# Lecture 4: Ventilation-Perfusion Relationships ## Unit 1.8: Respiratory System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Define ventilation-perfusion ratio and its importance in gas exchange 2. Describe the regional distribution of ventilation and perfusion 3. Explain the V/Q ratio at different lung zones 4. Describe the effects of V/Q mismatch on gas exchange 5. Explain compensatory mechanisms for V/Q mismatch 6. Apply V/Q concepts to clinical scenarios --- ## Lecture Content ### I. Ventilation-Perfusion Matching The efficiency of pulmonary gas exchange depends not only on adequate total ventilation and perfusion but critically on the matching of these two processes at the alveolar level. Oxygen transfer requires that blood flows past alveoli that receive fresh air, while carbon dioxide elimination requires that expired gas comes from alveoli that received blood. When ventilation and perfusion are mismatched, gas exchange efficiency decreases and hypoxemia may result. The ventilation-perfusion ratio, abbreviated V/Q, quantifies this matching for any lung unit. It represents the ratio of alveolar ventilation to pulmonary blood flow. For the entire lung, normal alveolar ventilation approximates 4 liters per minute while pulmonary blood flow equals cardiac output at 5 liters per minute. The resulting ideal V/Q ratio for the whole lung equals 0.8. Understanding the extreme values of V/Q ratio illuminates the concept. A V/Q ratio of zero occurs when blood flows past completely unventilated alveoli, representing a shunt. Blood leaves these units with gas tensions identical to mixed venous blood: oxygen partial pressure around 40 mmHg and carbon dioxide partial pressure around 46 mmHg. No gas exchange occurs because no fresh air reaches the alveolus. A V/Q ratio of infinity occurs when ventilation reaches alveoli with no blood flow, representing dead space. Gas tensions in these units approach those of inspired air: oxygen partial pressure around 150 mmHg and carbon dioxide partial pressure approaching zero. Although ventilation is adequate, no blood flows past to participate in gas exchange. The ideal V/Q ratio of approximately 0.8 produces alveolar gas tensions of approximately 100 mmHg for oxygen and 40 mmHg for carbon dioxide. At this ratio, ventilation supplies oxygen and removes carbon dioxide at rates perfectly matched to metabolic demands carried by the blood. Panel A: Horizontal V/Q ratio axis from 0 to infinity with color gradient transitioning from blue through purple to red indicating oxygenation state. Panel B: Shunt alveolus at V/Q=0 with blood vessels but no airway connection showing mixed venous gas values PO2=40 and PCO2=46. Panel C: Normal alveolus at V/Q=0.8 with balanced ventilation and perfusion showing ideal gas values PO2=100 and PCO2=40. Panel D: Dead space alveolus at V/Q=infinity with airway but no blood vessels showing inspired air values PO2=150 and PCO2=0. --- ### II. Regional Distribution of Ventilation Ventilation distributes unevenly throughout the lung, with gravity playing a dominant role in determining regional ventilation in upright posture. Understanding this distribution explains why different lung regions contribute differently to gas exchange. In the upright position, the lung bases receive substantially more ventilation per unit volume than the apices. This distribution reverses with body position: in the supine position, the posterior (dependent) lung regions receive more ventilation than anterior regions. These patterns reflect the effect of gravity on lung mechanics. The mechanism involves intrapleural pressure and the compliance curve of the lung. Gravity causes the weight of lung tissue to create a gradient in intrapleural pressure from apex to base. At the apex, intrapleural pressure is more negative (approximately minus 10 centimeters of water) because less lung weight compresses the pleural space. At the base, intrapleural pressure is less negative (approximately minus 2.5 centimeters of water) because the overlying lung tissue compresses the dependent pleural space. This pressure gradient affects alveolar size at rest. Apical alveoli, exposed to more negative intrapleural pressure, begin inspiration already relatively inflated. Basal alveoli, exposed to less negative pressure, are smaller at rest. The critical consequence arises from the shape of the compliance curve: basal alveoli, starting at a lower volume, rest on the steeper portion of the pressure-volume curve. A given change in intrapleural pressure produces a larger volume change in these dependent alveoli. Apical alveoli, already near the flat upper portion of the compliance curve, change volume less for the same pressure change. Quantitatively, the base receives approximately three times more ventilation per unit volume than the apex. This difference, though seemingly disadvantageous for uniform gas exchange, matches a similar though more pronounced gradient in perfusion, maintaining reasonable V/Q matching in healthy lungs. Panel A: Upright lung diagram with intrapleural pressure gradient from apex (-10 cmH2O) to base (-2.5 cmH2O) with arrows showing lung weight effect. Panel B: Compliance curve plotting lung volume versus transpulmonary pressure with apical alveoli on flat upper portion and basal alveoli on steeper middle portion. Panel C: Graph of ventilation per unit volume from apex to base showing values of 0.07 at apex and 0.13 at base. Panel D: Anatomical cross-section comparing alveolar sizes with larger inflated alveoli at apex and smaller less-inflated alveoli at base. --- ### III. Regional Distribution of Perfusion Pulmonary perfusion also distributes unevenly in the upright lung, with gravity creating an even more pronounced gradient than for ventilation. This distribution follows from the low-pressure characteristics of the pulmonary circulation and the effect of alveolar pressure on capillary blood flow. In the upright position, the lung bases receive dramatically more perfusion than the apices, with the base receiving approximately ten times more blood flow per unit volume than the apex. In the supine position, this gradient from base to apex diminishes, replaced by a smaller gradient from posterior to anterior regions. The West zones model explains this distribution by describing three zones based on the relative magnitudes of alveolar pressure, pulmonary arterial pressure, and pulmonary venous pressure. Zone 1 occurs where alveolar pressure exceeds pulmonary arterial pressure. When alveolar pressure is greater than arterial pressure, capillaries collapse and no blood flows. In healthy individuals at rest, Zone 1 essentially does not exist because pulmonary arterial pressure, though low, still exceeds alveolar pressure throughout the lung. However, Zone 1 conditions may develop with positive pressure ventilation, hemorrhage causing low pulmonary arterial pressure, or at very high altitudes. Zone 2 occurs in the mid-lung where pulmonary arterial pressure exceeds alveolar pressure, but alveolar pressure exceeds pulmonary venous pressure. In this region, blood flow depends on the difference between arterial and alveolar pressure rather than the arteriovenous gradient. The downstream pressure is effectively alveolar pressure because capillaries collapse at the venous end where venous pressure falls below alveolar pressure. This creates a waterfall effect where flow is independent of downstream venous pressure. Zone 3 occurs in the dependent portions of the lung where both arterial and venous pressures exceed alveolar pressure. Capillaries remain patent throughout, and blood flow depends on the conventional arteriovenous pressure gradient. Most blood flow occurs in Zone 3, where distended capillaries offer the lowest resistance. Panel A: Upright lung divided into three zones with pressure relationships labeled as Zone 1 apex (PA>Pa>Pv), Zone 2 mid-lung (Pa>PA>Pv), and Zone 3 base (Pa>Pv>PA). Panel B: Magnified capillary cross-sections showing collapsed capillaries in Zone 1, waterfall effect in Zone 2, and distended capillaries in Zone 3. Panel C: Perfusion graph showing exponential increase from apex (0.03) to base (0.26). Panel D: Pressure legend defining PA as alveolar, Pa as pulmonary arterial, and Pv as pulmonary venous pressure. --- ### IV. V/Q Ratio Distribution The uneven distributions of ventilation and perfusion combine to produce regional variation in V/Q ratios throughout the lung. Because perfusion varies more with gravity than ventilation, V/Q ratios differ substantially from apex to base, with important implications for gas exchange and disease localization. At the lung apex, both ventilation and perfusion are reduced compared to the base, but perfusion is reduced proportionally much more. The apical V/Q ratio therefore exceeds unity, reaching approximately 3.3 in an upright individual. This represents relative dead space: ventilation exceeds what perfusion can utilize. At the lung base, both ventilation and perfusion increase, but perfusion increases more than ventilation. The basal V/Q ratio falls below unity, reaching approximately 0.6. This represents relative shunt: perfusion exceeds what ventilation can oxygenate. These regional V/Q differences produce regional variation in gas tensions. High V/Q regions at the apex, approaching dead space conditions, have alveolar gas tensions trending toward inspired air: oxygen partial pressure rises to approximately 130 mmHg while carbon dioxide partial pressure falls to approximately 28 mmHg. Low V/Q regions at the base, approaching shunt conditions, have alveolar gas tensions trending toward mixed venous blood: oxygen partial pressure falls to approximately 89 mmHg while carbon dioxide partial pressure rises to approximately 42 mmHg. The blood leaving the lungs represents a flow-weighted average of contributions from all regions. Because the base contributes more blood flow than the apex, basal gas tensions influence arterial blood more heavily. However, the oxygen-hemoglobin dissociation curve's shape means that even the well-oxygenated apical blood cannot fully compensate for the desaturated basal blood. This physiological V/Q heterogeneity contributes to the normal alveolar-arterial oxygen gradient. These regional gas tension differences have clinical significance. Tuberculosis characteristically affects the lung apex, perhaps because the high oxygen tensions there favor the obligate aerobe Mycobacterium tuberculosis. Pulmonary edema preferentially involves the lung bases, where higher perfusion and lower lymphatic drainage create favorable conditions for fluid accumulation. Aspiration pneumonia typically involves posterior and lower lobe segments, reflecting gravity-dependent particle deposition. Panel A: Upright lung with regional V/Q ratios showing apex at 3.3, mid-lung at 1.0, and base at 0.6 with bar graphs comparing ventilation and perfusion. Panel B: Gas tension values at each region with apex PO2=130 PCO2=28, mid-lung PO2=100 PCO2=40, and base PO2=89 PCO2=42. Panel C: Clinical correlations showing tuberculosis at high-oxygen apex, normal exchange in mid-lung, and pulmonary edema at dependent base. Panel D: Summary emphasizing perfusion varies more than ventilation creating the V/Q gradient from apex to base. --- ### V. Effects of V/Q Mismatch Ventilation-perfusion mismatch represents the most common cause of hypoxemia in clinical practice. Understanding how V/Q mismatch impairs gas exchange explains the arterial blood gas abnormalities seen in various pulmonary diseases. Low V/Q regions, where perfusion exceeds ventilation, function as partial shunts. Blood passing through these regions equilibrates with poorly ventilated alveolar gas, leaving with reduced oxygen content and elevated carbon dioxide content. Common causes include airway obstruction from mucus, bronchoconstriction, or atelectasis, which reduce ventilation to perfused regions. High V/Q regions, where ventilation exceeds perfusion, function as partial dead space. Fresh air enters alveoli that receive little blood flow, wasting ventilation and work of breathing. Common causes include pulmonary embolism, low perfusion states, and destruction of pulmonary capillaries. The ventilated but unperfused gas contributes minimally to arterial blood composition because little blood flows past to participate in exchange. V/Q mismatch causes hypoxemia more readily than hypercapnia due to the shapes of the respective dissociation curves. The oxygen-hemoglobin curve has a flat upper portion, meaning that well-oxygenated blood from high V/Q regions already approaches maximum saturation and cannot compensate for desaturated blood from low V/Q regions by carrying extra oxygen. Mixing desaturated blood with fully saturated blood inevitably reduces average oxygen content. In contrast, the carbon dioxide dissociation curve is nearly linear in the physiological range. High V/Q regions can eliminate extra carbon dioxide, partially compensating for elevated carbon dioxide content in blood from low V/Q regions. Additionally, hypoxemia and early hypercapnia stimulate ventilation, increasing overall alveolar ventilation and restoring carbon dioxide elimination. Thus, patients with V/Q mismatch typically present with hypoxemia and normal or even low arterial carbon dioxide partial pressure reflecting compensatory hyperventilation. Only when V/Q mismatch becomes severe or when ventilatory reserve is exhausted does hypercapnia develop. This occurs because the dead space effect of high V/Q regions wastes a fraction of each breath, requiring increased minute ventilation to maintain adequate alveolar ventilation. Panel A: Low V/Q unit showing constricted airway with patent blood vessel and blood leaving with reduced oxygen and elevated carbon dioxide. Panel B: High V/Q unit showing patent airway with obstructed vessel depicting wasted ventilation. Panel C: Oxygen-hemoglobin and CO2 dissociation curves demonstrating flat O2 curve upper portion preventing compensation versus linear CO2 curve allowing partial compensation. Panel D: Typical ABG findings in V/Q mismatch showing low PaO2, normal or low PaCO2, and increased A-a gradient. --- ### VI. V/Q Diagram The oxygen-carbon dioxide diagram provides a graphical representation of all possible combinations of alveolar gas tensions across the range of V/Q ratios. This diagram helps visualize how V/Q mismatch distributes alveolar units away from the ideal point, reducing overall gas exchange efficiency. The diagram plots oxygen partial pressure on the horizontal axis and carbon dioxide partial pressure on the vertical axis. A curved line connects all possible alveolar gas compositions as V/Q ratio varies from zero to infinity. This V/Q line represents the continuum of possible alveolar gas tensions. Three key points anchor the V/Q line. The venous point, at the left end where V/Q equals zero, represents the gas composition that would exist in a completely unventilated alveolus equilibrated with mixed venous blood: oxygen partial pressure of 40 mmHg and carbon dioxide partial pressure of 46 mmHg. The ideal alveolar point, at V/Q of approximately 0.8, represents optimal gas exchange with oxygen partial pressure of 100 mmHg and carbon dioxide partial pressure of 40 mmHg. The inspired air point, at the right where V/Q approaches infinity, represents gas composition in a completely unperfused alveolus filled with humidified inspired air: oxygen partial pressure of 150 mmHg and carbon dioxide partial pressure of zero. In a healthy lung, most alveolar units cluster tightly around the ideal point, with minimal dispersion along the V/Q line. In disease states with V/Q mismatch, alveolar units scatter along the curve: shunt-like units distribute toward the venous point while dead space-like units distribute toward the inspired air point. The overall effect reduces gas exchange efficiency. Mathematical analysis using the V/Q diagram allows quantification of V/Q mismatch severity and its contributions to hypoxemia. This approach has been crucial in understanding the gas exchange abnormalities in conditions such as chronic obstructive pulmonary disease, acute respiratory distress syndrome, and pulmonary embolism. Panel A: O2-CO2 diagram with PO2 horizontal axis (0-150 mmHg) and PCO2 vertical axis (0-50 mmHg) showing curved V/Q line. Panel B: Three key points on V/Q line with venous point v at coordinates 40,46, ideal point i at 100,40, and inspired point I at 150,0. Panel C: Normal lung distribution narrowly clustered around ideal point compared to diseased lung distribution spread widely along V/Q line. Panel D: Directional arrows showing low V/Q regions shifting toward venous point and high V/Q regions toward inspired point with impairment severity correlating to distribution spread. --- ### VII. Compensatory Mechanisms The pulmonary circulation possesses intrinsic mechanisms to optimize V/Q matching, reducing the impact of regional variations in ventilation. The most important of these is hypoxic pulmonary vasoconstriction, a response unique to the pulmonary circulation that diverts blood flow away from poorly ventilated lung regions. Hypoxic pulmonary vasoconstriction represents a local response of pulmonary arterial smooth muscle to low alveolar oxygen partial pressure. When oxygen partial pressure in the alveolus falls below approximately 60 mmHg, the adjacent pulmonary arterioles constrict, reducing blood flow to that region. This response requires no neural or hormonal input and operates through direct effects of oxygen tension on smooth muscle ion channels. The physiological significance of hypoxic pulmonary vasoconstriction lies in V/Q matching. By reducing perfusion to poorly ventilated areas, the response diverts blood toward better-ventilated regions where gas exchange can occur effectively. This improves overall V/Q matching and reduces the impact of regional hypoventilation on arterial oxygenation. Hypoxic pulmonary vasoconstriction differs fundamentally from systemic vascular responses. In the systemic circulation, hypoxia causes vasodilation to increase oxygen delivery to tissues. In the pulmonary circulation, the opposite response makes physiological sense: the goal is not to deliver more blood to hypoxic lung regions but to send blood where oxygen is available for uptake. A secondary compensatory mechanism involves regional changes in bronchial tone. Low carbon dioxide partial pressure, as occurs in high V/Q regions, causes bronchoconstriction that reduces ventilation to these areas. High carbon dioxide partial pressure causes bronchodilation. However, this mechanism plays a less important role than hypoxic pulmonary vasoconstriction in clinical conditions. Compensatory mechanisms can fail or become problematic under certain circumstances. When hypoxia is widespread rather than localized, hypoxic pulmonary vasoconstriction causes generalized pulmonary vasoconstriction and pulmonary hypertension. This occurs at high altitude and in diffuse lung diseases such as advanced chronic obstructive pulmonary disease. Several anesthetic agents inhibit hypoxic pulmonary vasoconstriction, which may worsen oxygenation during surgery. In some diseases, damaged regions lose the capacity for normal vasoconstriction, allowing blood to flow to poorly ventilated areas and worsening V/Q mismatch. Panel A: Two alveoli comparison showing normally ventilated alveolus with relaxed arteriole versus poorly ventilated alveolus with constricted arteriole and arrows indicating blood diversion. Panel B: Contrast diagram showing systemic hypoxic vasodilation to increase tissue oxygen delivery versus pulmonary hypoxic vasoconstriction to optimize V/Q matching. Panel C: Failed compensation with widespread hypoxia causing pulmonary hypertension shown by thickened vessels and elevated pressure. Panel D: Impaired response from anesthetics or disease allowing blood flow to hypoxic regions despite low alveolar PO2. --- ### VIII. Shunt Shunt represents the extreme case of V/Q mismatch where blood completely bypasses ventilated alveoli, passing through the pulmonary circulation without participating in gas exchange. Understanding shunt physiology explains why certain forms of hypoxemia respond poorly to supplemental oxygen therapy. Anatomic shunts involve blood that never enters pulmonary capillaries. Normal anatomic shunts include bronchial venous drainage into pulmonary veins and Thebesian veins draining directly into the left ventricle. These account for a physiological shunt of approximately 2 to 5 percent of cardiac output. Pathological anatomic shunts include intracardiac defects with right-to-left flow (such as atrial or ventricular septal defects with Eisenmenger physiology) and pulmonary arteriovenous malformations. Physiologic shunt includes anatomic shunt plus any intrapulmonary shunt where blood flows past completely unventilated alveoli. Common causes include atelectasis, where collapsed alveoli receive perfusion but no ventilation; consolidation from pneumonia, where fluid-filled alveoli exclude air; and acute respiratory distress syndrome, where widespread alveolar flooding creates extensive shunt. The shunt equation quantifies the fraction of cardiac output bypassing gas exchange. The equation compares oxygen content differences: the numerator is the difference between ideal end-capillary content and actual arterial content, while the denominator is the difference between end-capillary and mixed venous content. End-capillary content assumes equilibration with ideal alveolar oxygen. A key characteristic distinguishing shunt from other causes of hypoxemia is the poor response to supplemental oxygen. When blood completely bypasses ventilated alveoli, increasing the oxygen concentration in those alveoli cannot improve oxygenation of the shunted blood. The shunted blood remains venous, and mixing this desaturated blood with well-oxygenated blood from ventilated regions limits overall arterial oxygen content. Clinical estimation of shunt fraction uses the arterial oxygen partial pressure achieved during 100 percent oxygen breathing. Values above 500 mmHg suggest less than 5 percent shunt, representing normal physiological shunt. Values between 300 and 500 mmHg suggest 5 to 10 percent shunt. Values between 150 and 300 mmHg suggest 10 to 20 percent shunt. Values below 150 mmHg indicate greater than 20 percent shunt, representing severe intrapulmonary shunting. Panel A: Anatomic shunt types including bronchial veins, Thebesian veins, and intracardiac defects with right-to-left flow. Panel B: Physiologic shunt causes including atelectasis, consolidation, and ARDS with the shunt equation and variable definitions. Panel C: Graph showing poor PaO2 response to increasing FiO2 in significant shunt versus normal response with clinical estimation table relating PaO2 on 100% O2 to shunt percentage. Panel D: Illustration of oxygenated blood mixing with shunted venous blood resulting in reduced final arterial oxygen content. --- ### IX. Dead Space Dead space represents the portion of ventilation that does not participate in gas exchange. While shunt involves perfusion without ventilation, dead space involves ventilation without perfusion. Understanding dead space explains inefficient carbon dioxide elimination and the increased ventilatory requirements of certain disease states. Anatomic dead space comprises the conducting airways from nose and mouth to terminal bronchioles. This volume, approximately 150 milliliters in an adult, receives fresh air with each breath but contains no alveoli for gas exchange. Anatomic dead space remains relatively constant and represents an obligate inefficiency in ventilation. Alveolar dead space consists of alveoli that receive ventilation but lack adequate perfusion for gas exchange. In healthy individuals, alveolar dead space approaches zero. Pathological increases occur when pulmonary perfusion is reduced, whether focally (as with pulmonary embolism) or globally (as with low cardiac output states). Physiological dead space equals the sum of anatomic and alveolar components, representing total wasted ventilation. In health, physiological dead space approximately equals anatomic dead space. In disease, physiological dead space may increase substantially. The Bohr equation calculates physiological dead space by comparing arterial and mixed expired carbon dioxide partial pressures. The ratio of dead space to tidal volume normally approximates 0.3, meaning about 30 percent of each breath does not contribute to gas exchange. When this ratio increases, more minute ventilation is required to maintain the same alveolar ventilation and the same arterial carbon dioxide partial pressure. Causes of increased dead space include pulmonary embolism, where clots obstruct perfusion to ventilated regions; pulmonary hypertension, where elevated resistance reduces capillary perfusion; emphysema, where destruction of alveolar septa reduces the capillary bed; and positive pressure ventilation, where overdistension of alveoli compresses adjacent capillaries. The clinical consequences of increased dead space center on ventilatory inefficiency. Patients must increase minute ventilation to maintain normal carbon dioxide elimination, increasing the work of breathing. When ventilatory reserve is exceeded, hypercapnia develops. Patients with large dead space fractions may fatigue from the increased respiratory work, progressing to respiratory failure. Panel A: Anatomic dead space showing conducting airways from nose to terminal bronchioles with 150mL volume labeled plus alveolar dead space as ventilated but unperfused alveoli. Panel B: Bohr equation with variable definitions showing physiological dead space equals anatomic plus alveolar components with normal VD/VT ratio of 0.3. Panel C: Causes of increased dead space including pulmonary embolism with blocked vessels, emphysema with destroyed alveoli, and positive pressure ventilation compressing capillaries. Panel D: Clinical consequences showing increased minute ventilation requirement, increased work of breathing, and potential hypercapnia when compensation fails. --- ### X. Clinical Applications The concepts of V/Q mismatch, shunt, and dead space apply directly to understanding and managing common pulmonary diseases. Each condition produces characteristic patterns of V/Q abnormality that explain the clinical findings and guide therapeutic interventions. Chronic obstructive pulmonary disease produces both high and low V/Q regions in a heterogeneous pattern. Emphysematous regions lose both ventilation and perfusion as alveolar septa are destroyed, creating dead space. Bronchitic regions with mucus and inflammation have reduced ventilation but preserved perfusion, creating low V/Q units. The net result is hypoxemia with variable hypercapnia depending on disease severity and ventilatory compensation. Hypoxic pulmonary vasoconstriction may become impaired, worsening V/Q mismatch. Chronic pulmonary hypertension develops from prolonged hypoxic vasoconstriction in remaining functional regions. Acute respiratory distress syndrome causes severe shunt physiology. Inflammatory injury floods alveoli with protein-rich edema, creating lung regions that receive perfusion but no ventilation. The shunt fraction may exceed 20 to 30 percent, causing profound hypoxemia that responds poorly to increasing inspired oxygen concentration. Treatment requires positive end-expiratory pressure to recruit collapsed alveoli and redistribute ventilation toward perfused regions, improving V/Q matching. Pulmonary embolism creates dead space by obstructing blood flow to ventilated lung regions. Perfusion distal to the embolus ceases while ventilation continues, producing high V/Q units or complete dead space. The alveolar-arterial gradient increases despite the primary perfusion defect because blood is redistributed to remaining perfused areas, potentially creating low V/Q regions. Ventilation-perfusion scanning exploits this pathophysiology: mismatched defects where perfusion is absent but ventilation preserved provide classic evidence for pulmonary embolism. Asthma causes low V/Q units through bronchoconstriction, which reduces ventilation to affected lung regions while perfusion continues. Hypoxemia results but typically responds well to supplemental oxygen because shunt is not a major component. Arterial carbon dioxide partial pressure often falls initially due to hypoxemia-driven hyperventilation; rising carbon dioxide during an asthma exacerbation signals impending respiratory failure as the patient fatigues. Panel A: COPD showing heterogeneous high V/Q emphysematous regions with destroyed alveoli and low V/Q bronchitic regions with mucus plugging causing hypoxemia and variable hypercapnia. Panel B: ARDS with flooded alveoli creating shunt physiology and severe hypoxemia poorly responsive to oxygen with PEEP treatment expanding collapsed alveoli. Panel C: Pulmonary embolism with blocked artery creating dead space shown as wedge-shaped V/Q scan defect with mismatched ventilation and perfusion. Panel D: Asthma with bronchoconstriction creating low V/Q regions showing hypoxemia responsive to oxygen and bronchodilators with rising CO2 warning of respiratory failure. --- ## Summary Ventilation-perfusion matching fundamentally determines gas exchange efficiency. The V/Q ratio of 0.8 represents the ideal balance for the whole lung, where alveolar ventilation of 4 L/min meets pulmonary blood flow of 5 L/min. Extremes of this ratio define shunt (V/Q = 0, perfusion without ventilation) and dead space (V/Q = infinity, ventilation without perfusion). Regional distribution of ventilation and perfusion follows gravity, with both increasing from apex to base in the upright lung. However, perfusion varies more dramatically than ventilation, creating a gradient in V/Q ratios: approximately 3.3 at the apex (dead space-like) and 0.6 at the base (shunt-like). These regional differences produce variations in alveolar gas tensions and affect disease localization. V/Q mismatch causes hypoxemia more readily than hypercapnia because the oxygen-hemoglobin dissociation curve's flat upper portion prevents compensation from high V/Q regions. The linear carbon dioxide dissociation curve and ventilatory compensation usually maintain normal or low arterial carbon dioxide until disease becomes severe. Hypoxic pulmonary vasoconstriction provides the primary compensatory mechanism, diverting blood away from poorly ventilated regions toward better-ventilated areas. This unique pulmonary response improves V/Q matching but can cause pulmonary hypertension when hypoxia is widespread. Shunt, the extreme of low V/Q, produces hypoxemia that responds poorly to supplemental oxygen because blood never contacts ventilated alveoli. Dead space, the extreme of high V/Q, wastes ventilation and increases respiratory work, eventually causing hypercapnia when ventilatory compensation fails. Clinical application of V/Q concepts explains the gas exchange abnormalities in chronic obstructive pulmonary disease, acute respiratory distress syndrome, pulmonary embolism, and asthma, guiding both diagnosis and treatment strategies. --- ## Key Terms | Term | Definition | |------|------------| | V/Q ratio | Ratio of alveolar ventilation to pulmonary blood flow | | Shunt | Perfusion without ventilation (V/Q = 0) | | Dead space | Ventilation without perfusion (V/Q = ∞) | | Hypoxic pulmonary vasoconstriction | Vasoconstriction in response to low alveolar PO₂ | | West zones | Lung regions defined by pressure relationships | | Bohr equation | Calculation of physiological dead space | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*