Lung Volumes and Capacities-MHS F23.pptx

Lung Volumes and Capacities Paul McDonough, PhD Resources • Costanzo, Physiology Chapter 5, Pp. 195-250 • Cloutier, M. Respiratory Physiology Chapter 1-2 Pp. 1-28 Learning Objectives 1. How is the alveolar pressure different from the pleural pressure? 2. How is the transpleural pressure gradient...

Lung Volumes and Capacities Paul McDonough, PhD Resources • Costanzo, Physiology Chapter 5, Pp. 195-250 • Cloutier, M. Respiratory Physiology Chapter 1-2 Pp. 1-28 Learning Objectives 1. How is the alveolar pressure different from the pleural pressure? 2. How is the transpleural pressure gradient created? 3. What is the difference between a lung volume and a lung capacity? How is the vital capacity measured by spirometry? Why can’t residual volume be measured by spirometry? 4. Why do changes in the static mechanical properties of the lung cause measurable changes lung volume measurements? 5. What is lung compliance? 6. What is pulmonary surfactant, and how does it help maintain lung compliance? Structure of the respiratory system • The respiratory system • Series of airways that ultimately connect the lungs with the external environment • Conducting zone • Move air in and out • Nose, pharynx, bronchi, and bronchioles up through the terminal bronchioles • Respiratory zone • Gas exchange • Respiratory bronchioles, alveolar ducts Structure of the respiratory system •There are approx. 23 generations of airways in the lungs • Cartilage is present in the conducting zone up through the 10th generation • Airways are unsupported after that and are open or closed based on pressure gradients • Smooth muscle • Sympathetic receptors (β 2) • • Cause bronchodilation Parasympathetic receptors (muscarinic) • Cause bronchoconstriction Respirator y zone •Gas exchange region • Alveoli • About 300-400 million • Elastic fibers • Type I cells • Structural • Type II cells • Synthesize surfactant • Alveolar macrophages • Keep alveoli free on debris Introduction • To achieve it’s main function of gas exchange air must be moved into and out of the lungs • • • This is pulmonary mechanics Specifically, it’s the mechanical properties of the lung and chest wall that allow this • Muscular activity moves the chest wall • This moves the lung Disease can impact the mechanics of the lung and chest wall • COPD, restrictive disease • • Impact the lung Aging, fractures • Impact the chest wall Pressures in the respiratory system • In health, the chest wall and the lung move as a unit • Pleural space • • • • • Between them Typically a “potential” space However, it is a key contributor to normal lung mechanics Transpulmonary pressure (PL) or pressure across the lung • Difference between the pressure inside the lung or alveolar pressure (PA) and the pressure outside the lung or pleural pressure (Ppl) • Positive PL keeps lungs open Transmural pressure (PW) or pressure across the chest wall • Difference between pressure in pleural space Ppl and pressure outside the ribcage or atmospheric pressure (PB) • Negative PW helps to keep chest wall tethered to lung PmmHg = PcmH2O x 1.35 PL = 760-756 PL = 4 mmHg PW = 756 – 760 PW = -4 mmHg Pressures in the respiratory system • Pressure across the whole system (Prs) is the sum of these pressures • Prs = PL + PW • Prs = (PA – Ppl) + (Ppl – Pb) • At rest, PL = 4 mmHg and Pw = -4 mmHg • So, at rest, Prs = 0 • The volume at which the respiratory system is at rest is called Functional Residual Capacity (FRC) Spirometry However, we can’t measure FRC, IC or RV without more involved tests ung Lung volumes and capacities • Typically measured using a spirometer • • • See Table of normal values at end of lecture) Explain the test (next slide) Measures 4 volumes • • Tidal volume, inspiratory and expiratory reserve volume and residual volume (estimated typically) Measures 4 capacities • Inspiratory capacity, vital capacity, functional residual capacity (estimated) and total lung capacity (estimated) Spirometry 1.0 •Panel A • Looking at volume changes over time • This is an expiratory curve • FEV1.0 • Forced expiratory volume in one second • FEF25-75 • Forced expiratory flow between 25 and 75 % of the expiratory response Spirometr y •Panel B • This is a flow-volume loop • Here we are looking at flow rates vs lung volume • Expiration is above the x-axis, inspiration below • PEFR • Peak expiratory flow rate • Vmax 25, 50 and 75 • Flow rates at which 25, 50 and 75% of Vital capacity Effort dependent and independent regions •This is another flow-volume loop • Here we look at three different levels of patient effort • Inspiratory volumes are always effort-dependent • Expiratory volumes • Two distinct areas • Effort dependent limb • From TLC until small airway closure occurs • Approximately the first 20% of expiration • Effort-independent limb • Most of expiratory curve • Note that for each effort level, they all resolve to the same slope Impact of disease on expiration •Top • Volume vs time • A is COPD • B is restrictive disease •Bottom • Flow rates vs volume • C is COPD • D is restrictive disease Helium dilution • Common technique for measuring FRC • Uses a known concentration of helium (C1) in a known volume (spirometer volume; V1) • Connect the patient to the spirometer; patients lung volume is V2 the new concentration (C2) is measured after a few breaths for equilibration Helium dilution • Now we solve for the unknown (V2) • C1 x V1 = C2 x (V1 + V2) • This converts to • V2 = FRC • RV can be calculated from FRC ERV Body Plethysmograph • Alternative way to measure FRC • Uses Boyle’s law • P x V = constant • So if volume decreases, pressure rises • Subject sits in an airtight box (Plethysmograph) • After expiring from tidal volume, the mouthpiece is closed • As the subject tries to inspire • • • Volume in subject’s lung increases Also, pressure in the lungs decreases This causes the opposite in the box • • Volume decreases, pressure increases The increase in box pressure is used to calculate the pre-inspiration lung volume • This is FRC Dead space • Volume of the lungs that does not participate in gas exchange • Anatomic vs physiologic • Anatomic dead space • • • Two ways to look at Dead space • • Volume of the conducting airways So, for each breath of 500 mL, 70% reaches the alveoli and 30% is in the conducting airways Volumes or flow rates During exercise, the dead space fraction falls as VT rises Conducting airways and anatomic dead space • Note how for the first 16 generations • • • No alveoli Supportive cartilage rings strop around generation 10 In health this is the entirety of dead space and it’s called anatomic because no alveoli Physiologic Dead Space • Total volume of lung that does not participate in gas exchange • • Anatomic dead space + functional dead space Some alveoli are ventilated but not perfused and vice versa • • This is a consequence of a Ventilation-perfusion mismatch (VA/Q) In normal physiology, total dead space is primarily anatomic dead space • Physiological dead space increases with disease Ventilation rates • Ventilation is the act of moving air into and out of the lungs per unit time • Alveolar ventilation is the total ventilation minus dead space ventilation Alveolar ventilation equation • This is a very important equation in respiratory physiology • • It describes the inverse relationship between alveolar ventilation and alveolar Pco2 (1 and 2) Basically in equation 1 • • Alveolar ventilation rises with metabolism (VCo2) Equation 2 is rearranged to solve for alveolar Pco2 • Alveolar and arterial Pco2 are essentially the same • So, PaCo2 rises with a decrease in VA 1 2 Blood gases vs VA • Looking at the above equation and the graph • • With a constant Pco2 • If metabolic rate doubles (Vco2) • Alveolar ventilation must rise With a halving of Pco2 (hyperventilation) at a constant metabolic rate • Alveolar ventilation must double Alveolar gas equation • This equation is useful in predicting the alveolar Po2 • Note that PaCO2 and PAO2 are inversely related • So if we hypoventilate (PaCO2 rises) • • PAO2 falls If we hyperventilate (PaCO2 falls) • PAO2 rises Forced expiratory volumes • Forced vital capacity • • • Normal physiology • • the maximal air that you can expire from a full breath and how fast you can achieve that In this case the FVC is about 5 L and this patient gets about 80% out in the first second (FEV1.0) COPD • • FVC is reduced compared to normal FEV1.0 is reduced compared to normal • • Now about 55-60% Restrictive disease • Both FVC and FEV1.0 are reduced compared to normal • Now about 90% Useful abbreviations and values in respiratory physiology