Ventilation-perfusion ratio

In advanced COPD, airflow obstruction, damaged bronchioles and alveoli, and pulmonary vascular abnormalities lead to impaired gas exchange. This results in hypoxemia and eventually hypercapnia. Hypoxemia is initially present only during exercise but occurs at rest as the disease progresses. Inequality in the ventilation/perfusion ratio (VAQ) is the major mechanism behind hypoxemia in COPD. 

Ventilation-perfusion ratio (VA/Q) A comparison of the proportion of lung tissue being ventilated by inhaled air to the rate of oxygenation of pulmonary blood. 

Stewart, W. E., and S. M. Mastenbrook, Parametric estimation of ventilation-perfusion ratio distributions, J. Appl. Physiol Respir., Environ. Exerc. Physiol, 55, 37-51 (1983b) Errata, 56, No. 6 (1984). 

Normal ventilation- perfusion ratio. The function of the lung is to maintain P02 and PC02 within the normal range. This is accomplished by matching 1 ml mixed venous blood with 1 mL fresh air (V/Q = 1). Normally, there is less ventilation (V) than perfusion (Q), and the V/Q ratio is 0.8. 

Biological Samples. Multiple gas equilibration has been used to measure up to 8 foreign inert gases in blood, plasma, and dextrose solutions (11). The method is used to measure distributions of ventilation-perfusion ratios based upon the simultaneous preliminary clearance of several inert gases (28). It is well suited to monitoring anesthetics and trace blood levels of anesthetic agents. 

Distributions of Ventilation-Perfusion Ratios Theory, J. Appl. Physiol. 1974,36, 588-599. 

Vidal Melo MF, Layheld D, Harris RS, O Neill K, Musch G, Richter T, Winkler T, Fischman AJ, Venegas JG. Quantification of regional ventilation-perfusion ratios with PET. J Nucl Med 2003 44(12) 1982-1991. 

Figure 2 Model of the respiratory neuromuscular control system. Potential mechanisms of respiratory failure are listed on the right side and afferent stimuli arising at each site are shown on the left side. Abbreviations Rrs, respiratory system resistance Ers, respiratory system elastance PV, pressure-volume V/Q, ventilation-perfusion ratio Qt, cardiac output PCO2 carbon dioxide tension PO2 oxygen tension. Source From Ref. 127.

In relaxing bronchial smooth muscle, iso-prenaline may impair ventilation-perfusion ratio and deepen hypoxaemia even as it diminishes airway resistance. The patient may feel better but be worse off (2 ). 

Labs Cr 1.2 mg/dL, international normalized ratio (INR) 1.1, activated partial thromboplastin time (aPTT) 35 seconds, hematocrit (Hct) 38%, platelets 247,000/mm. Doppler ultrasound Partial noncompressibility of the left popliteal vein, nonocclusive clot. Ventilation-perfusion scintigraphy High probability for acute pulmonary embolism. Unmatched ventilation perfusion defect is seen in the left lower lobe. 

Although the reversibility of airways obstruction is an important characteristic of asthma, those asthmatics with chronic symptoms do not reverse completely. The chronic airways obstruction is probably related to mucus plugging of bronchi and hypertrophy of the bronchial smooth muscle. These changes are not reversible by bronchodilators, and it is not clear that they can be reversed with any specific therapy. Both types of obstruction contribute to an imbalance of the ventilation to perfusion ratio in the lung and can result in hypoxemia and hypocarbia. The decreased carbon dioxide content of the blood is the result of the asthmatic s hyperventilation and will persist until the respiratory muscles fatigue and hypoventilation becomes prominent. Asthmatics can die quite quickly when this occurs. 

TABLE 7.3 Ventilation-to-Perfusion Ratios from the Top to Bottom of the Lung of Normal Man in the Sitting Position... 

There has been considerable debate about the rationale for the use of systemic and inhaled steroids in the treatment of toxic PE. Theoretically, there should be many advantages such as the inhibition of phospholipase A2 via induced lipomodulin and macrocortin, inhibition of macrophages, inhibition of the production of prostanoids and leukotrienes and stimulation of surfactant production in type II cells. However, beneficial effects only follow early administration while late administration may be deleterious (inhibition of production of type I cells and enhanced fibroblast production). More recently, studies have been published on animal models. A number of studies on pigs have shown variable results including one where inhaled beclomethasone showed improved Pa02, improved ventilation to perfusion ratio and less histological damage. However, species differences exist, and results should be applied with caution to humans. 

Alveolar ventilation supplies O2 to the bloodstream while alveolar capillary perfusion provides alveolar gas with COj. Resting individuals consume approximately 250 mL 02/min and produce approximately 200 ml. COi/min because, stoichiometrically, metabolic processes require a greater supply of O, than the quantity of CO2 produced. Defining the respiratory exchange ratio, R, as... 

Bronchiolar smooth muscle is sensitive to changes in carbon dioxide levels. Excess carbon dioxide causes bronchodilation and reduced carbon dioxide causes bronchoconstriction. Pulmonary vascular smooth muscle is sensitive to changes in oxygen levels excess oxygen causes vasodilation and insufficient oxygen (hypoxia) causes vasoconstriction. The changes in bronchiolar and vascular smooth muscle tone alter the amount of ventilation and perfusion in a lung unit to return the V/Q ratio to one. 

In a lung unit with high blood flow and low ventilation (airway obstruction), the level of carbon dioxide is increased and the level of oxygen is decreased. The excess carbon dioxide causes bronchodilation and an increase in ventilation. The reduced oxygen causes vasoconstriction and a decrease in perfusion. In this way, the V/Q ratio is brought closer to one and gas exchange is improved. 

The V/Q term describes the imbalance between ventilation (V) and perfusion (Q) in different areas of the lung. Given that alveolar ventilation is 4.5 l.min and pulmonary arterial blood flow is 5.0 l.min 1, the overall V/Q ratio is 0.9. Both ventilation and perfusion increase from top to bottom of the lung, but perfusion by much more than ventilation. 

The overall effect of nonuniform ventilation and perfusion is that both decrease as one progresses vertically upward in the upright lung. But perfusion decreases more rapidly so that the dimensionless ratio of ventilation to perfusion, VJQ, decreases upward, and can vary fin>m approximately 0.5 at the lung s bottom to 3 or more at the lung s top. Extremes of this ratio are ventilated regions with no blood flow, called dead space, where VJQ — , and perfused regions with no ventilation, called... 

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