12-4

Answers to Workbook Questions

Beachey: Respiratory Care Anatomy and Physiology, 3rd Edition

Chapter 12: Ventilation-Perfusion Relationships

Answers to Workbook Questions

Key Terms and Definitions

1. VA/Qc ratioMeasurement used to assess the efficiency and adequacy of the matching of alveolar ventilation and pulmonary perfusion. It is defined as the ratio of the amount of air reaching the alveoli to the amount of blood reaching the alveoli.

2. V/Q mismatchingDefect that occurs in the lungs where ventilation (the exchange of air between the lungs and the environment) and perfusion (the passage of blood through the lungs) are not evenly matched.

3. Absolute shuntAn extreme condition where there is blood flow (perfusion) but no alveolar ventilation: VA/Qc = 0.

4. Absolute dead spaceAn extreme condition where there is ventilation but no perfusion: VA/Qc = .

5. Relative dead spaceA condition where blood flow is present but is abnormally low with respect to ventilation, implying a blood flow deficiency: high VA/Qc.

6. Relative shuntA condition where ventilation is present but is abnormally low with respect to blood flow, implying a ventilatory deficiency: low VA/Qc.

7. Venous admixtureThe result of mixing of shunted non-reoxygenated blood with reoxygenated blood distal to the alveoli. The shunted blood is a result of (1) anatomic shunts and (2) shunt-like effects.

8. A-a gradient, P(A-a)o2; The best-known index of oxygen-transfer efficiency; used to assess the degree of shunting and V/Q mismatch.

9. a-A ratio, Pa o 2/PA o 2 Index representing the percentage of the alveolar oxygen that is transferred to the arterial blood.

10. Oxygen ratio, Pao2/Fi o2 ratio; aAnother measure of oxygen efficiency, used for its ease of calculation; however, it is not a good indicator of shunt.

Matching

1. B

2. A

3. C

4.

P Atelectasis

P Pulmonary edema

P Pneumonia

A Ventricular septal defect

P Bronchial occlusion

P Adult respiratory distress syndrome (ARDS)

PAirway mucus plug

P Pneumothorax

A Bronchial venous admixture

Short Answer/Critical Thinking Questions

1.

A. The reciprocal relationship between Paco2 and PaO2 (high Pac and low Pao2) indicates that this patients hypoxemia is due to hypoventilation.

B. PA o2 = 0.21(713) 63 1.2 = 74.13

P(A-a) o2 = 74 59 = 15 mm Hg (normal = 114 mm Hg)

C. No, calculation of P(A-a) o2 (as shown above) is normal, so there is not an oxygen transfer problem. The basic defect is a ventilation problem due to drug overdose. The patient will most likely require mechanical ventilation until the drug used for the overdose has been eliminated from the body and the patient is able to breathe (ventilate) effectively on her own.

2.

A. Overall hypoventilation can be ruled out because Pco2 is below normal. Therefore, shunt or V/Q mismatch is the problem.

B. The PaO2 rose by 30 mm Hg, which is a good response to oxygen therapy; therefore, V/Q mismatching must be the predominant problem.

3.

A. Because oxygen cannot reach the collapsed alveoli and blood from ventilated units is near maximum saturation.

B. Treatment is aimed at restoring ventilation to the airless alveoli by the application of positive end-expiratory pressure (PEEP) or continuous positive airway pressure (CPAP).

4. Because of the shape of the oxyhemoglobin dissociation curve. A Pao2 of 60 mm Hg is on the flat portion of the oxyhemoglobin curve, meaning that hemoglobin is almost completely saturated with oxygen. Therefore, increasing ventilation will not have an effect on PaO2s of poorly ventilated alveoli.

5.

A.

When Paco2 begins to rise as a result of alveolar dead space, medullary chemoreceptors stimulate an increased in minute ventilation, returning Paco2 to normal. This results in an increased work of breathing to maintain a normal Paco2. An increased work of breathing along with minute ventilation that is disproportionate to Paco2 are the major characteristics of alveolar dead space.

B.

The Pco2 of mixed alveolar gas is decreased because it has no blood flow has a PAco2 of 0 because it has not blood flow.

6.

A. Pao2 /PAo2

B. Pao2 /PAo2

C. Pao2 /Fio2

7. A decreased cardiac output allows the blood to stay in contact with the tissues for a longer period of time, allowing them to extract more oxygen from the blood, which reduces Pao2. This decreased Pao2 will cause all three shunt indicators to show that an oxygen transfer problem exists even when oxygen transfer is normal.

8. PAo2 = FiO2(Pb 47) Paco2 Cao2 = (Hb 1.34 Sao2) + (Pao2 0.003)

= (0.80 700) 35 = (15 1.34 0.985) + (100 0.003)

= 525 = 19.80 + 0.3

= 19.83

Cco2 = (Hb 1.34 Sco2) + (PAo2 0.003)Cvo2 = (Hb 1.34 Svo2) + (Pvo2 0.003)

= (15 1.34 1) + (525 0.003) = (15 1.34 .60) + (30 0.003)

= 20.1 + 1.56 = 12.06 + .09

= 21.68 = 12.15

Qs = Cco2 Cao2= 21.68 19.83 = 1.85 = 0.19 or 19% shunt (so 19% of the cardiac

QT Cco2 Cvo2 21.68 12.15 9.53 output perfuses nonventilated alveoli)

Case Studies

A.

Patient A:

Cao2 = (Hb 1.34 Sao2) + (Pao2 0.003)

= (15 1.34 .90) + (60 0.003)

= 18.09 + 0.18

= 18.27

Patient B:

Cao2 = (Hb 1.34 Sao2) + (Pao2 0.003)

= (10 1.34 0.97) + (90 0.003)

= 12.998 + 0.27

= 13.27

Although both have decreased Cao2 (normal = 20 mL/dL), patient A has better oxygenation because of the higher Hb. Therefore, Pao2 and Sao2 alone will not provide an adequate assessment of a patients oxygenation.

Key Concept Questions

1. A. Mixed venous Pco2 comes into contact with alveolar Pco2, which will decrease alveolar Pco2 as diffusion and equilibrium occur between blood and alveolar gas pressures.

2. D. Arterial blood is a mixture of end-capillary blood from many different VA/Qc regions. Since the lung bases receive more blood flow than the apices, Pao2 can be determined only by using the average of all end-capillary O2 contents.

3. B. Hypoxemia is not an inherent feature of dead-space disease unless the ability to increase overall ventilation is impaired (i.e., COPD)

4. C. Unlike shunt, V/Q mismatch responds well to oxygen therapy because oxygen can enter underventilated units and raise Po2s to near-maximum values.

Copyright 2013, 2007, 1998 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

Copyright 2013, 2007, 1998 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.