Alveolar Ventilation.pdf

Alveolar Ventilation Thomas Ferrari, Ph.D. [email protected] Respiratory, P1, Winter 2021 1  Objectives PUL 17. List the normal airway, alveolar, arterial, and mixed venous PO2 and PCO2 values. PUL 18. D...

Alveolar Ventilation Thomas Ferrari, Ph.D. [email protected] Respiratory, P1, Winter 2021 1  Objectives PUL 17. List the normal airway, alveolar, arterial, and mixed venous PO2 and PCO2 values. PUL 18. Define and contrast the following terms: anatomic dead space, physiologic dead space, wasted (dead space) ventilation, total minute ventilation and alveolar minute ventilation. PUL 19. Describe the concept by which physiological dead space can be measured. PUL 20. Define and contrast the relationships between alveolar ventilation and the arterial PCO2 and PO2. PUL 21. Describe in quantitative terms the effect of ventilation on PCO2 according to the alveolar ventilation equation. PUL 22. Be able to estimate the alveolar oxygen partial pressure (PAO2) using the simplified form of the alveolar gas equation. Be able to use the equation to calculate the amount of supplemental O2 required to overcome a reduction in PAO2 caused by hypoventilation or high altitude. PUL 23. Define the following terms: hypoventilation, hyperventilation, hypercapnea, eupnea, hypopnea, and hyperpnea. PUL 15. Describe the regional differences in alveolar ventilation in healthy and diseased lungs and explain the basis for these differences. 2  Lung Volumes TLC = Total Lung Capacity INSPIRATORY CAPACITY VC = Vital Capacity 3600 ml (60%) Spirometry IRV IRV = Inspiratory 3100 ml VITAL CAPACITY Reserve Volume Tracing TOTAL LUNG CAPACITY 4800 (80%) (52%) 6000 ml (100%) TV = Tidal Volume ERV = Expiratory TV R =1 Reserve Volume 500 ml (8%) RV = Residual 30%ERVof 500 ml = 150 ml Volume =physiologic 1200 ml dead space under normal FRC (20%) conditions IC = Inspiratory 2400 ml Capacity RV (40%) 1200 ml FRC = Functional (20%) Residual Capacity 3 Any volume that is a combination of other volumes is termed “capacity” ANATOMIC DEAD SPACE Conducting Zone Airways from nose or mouth through generation #16- terminal bronchioles (volume = 150 ml) R=1 Factors influencing anatomic dead space: Increases with body size Infants have higher values per body weight Posture (supine less than sitting) Decreases by 20 ml for each liter of lung volume Why would medications (e.g. bronchodilators) increase dead space? 4  Ventilation THINK OF THIS AS FUNCTIONAL DEAD SPACE (NOT INVOLVED IN GAS EXCHANGE) ANATOMIC DEAD SPACE PHYSIOLOGIC VOLUME (VD) DEAD SPACE (normally 30% of Tidal Volume, TV) ALVEOLAR DEAD SPACE VOLUME In healthy individuals the Physiologic Dead Space Volume is roughly equivalent to the Anatomic Dead Space Volume (VD) since most areas of the lung are well perfused = no alveolar dead space volume. 5  Airway Components Branch Ciliated Hyaline Smooth Elastic Structure Image Epithelial Cells Goblet Cells # Cells Cartilage Muscle Fibers C-Shaped 0 Trachea Respiratory Yes Yes Yes Throughout Rings Primary Begins to CONDUCTING ZONE 1 Respiratory Yes Yes Yes Throughout Bronchi (L & R) disappear Secondary Irregular Frequency more 2 Bronchi Respiratory Yes and Throughout (Lobar) + nose and mouth lessens sporadic predominate 3 Segmental 3o Bronchi =columnar Anatomic Respiratory to ciliated reducedDeadYesSpace Significantly FewVolume (VDThroughout dominate ) Ciliated columnar to Surrounds 4-11 Bronchioles No Less None Throughout Simple Cuboidal bronchiole Terminal Starts to 12-16 Simple Cuboidal No No None Throughout Bronchioles diminish Respiratory Stratified to Continues Starts to No No None Bronchioles Simple Squamous to diminish Diminish RESPIRATORY ZONE (Gas Exchange) 17-23 Stratified to Alveolar Continues Simple Squamous No No None Sparse Ducts (AD) to Diminish Alveolar Sacs Simple Squamous No No None Very Sparse Sparse (AS) 24 Very Sparse Alveoli Simple Squamous No No None None and 6fine  Ventilation Anatomic Dead Space Volume Can Increase When: ü Swimmers use a snorkel ü Patients are connected to a mechanical ventilator ANATOMIC DEAD SPACE PHYSIOLOGIC VOLUME (VD) DEAD SPACE VOLUME ALVEOLAR DEAD SPACE VOLUME 7  Ventilation ANATOMIC DEAD SPACE PHYSIOLOGIC VOLUME (VD) DEAD SPACE VOLUME ALVEOLAR DEAD SPACE VOLUME (alveoli without blood flow—may be variable amount depending on physiological/pathological conditions) 8  Contributes to Alveolar Dead Space Volume Anatomic Dead No Blood Space Flow Blood Flow Normal Alveolar Alveolus Dead Space Portion of (no Dead Space) respiratory zone not involved in gas exchange Alveolar Dead Space is caused by Ventilation (V)/Perfusion (Q) inequalities at the alveolar level  Fresh Air. = 500 ml Alveolar Ventilation Rate (VA) 50 ml 50 ml 50 ml 50 ml 50 ml 50 ml Tidal 50 ml 50 ml 50 ml Expired Volume 50 ml 50 ml 50 ml mixed Air (TV) = 500ml 50 ml 50 ml 50 50ml ml (350 ml) 50 50ml ml 50 ml 50 ml 50ml 50 ml 50 ml 50 ml 50 ml Anatomic Dead 50 ml 50 ml 50 ml Air from 50 ml Space(VD) 50 ml 50 ml 50 ml previous 50 ml =150 ml 50 ml 50 ml 50 ml breath 50 50 ml ml (150 ml) 50 50ml ml 50ml 50 ml Alveolar 50ml 50 ml Volume 50ml 50 ml (350 ml) 50 ml 50 ml Pre- End- End- Inspiration Inspiration Inspiration Expiration Alveolar Volume (VA) = Tidal Volume (TV) – Anatomic Dead Space Volume (VD) Alveolar Ventilation = the volume of fresh air introduced into the gas exchanging regions of the lung/min. during normal breathing. Rate (VA) = VA x breaths/min = ?? ml/min  Fresh Air. 50 ml = 500 ml Alveolar Ventilation Rate (VA) 50 ml 50 ml 50 ml 50 ml 50 ml 50 ml 50 ml 50 ml 50 ml 50 ml 50 ml Tidal Fresh Air Volume 50 ml 50 50 ml ml 50 ml (TV) (350 ml) = 500ml 50 ml 50 50ml ml 50 ml 50ml 50 ml 50 ml 50 ml 50ml 50 ml 50 ml 50 ml 50 ml Anatomic Dead 50 ml 50ml 50 ml 50 ml Air from 50 ml Space(VD) 50 ml 50 ml 50 ml previous 50 ml =150 ml 50 ml 50 ml 50 ml breath 50 ml (150 ml) 50 ml 50 ml Alveolar 50 ml Volume 50 ml (350 ml) 50 ml 50 ml Pre- End- End- Inspiration Inspiration Inspiration Expiration Alveolar Volume (VA) = Tidal Volume (TV) – Anatomic Dead Space Volume (VD) Alveolar Ventilation = the volume of fresh air introduced into the gas exchanging regions of the lung/min. during normal breathing. Rate (VA) = VA x breaths/min = ?? ml/min  Minute Ventilation (VE) Rate (VE) = tidal volume (TV) x breathing frequency (f) Alveolar Ventilation (VA) Rate (VA) = tidal volume (TV) – anatomic dead space volume (VD) x breathing frequency (f) Note: normal breathing frequency = 12-15 breaths/min 12 Measuring Dead Space 13 Measuring Dead Space 14 Measuring Dead Space Fowler’s Method 15  Regional Ventilation differences Zone V APEX Zone 2 generally has the 1 Lower standard blood gas values: PaO2 =100 mmHg Hilum PaCO2 = 40 mmHg 2 3 Higher BASE What will be the effect of gravity on breathing mechanics and hence ventilation? Regional Ventilation differences Regional Ventilation differences Apex Base 18 What Happens to Partial Pressures of of O2 and CO2 with Hypo- and Hyper- Ventilation? 19  Some basic definitions Eupnea – normal, natural breathing and ventilation Hypoventilation- lower than normal (insufficient) ventilation Hyperventilation – higher than normal (overbreathing) ventilation Hypercapnia – higher than normal CO2 levels Hypocapnia - lower than normal CO2 levels Hyperpnea – elevated VT and/or rate (frequency) of breathing Hypopnea – decreased VT and/or rate (frequency) of breathing Tachypnea – higher than normal rate of breathing Bradypnea – lower than normal rate of breathing 20  Normal Ventilation ~12 breaths/min 120 PO2 100 Alveolar Gas Partial Pressures Alveolar Partial Pressure Gas % Pressure 80 (Pgas) in mm Hg (mmHg) O2 13 100 = 140 mmHg 60 CO2 H2O 5 6 40 47 N2 76 573 40 Total 10 0 760 PCO2 20 VA = 4.2 L/min 2 3 4 5 6 7 8 9 10 VA (liters/min) 21  Normal Ventilation ~12 breaths/min Hypoventilation e.g. 6 breaths/min 120 PO2 100 Hypoventilation causes PACO2 > 40 mmHg Alveolar Partial Pressure WHY? 80 (Pgas) in mm Hg If PACO2 = 50 60 Then PAO2 = ?? 40 PCO2 20 2 3 4 5 6 7 8 9 10 VA (liters/min) 22  Normal Ventilation ~12 breaths/min Hyperventilation e.g. 30 breaths/min 120 PO2 100 Hyperventilation causes PACO2 < 40 Alveolar Partial Pressure WHY? 80 (Pgas) in mm Hg If PACO2 = 32 60 Then PAO2 = ?? 40 PCO2 20 2 3 4 5 6 7 8 9 10 VA (liters/min) 23  Revisiting the Alveolar Gas Equation PAO2= (FiO2 X (Patm – PH20)) – PaCO2 RespQ What is PAO2 with a normal diet at sea level? Abbreviated equation: PAO2= 150 – [ABG PaCO2/0.8] 24  Anchor Point Gas Values PO2 PCO2 (mmHg) (mmHg) Location Abbreviation Abbreviation Sea Level, No Sea Level, No Pathology Pathology Total Pressure of combined gasses = 760 mmHg at sea level Ambient Air (would be lower at higher altitudes) O2 fraction= 21% Ambient Air atm 160 atm 0 Alveoli PAO2 100 (Calculated from PACO2 40 (prior to exhalation) (Large A is alveolar) Alveolar Gas Equation) Partial pressure of 47 mmHg water vapor Pulmonary capillary PaO2 40 100 PaCO2 arterial Blood (Small a is arterial) (obtained from ABG) Systemic arterial PaO2 95 PaCO2 40 Blood Pulmonary capillary venous blood PVO2 40 PVCO2 45 (single mixture returning from many tissues) 25  Clinical Case Study A 53 year old ALS patient has progressed to the point of muscle weakness resulting in hypoventilation. The PaCO2 is 65 mmHg. What do you predict will be the PAO2? PAO2 = 150 – [ABG PaCO2/0.8] PAO2 = 150 – [65/0.8] PAO2 = 68.75 Should this patient be put on supplemental O2? If so, what should the concentration be to return the PAO2 back to normal? PAO2 = (FiO2 X (Patm – PH20)) – PaCO2 RespQ Note: In clinical practice, 100 = (FiO2 X 713 ) – 81.25 canisters of 100% oxygen are used; the flow rate is 181.25 = (FiO2 X 713 ) adjusted to achieve the desired FiO2 0.254 = 181.25/713 = FiO2 26