THE RESPIRATORY SYSTEM

Emphysema with Respiratory Failure

A 60-year-old woman is in the intensive care unit for treatment of respiratory tract failure. Her underlying

disease is emphysema, caused by years of smoking. Her trachea is intubated, and the tube is connected to a

mechanical ventilator that has taken over her breathing. The fraction of inspired oxygen is 0.6. The machine-

delivered tidal volume is 700 ml and the rate is 12 breaths/minute. She is afebrile and has normal blood

pressure. The following data are obtained:

1. Write the formula you would use to calculate the following:

a. Arterial oxygen content

b. Arterial oxygen delivery

c. Venous oxygen content

d. Oxygen uptake

e. Venous oxygen delivery

a. Arterial blood oxygen content (CaO2) = (SaO2 x Hb x 1.34) + (0.003 x PaO2)

b. Arterial oxygen delivery = CaO2 x cardiac output

c. Venous blood oxygen content (CVO2) = (SVO2 x Hb x 1.34) + (0.003 x PVO2)

d. Oxygen uptake = (CaO2 - CVO2) x cardiac output*

*This important relationship is also known as the Fick Equation.

e. Venous oxygen delivery = arterial oxygen delivery - oxygen uptake

2. Assuming that all other factors remain the same, explain the result of each of the following changes on

her arterial oxygen content and arterial oxygen delivery. In each instance where the HbO2 equilibrium

curve is altered, state whether the P50 is lower or higher than normal.

a. Increase in PaCO2 to 50 mm Hg

b. Increase in body temperature to 101 0F

c. Increase in pH to 7.58

d. Increase in PaO2 to 80 mm Hg

e. Increase in hemoglobin to 12 g/dl blood

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f. Decrease in cardiac output to 4 L/min

a.-b. Both a high PaCO2 and a high temperature will shift the HbO2 equilibrium curve to the right,

thereby causing decreased oxygen uptake in the pulmonary capillaries and a reduced SaO2. Because

oxygen content is directly related to SaO2, oxygen content will fall. Because arterial oxygen delivery is

directly related to oxygen content, it too will fall. Note that the higher the PaO2, the flatter the curve;

compared to changes in the steep part of the curve, right and left shifts in the flat region affect

pulmonary capillary oxygen uptake relatively slightly. The P50 of a right-shifted curve is higher than

normal.

Changes that shift the curve toward the right while causing reductions in pulmonary capillary oxygen

uptake, arterial oxygen content, and arterial oxygen delivery also cause oxygen to be unloaded more

easily from the venous end of the systemic capillaries, where PO2 is low. Thus, although there may be

less oxygen delivered to the systemic capillaries per minute, it is released to the tissues more easily; as

a result, there may be no net reduction in oxygen actually delivered to the tissue cells.

c. An increase in pH shifts the HbO2 equilibrium curve to the left; the P50 will be reduced. Blood

changes that shift the HbO2 equilibrium curve toward the left lead to a higher SaO2 at the pulmonary

capillary level, although for reasons previously stated, the increase is slight in the flat portion of the

curve. In this example, where pH changes from 7.5 to 7.58, SaO2 increases only from 92% to 94%. This

increase will lead to a modest increase in arterial O2 content and delivery, which may be offset by the

fact that oxygen will be held more tightly by hemoglobin at low PO2 values (i.e., in the systemic

capillaries).

d. A 20 mm Hg increase in PaO2 (a 33% increase over baseline) increases SaO2 a small amount,

approximately 4%. Because this patient's baseline pH is 7.5, the left-shifted (solid black line) curve in

the figure more closely approximates her curve than the standard (red line) curve of pH 7.4. As a result

of increased SaO2, arterial oxygen content and arterial oxygen delivery will increase slightly. A change

in PaO2 by itself will not alter the position of the curve, so P50 will not change.

e. Increasing hemoglobin from 9 to 12 g/dl provides a 33% increase in hemoglobin-bound oxygen

content, and a corresponding increase in arterial oxygen delivery. Clearly, in the region of the flat part

of the HbO2 equilibrium curve, raising a low hemoglobin content can have a more profound effect on

oxygen content and delivery than increasing PaO2.

f. A 20% decrease in cardiac output from 5 to 4 L/min will have no effect on oxygen content (assuming

nothing else changes) but will reduce systemic oxygen delivery by the same degree, 20%. The position

of the HbO2 equilibrium curve, and hence the P50' will be unaffected. (When cardiac output falls, the

body usually compensates by increasing the percentage of oxygen extracted at the tissue level.)

3. Based on the information provided:

a. Approximately what percentage of oxygen delivered to the systemic tissue capillaries is taken up and

metabolized by the tissues?

b. Approximately what percentage of oxygen delivered to the systemic tissue capillaries returns to the

right heart?

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a. The percentage of oxygen taken up is the difference between arterial and venous oxygen contents

divided by arterial oxygen content:

(CaO2 ¨C CvO2)/CaO2

Oxygen content = (HbO2 saturation x Hb content x 1.34) + dissolved oxygen.

If you used a normal hemoglobin content of 15 g/dl blood when you did the calculation, you would find

the following values (approximate):

CaO2 = 20 ml O2/dl

CvO2 = 15 ml O2/dl

In this manner the percentage of oxygen uptake is 5/20 = 25%. Note that the content can be stated

as ml O2/dl or ml O2/L; either way the percentage of uptake is the same.

Because hemoglobin content is the same in arterial and venous blood, and dissolved oxygen is a very

small amount in both arterial and venous blood, the difference in oxygen saturations also reflects the

difference in contents. Normally, arterial oxygen saturation is 98% and mixed venous saturation is 75%.

This difference reflects the fact that approximately 25% of arterial oxygen delivery is taken up by the

tissues.

In this patient the difference in oxygen saturation (92% - 72%) indicates that about 20% of the

delivered oxygen is metabolized by the tissues (i.e., about the normal amount).

b. The percentage of total arterial oxygen delivery that returns to the right heart is 100 minus the

percentage of oxygen uptake, or approximately 75%.

These values¨C25% oxygen uptake, 75% oxygen return to the right heart¨Care normal for resting

individuals. The percentage of delivered oxygen used by the tissues increases in anemia, exercise, or

heart failure.

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