SECTION IV THE RESPIRATORY SYSTEM Emphysema with Respiratory Failure A 60-yearold 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 nechanical ventilator that has taken over ber 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 comtent b.Arterial oxygen delivery c.Venous oxygen content d.Oxygen uptake e.Venous oxygen delivery 2.Assuming that all other factors renain the sare,explain the result of each of the following changes on her arterial oxygen content and arterial oxygem delfvery. In each instance where the bo,equilibriun curve is altered,state whether the Pe is lower or higher than normal. a.Increase in PaCD:to 50 m Hg b.Increase in body teaperature to 101 F c.Increase in pl to 7.58 d.Increase in Pa0 to 80 m Hg e.Increase in henoglobin to 12 g/dl blood f.Decrease in cardiac output to 4 L/nin 3.Based on the information provided: a.Approximately what percentage of oxygen delivered to the systemic tissue capillaries is taken up and netabolized by the tissues?
SECTION IV 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 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 0 F c. Increase in pH to 7.58 d. Increase in PaO2 to 80 mm Hg e. Increase in hemoglobin to 12 g/dl blood f. Decrease in cardiac output to 4 L/min 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 perceatage of oxygen delivered to the systemic tissue capillaries returns to the right heart? ANSVER 1.a.Arterial blood oxygen content (CaD:)"(Sa0:x Hb x 1.34)+(0.003 x Pa0:) b.Arterial oxygen delivery CaD:x cardiac output c.Venous blood oxygen content (CVO)(SVO x Hb x 1.34)+(0.003 x PVO.) d.Oxygen uptake (Ca0 CVO)x cardiac output* *This important relationship is also known as the Fick Equation. e.Venous oxygen delivery arterial oxygen delivery oxygen uptake 2.a.-b.Both a high Pacoy and a high terperature will shift the Hbo equilibriun curve to the right,thereby causing decreased oxygen uptake in the pulmonary capillaries and a reduced Sat.Because oxygen content is directly related to Sa.oxygen content will fall.Because arterial oxygen delivery is directly related to oxygen content,it too will fall.Note that the higher the PaD.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 Pm of a right-shifted curve is higher than normal. Changes that shift the curve toward the right while causing reductions in pulnonary 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 PO.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 nay be no net reduction in oxygen actually delivered to the tissue cells. c.An increase in pH shifts the Hbo equilibriun curve to the left:the P will be reduced.Blood changes that shift the bo:equilibriun curve toward the left lead
b. Approximately what percentage of oxygen delivered to the systemic tissue capillaries returns to the right heart? ANSWER 1. 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. 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 Sad at the pulmonary capfllary level,although for reasons previously stated,the increase is slight in the flat portion of the curve.In this example, where pH changes fron 7.5 to 7.58,Sa0 increases only froa 92%to 94%.This increase will lead to a modest inerease in arterial 0,content and delivery,which nay be offset by the fact that oxygen will be held more tightly by henoglobin at low PO: values (i.e.,in the systemic capillaries). d.A 20 m lg increase in Pa0:(a 336 increase over baseline)increases Sa0. a small amoumt,approximately 4%Because this patient's baseline pH is 7.5,the left-shifted (solid black line)curve in the figure more closely approxinates her curve than the standard (red line)curve of pll 7.4.As a result of increased Sa02, arterial oxygen content and arterfal oxygen delfvery will increase slightly.A change in Pa02 by itself will not alter the position of the curve.so P will not change. e.Increasing hemoglobin from 9 to 12 g/dl prowides 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 Hbo equilibriun curve, raising a low henoglobin content can have a more profound effect on oxygen content and delivery than increasing Pad 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 HbO:equilihrium curve,and hence the Pa vill be unaffected.(When cardiac output falls,the body usually compensates by increasing the percentage of oxygen extracted at the tissue level. 3 a.The percentage of oxygen taken up is the difference between arterial and venous oxygen contents divided by arterial oxygen content: (Ca0 Cv0:)/Ca0: Oxygen content (HbO saturation x lb content x 1.34)+dissolved oxygen
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 a. The percentage of oxygen taken up is the difference between arterial and venous oxygen contents divided by arterial oxygen content: (CaO2 – 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): C0=20l0/d1 C0=15nl0/d1 In this manner the percentage of oxygen uptake is 5/20 =25%.Note that the content can be stated as ml 0/dl or ml 0/L:either way the percentage of uptake is the sane. Because henoglobin conteat 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.Normlly. 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 (f.e.,about the normal amount). b.The percentage of total arterial oxygen delivery that returms to the right heart is 100 minus the percentage of oxygen uptake,or approximately 75%. These values-25%oxygen uptake,75%oxygem return to the right heart are normal for resting individuals.The percentage of delivered oxygen used by the tissues increases in anenia,exercise.or heart failure
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–25% oxygen uptake, 75% oxygen return to the right heart–are normal for resting individuals. The percentage of delivered oxygen used by the tissues increases in anemia, exercise, or heart failure