to the right, which facilitates the unloading of carbon dioxide by the blood. In systemic capillaries, the decrease in P02 causes the carbon dioxide blood dissociation curve to shift leftward, which facilitates the loading of carbon dioxide by the blood.

Question 53

What is the total CO2 content of arterial blood? Venous blood?

Answer 53

The arterial blood C02 content is approximately 48 ml CO2/100 ml blood. The venous blood C02 content is approximately 52 ml CO2/100 ml blood.

Question 54

What is the normal venous-arterial carbon dioxide content difference (CvC02-CaC02)? How many ml of CO2 are eliminated from each 100 ml of blood?

Answer 54

The normal venous-arterial carbon dioxide content difference (CvCO2-CaCO2) is 4 ml CO2/l00 ml blood. Hence, 4 ml CO2 are eliminated from each 100 ml of venous blood.

Question 55

Compare the amounts of O2 and C02 carried in arterial blood.

Answer 55

Approximately 20 ml 02 are carried in each 100 ml of arterial blood. The amount of C02 carried in each 100 ml of arterial blood of 48 ml C02 is almost 2 1/2 times greater than the amount of 02 carried.

Question 56

What are the four ways CO2 is transported in the blood? What percent does each exist?

Answer 56

Carbon dioxide is carried: (1) physically dissolved in solution, 5%; (2) as carbonic acid (H2CO3), <1%; (3) as bicarbonate ion (HCO3-), 90%; (4) protein bound (plasma proteins and hemoglobin), 5%.

Question 57

What is the role of carbonic anhydrase in the red blood cell?

Answer 57

Carbonic anhydrase is an enzyme in red blood cells that accelerates the conversion of H2O and CO2 to carbonic acid (H2CO3) and then to bicarbonate ions. Carbonic anhydrase is responsible for converting CO2 to bicarbonate ions.

Question 58

What laboratory value will exclude CO2 retention from a diagnosis?

Answer 58

Normal bicarbonate (HCO3) values will rule out CO2 retention. For every 10 mmHg increase in PCO2, serum [HCO3] will increase by 1 mmol/L.

Question 59

After it is formed, bicarbonate moves out of the red cell into
the plasma in exchange for what? What is this called?

Answer 59

Bicarbonate diffuses out of red blood cells in exchange for chloride ions. This is the chloride shift. It is also known as the Hamburger shift.