# Blood Gases: O2 & CO2 Flashcards

1
Q

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

A

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.

2
Q

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

A

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.

3
Q

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

A

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

4
Q

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

A

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

5
Q

What is normal Pa02 in the adult breathing room air?

A

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

6
Q

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

A

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.

7
Q

What is the normal Sa02?

A

Normal SaO2 is 90-97%.

8
Q

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

A

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

9
Q

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

Where in the circulation is blood with this PO2 found?

A

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

10
Q

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

A

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

11
Q

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

A

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.

12
Q

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

A

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).

13
Q

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

A

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.

14
Q

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

A

0.003x200= 0.6 ml 02/l00 ml blood.

15
Q

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

A

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.

16
Q

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.

A

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).

17
Q

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

A

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

18
Q

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

A

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

19
Q

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.

A

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.

20
Q

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

A

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).

21
Q

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

A

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.

22
Q

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

A

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

23
Q

Define P50. The normal P50 is how many mmHg?

A

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

24
Q

**What happens to the P50 when the oxyhemoglobin

dissociation curve shifts rightward? Leftward?

A

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

25
Q

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

A

(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.

26
Q

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

A

(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.

27
Q

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?

A

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.

28
Q

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?

A

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.

29
Q

What is the Bohr effect?

A

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.

30
Q

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

A

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.

31
Q

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

A

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

32
Q

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

A

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.

33
Q

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

A

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.

34
Q

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

A

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

35
Q

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

A

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.

36
Q

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

A

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.

37
Q

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?

A

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.

38
Q

What determines mixed venous oxygen content (Mv02)?

A

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.

39
Q

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

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

A

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.

40
Q

Venous blood oxygen saturation provides what information?

A

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

41
Q

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

A

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.

42
Q

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

A

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.

43
Q

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

A

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.

44
Q

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

A

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

45
Q

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).

A

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.

46
Q

Compare the solubilities of 02 and C02 in blood.

A

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.

47
Q

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

A

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.

48
Q

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

A

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.

49
Q

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

A

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

50
Q

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?

A

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.

51
Q

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?

A

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.

52
Q

What is the Haldane effect?

A

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.

53
Q

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

A

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.

54
Q

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?

A

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.

55
Q

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

A

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.

56
Q

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

A

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%.

57
Q

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

A

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.

58
Q

What laboratory value will exclude CO2 retention from a diagnosis?

A

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

59
Q

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

A

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.