2.2 Alveolar Ventilation(1).pptx

Alveolar Ventilation Lecture Outline I. Lung Volumes II. Anatomic Dead Space and Alveolar Ventilation III. Alveolar Ventilation and Alveolar Oxygen and Carbon Dioxide IV. Regional Distribution of Alveolar Ventilation V. Closing Volume 1 Alveolar Ventilation Objectives 1. Identify standard lung volum...

Alveolar Ventilation Lecture Outline I. Lung Volumes II. Anatomic Dead Space and Alveolar Ventilation III. Alveolar Ventilation and Alveolar Oxygen and Carbon Dioxide IV. Regional Distribution of Alveolar Ventilation V. Closing Volume 1 Alveolar Ventilation Objectives 1. Identify standard lung volumes and capacities 2. Identify the two purposes of ventilation 3. List factors that affect anatomic dead space 4. Compare types of dead space including anatomic, alveolar, and physiologic 5. Understand the Bohr equation for measuring physiologic dead space 6. Describe the renewal of alveolar air 7. Explain Dalton’s law and the composition of respiratory gases 8. Explain how alveolar ventilation affects alveolar gas tensions 9. Understand the alveolar air equation 10. Describe regional differences in alveolar ventilation at FRC and RV 11. Describe the closing volume 2 References Assigned reading from your text: Levitzky Chapter 3 3 I. Lung Volumes 4 Standard Lung Volumes and Capacities  Lung Volumes are normally expressed at BTPS Body Temperature, ambient Pressure, Saturated with water vapor VT = Contains a volume of gas in both the conducting zone and respiratory zone ERV= FRC- RV (a forced expiration) IRV= Determined by strength of contraction of inspiratory muscles; starts at FRC + TV RV= Force generated by the muscles of expiration and inward elastic recoil: – Dynamic compression may increase 5 Lung Capacities = 2 or More Volumes  FRC (“relaxation volume”)- Volume of gas remaining in lungs at end of a normal tidal expiration IC- inhaled volume beginning at FRC TLC- determined by the strength of contraction of the inspiratory muscles VC= Measured during forced expiration 6 Changes in Volumes/Capacities from Standing to Supine  Lung volumes change for physiologic circumstances such as posture -FRC decreases- gravity not pulling abdominal contents away from the diaphragm -Decreased FRC decreases ERV and increases IRV -RV & TLC slight decrease or no change 7 Two Major Pathological Changes to Lung Volumes  Restrictive disease Reduced compliance and compressed volumes Increased recoil To minimize work of breathing: Increased frequency and decreased VT  Obstructive disease Increased resistance to airflow due to mucus plugs and high IPP needed to overcome high airway resistance of forced expiration Destruction of alveolar septa and decreased elastic recoil and decreased radial traction to hold open small airways RV, FRC, TLC greatly increased in obstructive disease To minimize work of breathing: Decreased frequency with an increased VT 8 Spirometer  Measures lung volumes the subject can exchange with it Traditionally a water-seal spirometer Only measures the volumes a subject can exchange with it Subject must be conscious and cooperative  RV cannot be measured by spirometry: RV, FRC, TLC  RV (FRC, TLC) can be measured by: Nitrogen-washout technique supposes 80% lung volume is N – Volume of N in lungs x 1.25 = FRC Helium-dilution technique (an indicator-dilution technique) – Helium does not diffuse/ is not taken up by pulmonary capillaries Body plethysmography can measure FRC (Boyle’s law) – Can measure gas trapped in “slow” alveoli Alveoli served by airways with high resistance to airflow 9 II. Anatomic Dead Space and Alveolar Ventilation 10 Ventilation  Ventilation is the exchange of gas between the lungs and the atmosphere for 2 purposes: 1. Acquire oxygen for metabolism 2. Remove carbon dioxide from blood  Pulmonary ventilation = minute volume Minute volume is the volume of air inhaled or exhaled per minute Determined as: VE = VT x frequency of breaths Average value = 6L/min Volume of air entering/leaving nose or mouth per minute exceeds the volume of air entering/leaving the alveoli per minute (not all gas goes to alveoli)  Alveolar ventilation Alveolar ventilation is the volume of fresh (new) air delivered to the alveoli per minute Less than minute volume Alveolar ventilation excludes that portion of air not involved in gas exchange  Dead space 11 Factors Affecting Anatomic Dead Space (ADS):  Factors include: Depth of inspiration stretches and expands conducting airways (minor effect) Height of individual – taller individual has longer airways Gender: males generally have increased anatomic dead space Age increases ADS due to loss of elastic recoil in conducting airways lacking cartilage At end-inspiration, the anatomic dead space contains ambient air At end-expiration, the anatomic dead space contains alveolar air 12 Factors Affecting Anatomic Dead Space (ADS): 13 Types of Dead Space: Anatomic, Alveolar, and Physiologic  Anatomic dead space is the volume of gas occupying the conducting airways: No gas exchange occurs in these airways since there are no alveoli Anatomic dead space volume can be: Measured clinically by Fowler’s method (not used clinically) Estimated from standard tables: 1 lb. ideal body weight = 1 ml anatomic dead space ex. 150 ml for 150 lb Physiological person ~1/3 VT 14 How does anatomic dead space affect alveolar ventilation?  VA = (VT - VD) x f where VD = ADS volume An individual has a tidal volume = 500 ml; ADS volume = 150 ml; rate = 12/min. VE = 500 ml x 12/min = 6.0 L/min. VA = (500-150) x 12/min = 4.2 L/min involved in gas exchange/min _______________________________________________________________________________  Which example below produces a larger alveolar ventilation? VE = 1000 ml x 6/min = 6.0 L/min VE = 250 ml x 24/min = 6.0 L/min VA = (1000 -150) ml x 6/min = 5100 L/min VE shows no change while VA shows substantial changes VA = (250 -150) ml x 24/min= 2400 L/min 15 Alveolar and Physiologic Dead Space  Alveolar dead space: volume of alveolar air not equilibrating with pulmonary capillary blood Ventilation is wasted here- this is ventilation reaching alveoli without perfusion There is minimal alveolar dead space in healthy individuals  Physiologic dead space: volume of all inhaled air not exchanging with pulmonary capillary blood Physiologic dead space = Anatomic dead space + Alveolar dead space This volume of air is inhaled for physiologic use but is not involved in gas exchange Since there is minimal alveolar dead space in a healthy individual, an increase in physiologic dead space is usually due to an increase in alveolar dead space 16 Bohr Equation  Bohr equation permits determination of physiologic dead space (equals alveolar + anatomic DS) Any measurable CO2 comes from ventilated/perfused alveoli Compares the partial pressure of CO2 in the blood (PaCO2) vs the exhaled CO2 (PeCO2) The greater the difference between these values, the greater the amount of dead space If dead space increases: minute ventilation (RR, TV, or both) must increase to maintain a constant PaCO2 In a circle system, dead space begins at the Y 17 FRC Resists Sudden Changes in Alveolar Air Composition  FRC resists sudden changes in alveolar air composition  arterial blood gas composition To blow off CO2, the body will normally increase VT first, then frequency if needed Inspiration VA is lower than VE due to dead space; PCO2 rapidly decreases to zero The last part of each inspiration remains in the conducting airways Expiration Air from the last part of a previous inspiration is added back to the ADS 150 ml in ADS is not fresh air Renewal of Alveolar Air  With each tidal breath of fresh air, approximately 350 ml new air is delivered to the alveoli. Using a VT of 500 ml, an ADS of 150 ml, and an FRC of 3000 ml prior to an inspiration: Approximately 350 ml “fresh” air is delivered to the alveoli (500 – 150) The % of the FRC being renewed with each breath is 350/3000 = 11.7% (12%) 20 III. Alveolar Ventilation and Alveolar O2 and CO2 21 Dalton’s Law  In a gas mixture, the pressure exerted by each individual gas is independent of the pressures of other gases in the mixture The partial pressure of a gas is = % total gas x P tot O2 = 20.93% of dry atmospheric air PO2 = 0.2093 x 760 mmHg = 159 mmHg PCO2 = 0.0004 x 760 mmHg – 0.3 mmHg The relative humidity of water vapor at body temp = 47 mmHg 1 L of gas at 760 mmHg is diluted by the added water vapor 22 Composition of Alveolar Air  Alveolar gas is in a compartment between ambient air and pulmonary capillary blood Alveolar gas tensions are affected by ventilation and perfusion  Components of alveolar air PH2O 47 mmHg PO2 104 mmHg PCO2 40 mmHg PN2 569 mmHg 760 mmHg  Components of tracheal air PH2O 47 mm Hg PO2 149 mm Hg PCO2 0.3 mm Hg PN2 564 mm Hg *149 104 *0.3 40 23 Predicted Alveolar Gas Tensions for Alveolar Ventilation  Concentration of CO2 in the alveolar gas is dependent on the alveolar ventilation and the rate of CO2 production  In healthy individuals: If alveolar ventilation is doubled – PACO2 and PaCO2 are reduced by one-half If alveolar ventilation is cut in half– PACO2 and PaCO2 will double 24 Alveolar Air Equation  Alveolar PO2 must be calculated with the alveolar air equation At constant CO2 production, alveolar PCO2 is approximately inversely proportional to alveolar ventilation  This equation illustrates: Hypoventilation causes hypercarbia and hypoxemia O2 can reverse hypoxemia; not hypercarbia Hypercarbia can go undetected in a patient breathing supplemental O2  As alveolar ventilation increases: PACO2 decreases, bringing the alveolar PO2 closer to the inspired PO2 So does PAO2  Variables: FiO2 is always higher than the PAO2 Hypoventilation increases PaCO2 which competes with O2 for space inside the alveolus Supplemental O2 masks hypoventilation (does not treat) 25 Alveolar Gas (Air) Equation  An Acceptable Means For Calculating PAO2 Of A Healthy Person PiO2 = partial pressure of inspired oxygen R PAO2 = [FiO2 x (PB-PH2O)] – (PaCO2/R) FiO2 = fractional percent of inspired oxygen = 21 or 0.21* PB PH2O = water vapor partial pressure = 47 mmHg PAO2 = [0.21 x (760-47)] – (40/0.8) = 150-50  = 100 mmHg Can be higher if O2 is being administered As alveolar ventilation increases, so does P AO2 Highest PAO2 one can achieve breathing air at sea level is 149 mmHg = respiratory quotient (a normal constant) = 0.8 = barometric pressure = 760 mmHg 26 IV. Regional Distribution of Alveolar Ventilation 27 Regional Differences in Alveolar Ventilation  Lower alveoli receive more ventilation/unit volume Non-Uniform distribution of inspired air All areas of lungs do not receive equal amounts of inspired air Variations due to: Body position Major cause of non-uniform distribution of inspired air normally Dependent areas of the lung receive a greater volume of fresh air per breath Mechanical advantageRibs in lower thorax are more oblique allowing greater expansion of rib cage Pulmonary abnormalitiesCompliance and/or resistance may be abnormally unequal throughout lungs 28 Dependent Lung Receives More Airflow  At or near FRC, basilar (dependent) alveoli are better ventilated than upper regions of the lung In left lateral position left lung receives more ventilation than the right (vs apex to base) Gravity is the major cause for the regional differences in ventilation/unit volume – Gravity produces unequal stretching  unequal expansion  Apical region = Gravity non-dependent More distended than basilar region More difficult to stretch further Apical IPP is more negative  Basilar region = Gravity dependent Less distended than apical region Basilar IPP is less negative 29 Explanation for Differences in Regional Ventilation During Eupnea At FRC  IPP is more negative in the upper regions of the lung compared to the lower regions IPP increases ~0.5 cm H2O for each cm from apical to basilar regions eg - 8.5 cm H2O in non-dependent apical region -1.5 cm H2O in gravity dependent basilar region Alveoli in upper regions also subjected to a greater distending pressure greater volume This difference in volume  difference in ventilation between upper and lower areas During a normal respiratory cycle greater change in volume in gravity-dependent alveoli  Compliance Apical alveoli less compliant Basilar alveoli more compliant Apical (non-dependent) alveoli: FRC – Most alveolar air is in the upper lung ERV Basilar (gravity-dependent) alveoli: IRV IC 30 Differences In Regional Ventilation After A Forced Expiration To The RV  After a forced expiration to RV Dynamic compression occurs in the smaller airways of the basilar region With inspiration from low lung volumes: The initial part of the breath goes to the upper alveoli The later part of the breath goes to the dependent alveoli As the inspiration approaches FRC from the RV The airways of the basilar dependent alveoli begin to open and these alveoli fill later in the inspiration 31 V. Closing Volume 32 Closing Volume  This is the volume at which airway closure begins to occur Normally only 10% of VC in young adults Loss of alveolar elastic recoil (from age or pathology) results in decreased traction on small airways – In emphysema and advanced age- airway closure occurs in the dependent lung even at higher lung volumes  First breath: Subject starts from RV, inspires single breath of 100% O 2 up to TLC, exhales back to RV At the end of forced expiration to RV, gas left in lungs is about 80% nitrogen Because of the IPP gradient from top to bottom of lungs: – Upper alveoli contain most of the RV and most of the N – Lower alveoli have smaller volumes and less N – At the bottom of the lung, airways are closed and trap a small volume of gas  Second breath: Subject starts from RV, inspires single breath of 100% O 2 up to TLC, exhales back to RV The initial part of the breath enters the upper alveoli most O2 enters the lower alveoli  Expired N % is measured during 2nd breath Phase I- gas from anatomic dead space Phase II- Mixture of dead space and alveolar gas Phase III- Mixed upper and lower alveolar gas Phase IV– Beginning of dependent airway closure 33 Effects Of Aging On Lung Volumes And Capacities  Aging causes increased static lung compliance and decreased chest wall compliance from: – A loss of alveolar elastic recoil – Increased chest wall recoil – Decreased muscle strength – Loss of alveolar surface area – Loss of pulmonary capillary blood volume  Results in increased FRC and Closing Volume 34 1. A. B. C. D. The tidal volume is: the amount of air that normally moves into (or out of) the lungs with each respiration the amount of air that enters the lungs but does not participate in gas exchange the amount of air expired after maximal expiratory effort the amount of gas that can be moved into and out of the lungs in 1 minute 2. Which of the following conditions are reasonable explanations for a patient’s functional residual capacity that is significantly less than predicted? A. Third trimester of pregnancy B. Pulmonary fibrosis C. Obesity D. Standing posture E. a, b, and c 3. A seated person begins to inspire from residual volume. Assuming this gas is labeled, most of the inhaled gas inspired after dead space will probably be found: A. in alveoli in lower portions of the lung B. in alveoli in upper portions of the lung C. uniformly distributed in all alveoli 35