Ventilation/Perfusion Relationship

B. The Influence of Gravity on Lung Perfusion How does the pulmonary circulation differ in important respects from the systemic circulation?

It is characterized by low vascular pressures and low flow resistance. The structure of the pulmonary vasculature reflects these characteristics; the arteries are relatively thin-walled and sparsely innervated, and the capillary bed is highly distensible.

What is a second consequence of Va/Q mismatch?

A second consequence of Va/Q mismatch results from the non-linear relationship between O2 content and PO2 in whole blood as defined by the oxyhemoglobin dissociation curve. Even if the volume of blood draining lung units with a high Va/Q ratio were the same as that from units with a low Va/Q ratio, the Po2 of the mixed blood would be unfavorably skewed toward a lower than expected value. For example, if equal volumes of blood from the left lung unit and right lung unit in Fig. 8 were mixed, the O2 content would be averaged to 18 ml/100ml [(16 + 20)/2]. This quantity of O2 in blood corresponds to a partial pressure of 55 mmHg, a value far lower than would result if partial pressures of O2 in the two blood samples were simply (and incorrectly) averaged (to 71.5mm Hg). Despite its high PO2, therefore, blood from lungs units with high Va/Q ratios has insufficient extra O2 content to offset the significantly lower O2 content of blood from units with very low Va/Q ratios.

What is the exchange of O2 and CO2 in the lungs determined by?

A)Ratio between alveolar ventilation (Va) and pulmonary blood flow (Q). B)The rate of O2 consumption and CO2 production. C)The gas tensions in inspired air and in mixed venous blood. D)Chemical processes in the blood.

Final Considerations

As mentioned earlier, in normal individuals ventilation perfusion mismatch accounts for the small 4 mm Hg (PA - Pa) difference. In the diseased lung, nonuniform distribution of ventilation and perfusion is the main cause of arterial hypoxemia and hypercapnia. In an upcoming discussion we will examine whether breathing 100% oxygen will correct an extreme form of Va/Q mismatch, a physiological shunt. While it is inappropriate to weight average PO2 values of mixed blood samples because of the sigmoidal shape of the oxyhemoglobin dissociation curve, the partial pressure of CO2 in combined blood samples can be approximated by weight averaging, because the relationship between CO2 content in blood and PCO2 is fairly linear.

Graphically, explain gradient of perfusion (top to bottom of lung).

Gravity, therefore, produces a gradient of perfusion from the top of the lung to the bottom, just as it causes a gradient of ventilation. However, since the gradient for perfusion is steeper from top to bottom of the lung, than for ventilation, the Va/Q ratio varies depending on the vertical position in the lung. The top of the lung is relatively poorly perfused (with respect to ventilation) (high Va/Q ratio) and the bottom of the lung is relatively poorly ventilated (with respect to perfusion) (low Va/Q ratio) as shown in figure 5.

What is Hypoventilation?(definition) What is it caused by commonly?

Hypoventilation is defined as a reduced or deficient ventilation of the alveoli. If O2 consumption is not correspondingly reduced, hypoventilation results in a reduction in alveolar PO2 and hypoxemia. Causes: Hypoventilation is commonly caused by diseases outside the lungs, such as a neuromuscular disorder; indeed, VERY OFTEN the lungs are NORMAL.

Introduction: What is hypoxemia and what might it result from?

Hypoxemia is low arterial Po2. At sea level, it may result from inadequate ventilation, diffusion impairment, a shunt in which venous blood bypasses the lung (right to left shunt) and finally an inadequate matching of ventilation and blood flow in various parts of the lung. (Will go into great detail about each one).

In patients with pulmonary disease, how are shunts different?

In patients with pulmonary disease shunts may be of considerable magnitude.

How does gravity relate to variation in perfusion in the lung?

Like ventilation, there is a regional variation in perfusion in the lung that is dependent to a large extent on gravity. Mean pulmonary artery pressure is about 15 mm Hg or about 20 cm H20 (S/D: 25/10 mm Hg or 34/13 cm H2O). As blood flows through the pulmonary circulation, frictional forces cause energy to be lost as heat, and pressures continually drop along the arterial tree, the arterioles, capillaries, venules and veins. Pressures are influenced by gravity as well. For each cm we move upward above the level of the heart, the hydrostatic pressure in the blood vessels decreases by 1 cm of H2O. If the apex of the lung were 20 cm above the heart, the hydrostatic pressure would be reduced in all vessels at that level by 20 cm H2O. Thus, 20 cm above the heart, the systolic pressure would be 14 cm H2O and the diastolic pressure would be -7 cm H2O. Since pulmonary vessels are highly distensible, the decreasing blood pressure at upper levels of the lung causes them to be distended less. The reduced radius increases resistance that contributes to a lower flow at the top of the lung than at the bottom.

What does overall gas exchange come from in lungs?

Overall gas exchange reflects the sum of heterogeneous gas exchanges in several hundred million alveoli. In the normal lung, regional differences in Va/Q ratios are responsible for an alveolar-arterial oxygen partial pressure difference (PAo2-Pao2) of about 4mmHg. In certain pulmonary diseases this mismatch can be much more significant.

What are shunts?

Right-to-left shunts, in which arterial blood is diluted with venous blood, are found to a small degree in normal subjects. Natural shunts occur as a result of the return of the bronchial circulation to the left heart via the pulmonary veins and from the return of some coronary venous blood to the left ventricle via the THEBESIAN veins.

Effect of Regional Differences of Va/Q ratios on mixed arterial blood

The effects of ventilation perfusion mismatching on the O2 content an Po2 of arterial blood can be demonstrated if we consider fig. 7 and the model shown in figure 8 below. One consequence of Va/Q mismatch, illustrated in fig 8, is that the lung unit with a high Va/Q ratio is contributing a smalervolume to the mixed arterial blood than the unit with the low Va/Q ratio. This situation occurs in the normal lung where at the apex the O2 content per ml of blood is high, but the volume/min draining the region is low; at the base perfusion is higher, but the O2 content/ml is lower. The result is that the small quantity of oxygen added by units with high Va/Q ratios is inadequate to compensate for the deficit created by the units with the low Va/A ratios.

True or false. In assessing the impact of a shunt is it important to recognize that the final Po2 that results from a mixing of blood volumes with different oxygen partial pressures cannot be calculated by simply weight averaging the respective PO2 values.

True! Because of the non-linearity of the oxy-hemoglobin dissociation curve, the content of oxygen in blood is not proportional to PO2. Calculations require that the content of each of the two streams of blood (venous and oxygenated) be determined first. After averaging O2 content, the PO2 of the blood mixture can then be estimated.

Ventilation-Perfusion Mismatching: True or false. The oxygen partial pressure of arterial blood (PaO2) is slightly less than that in alveoli (PAO2).

True! Not only because of natural right to left shunts but because of mismatching of blood flow and ventilation in various parts of the lung. Normal blood flow to the lung is equal to the cardiac output and is about 5 L/min. Resting alveolar ventilation is roughly 4L/min. The overall Va/Q ration for the lung, therefore, is 0.8. This ratio however is not uniform in the lung. There is non-hemogenicity in the ration of ventilation to perfusion in each alveolus.

True or false. The compliance of the lung decreases at larger volumes.

True. Since the compliance of the lung decreases at larger volumes, we would expect that larger alveoli at the top of the lung would be on a less compliant portion of the pressure-volume curve and would be less distensible than smaller ones at the bottom (Fig. 3). An inspiratory effort, therefore, that decreases the intrapleural pressure everywhere in the lung by a given amount, will cause the greatest volume change in the alveoli with the highest compliance. Thus, for a given change in distending pressure ventilation will be greater at the base of the lung than at the apex.

Explanations for regional variations in ventilation perfusion ratios: A. The influence of Gravity on Ventilation True or false, because of gravitational forces the intrapleural pressures are more negative at the top of the lung than at the bottom in an upright individual.

True. This has been explained by the weight of the lung pulling down from the apex of the thoracic cavity, where it lowers PIP, and by the weight of the lung compressing the base where PIP is increased. The result is a gradient of intrapleural pressures that creates a gradient of alveolar distending pressures (PALV -PIP) from top to bottom. At any given lung volume alveoli are more distended at the top of the lung than at the bottom.

True or false. Shunts can be either intrapulmonary or extrapulmonary.

True. Shunts can be INTRAPULMONARY: where blood perfuses unventilated alveoli (as a result of bronchial blockage or fluid filling the alveoli). or EXTRAPULMONARY: in which some venous blood by- passes the lungs and enters the arterial circuit. (The latter cases include some congenital heart diseases in which there are atrial or ventricular septal defects or a patent ductus arteriosus. Because of normal pressure gradients these are usually L-R shunts, but can become R-L shunts under some circumstances.)

True or false. Hypoxia resulting from sizeable shunts cannot be corrected by breathing pure O2.

True. The reason lies in the fact that hemoglobin carries most of the O2 in blood and is nearly saturated under normal conditions. Breathing 100% oxygen not only adds little additional O2 to hemoglobin, but since the O2 solubility in blood is low, it also adds only a small additional quantity of dissolved gas. Furthermore, breathing 100% oxygen contributes nothing to the shunted venous blood. Thus, though breathing 100% oxygen can significantly raise the partial pressure of O2 in the oxygenated blood, the additional content of O2 is quite small and may not compensate for the oxygen deficit in the venous blood with which it is mixed.

Ventilation Perfusion mismatch by examining figure 2 (attached).

We can understand the impact of ventilation-perfusion mismatch by examining Figure 2 which shows a normal lung unit (A) in which inspired fresh air PIo2 = 150 mmHg, PIco2 = 0 mm Hg) has equilibrated with venous blood (Pvo2 = 40 mm Hg, PVCO2 = 45 mmHg). The alveolar gas pressures (and those at the end of the capillary) following equilibration are PAo2 = 100 mm Hg and PAco2 = 40 mm Hg. Lung unit B has an extremely low ventilation (low Va/Q) resulting from a blocked respiratory tube. Since little or no fresh air enters the alveoli, the alveolar partial pressure of O2 naturally falls and that of CO2 rises; the alveolar gas partial pressures approach those of venous blood. Blood draining this lung unit has been unable to pick up O2 or unload CO2. A very high Va/Q ratio is seen in lung unit C. The significant reduction of perfusion results in alveolar gas pressures that reflect the composition of inspired gas. There is too little blood to deplete inspired air of O2 or to significantly enrich it in CO2. The small volume of blood equilibrating with alveolar gas in unit C will have gas partial pressures reflecting higher than normal levels of O2 and lower than normal levels of CO2. Although Figure 2 illustrates extremes of Va/Q mismatch, it should be clear that blood draining lung units with low Va/Q ratios (B) have gas partial pressures closer to those of venous blood, and those with high Va/Q ratios (C) have gas partial pressures that are closer to those of inspired gas.

What is diffusion impairment (definition). What are some causes of diffusion impairment?

With significant decrease in the diffusing capacity of the lung, equilibration may not occur between the PO2 in pulmonary capillary blood and alveolar gas. Causes:In some diseases the blood-gas barrier is thickened, and diffusion is so slow that equilibration is incomplete. Disease Examples: Examples are diffuse interstitial fibrosis and asbestosis or a build up of fluid in alveoli as a result of infection.