V/Q Mismatch

Course overview

Objectives Describe the processes involved with normal VQ Explain the concept of VQ mismatch and how to identify its signs and symptoms Content VQ Mismatch Introduction Ventilation and perfusion ratios are important to understand in daily patient care Understanding this subject and the pathological changes that cause abnormalities is very important Foundation We must refresh basic knowledge of the cardiovascular and respiratory systems and the important related factors Cardiovascular Right side of the heart Deoxygenated blood enters the right atrium through the tricuspid valve into the right ventricle The right ventricle pumps blood through the pulmonary valve into the pulmonary trunk Blood then separates into the right and left pulmonary arteries Pulmonary vascular pressure is approximately 25 mmHg systolic and 8 mmHg diastolic Pulmonary circulation to the right lung The right pulmonary artery separates into the superior lobar artery and the middle lobar artery The superior lobar artery supplies the upper right lobe The middle lobar artery branches into the inferior lobar artery The middle lobar artery supplies the middle lobe The inferior lobar artery supplies the lower right lobe Pulmonary circulation to the left lung The left lung vasculature is more complex The left pulmonary artery gives off several superior lobar arteries to the upper lobe It then branches into a variable number of inferior lobar arteries to supply the lower lobe Pulmonary arteries and capillary beds Pulmonary arteries continue branching until reaching pulmonary capillary beds Capillary beds surround the alveoli Diffusion occurs at the alveolar capillary membrane This membrane is one cell layer thick It has sufficient surface area for diffusion It has strength to resist mechanical failure and allows diffusion of carbon dioxide and oxygen Overall capillary pressure is approximately 7 mmHg Pulmonary veins Alveolar capillary beds merge into pulmonary venules and veins Pulmonary veins exit the lungs and deliver oxygenated blood to the left atrium There are two pulmonary veins from each lung Other factors Pulmonary circulation is significantly shorter than systemic circulation Arterial lumens are larger with thin distensible walls This allows maintenance of blood flow during low cardiac output This prevents rupture of microvessels Respiratory system Focus on the lower respiratory system and respiratory volumes Respiratory pathways The trachea brings air from the environment to participate in gas exchange The trachea extends from the larynx to the carina At the carina the trachea divides into the main bronchi Main bronchi divide into lobar bronchi supplying each lung lobe The right lung has three lobar bronchi The left lung has two lobar bronchi Lobar bronchi divide into smaller airways These reach alveolar ducts and alveolar sacs Gas exchange occurs in the alveoli Lung volumes An average adult has approximately 480 million alveoli Lung size affects alveolar count and respiratory volumes Lung volumes are important for diagnosis treatment and homeostasis Tidal volume is the amount of air moved during one breath approximately 500 milliliters Total lung capacity is approximately 6000 milliliters in adult males and one third less in females Inspiratory reserve volume is approximately 3000 milliliters Expiratory reserve volume is approximately 1200 milliliters Residual volume is approximately 1200 milliliters and prevents lung collapse Dead space Dead space is the portion of the respiratory system not involved in gas exchange Approximately 30 percent of tidal volume is lost to dead space Physiologic dead space can increase with lung disease affecting alveoli Normal VQ ratio Volumes for VQ Resting minute ventilation is approximately 6 liters per minute One third is lost to dead space resulting in approximately 4 liters per minute Pulmonary blood flow is approximately 5 liters per minute This produces a VQ ratio of 4 to 5 or 0.8 Lung zones