Week 10 - Pulmonary Phys and Pharm Flashcards
What is gas exchange in the lungs determined by?
Ventilation and Perfusion
- appropriate matching of these two independent variables
- gases must also be able to DIFFUSE across the alveolar membranes
What structures of the lungs are apart of the conducting airways?
Trachea
Segmental Bronchi
Non-respiratory Subsegmental Bronchi (Bronchioles)
What structures of the lungs are apart of the respiratory unit?
Respiratory Subsegmental Bronchi (Bronchioles)
Alveolar Ducts
What are the components of the respiratory membrane?
- A layer of fluid lining the alveolus that contains surfactant
- The alveolar epithelium, which is composed of thin epithelial cells
- A thin interstitial space between the alveolar epithelium and the capillary membrane
- A capillary basement membrane that fuses in places with the epithelial basement membrane
- The capillary endothelial membrane
What are the three types of alveolar cells?
Type I: squamous cells, compose the monolayered alveolar epithelium and cover 80% of the alveolar surface area (Structure)
Type II: contain lamellar bodies that produce surfactant – decrease surface tension and prevents alveoli from collapsing
Type III: alveolar macrophages
What is the purpose of surfactant and what law does it use?
Secreted by type II alveolar epithelial cells to reduce the surface tension within the alveoli and the work of breathing
*Keeps smaller alveoli from collapsing into larger alveoli
Law of LaPlace (if two bubbles have the same surface tension, the smaller bubble will have higher pressure)
What factors effect the rate of diffusion in the respiratory system?
Fick’s Law = AxDx(P1-P2)/T
T: Thickness of the respiratory membrane – rate of diffusion is inversely proportional to membrane thickness
A: Surface area of the respiratory membrane
D: Diffusion coefficient
*Pressure difference across the respiratory membrane (difference in partial pressure of gas in alveoli and in the blood)
How does edema in the interstitial space and alveoli affect diffusion of respiratory gases? How about fibrosis?
Edema decreases diffusion of respiratory gases
Fibrosis of the lungs can also increase the thickness of some portions of the membrane decreasing diffusion of respiratory gases
How does emphysema affect diffusion of respiratory gases?
Decreases surface area – decreased gas diffusion
- total surface area of the respiratory membrane = ~70 square meters in normal adults
- Emphysema can cause destruction of alveolar walls and cause the total surface area to decrease by as much as five fold
*When decreased to 1/3 to 1/4 normal – exchange of gases is impaired even at rest
What does the diffusion coefficient in respiratory gas diffusion depend on?
Depends on the solubility in the membrane and inversely the square root of its molecular weight (Graham’s law)
*large molecules diffuse slower – small molecules diffuse faster
- CO2 = 20x more soluble than oxygen
- O2 diffuses about 2x as rapidly as nitrogen
- N2O = 19x as diffusible as O2
- N2O = 36x as diffusible as N2
How does the pressure/concentration gradient affect respiratory gas diffusion?
Net diffusion of gases will occur from a high concentration (pressure) area to a low concentration (pressure) area
What is the partial pressure of water vapor at normal body temperature?
47 mmHg
What are the normal alveolar partial pressures on room air?
PO2 = 104 mmHg PCO2 = 40 mmHg PH2O = 47 mmHg PN2 = 569 mmHg
Total body oxygen delivery is the product of what?
O2 content of arterial blood (CaO2) and the rate of delivery of blood to the tissues (CO)
DO2 = CO x CaO2
CaO2 = Hbg x 1.39 x SaO2 + (0.0031 x PaO2)
What is the equation to figure out oxygen consumption?
VO2 = CO x (CaO2 - CvO2)
VO2: total body oxygen consumption
CO: cardiac output
CaO2: oxygen content of arterial blood (hbg x 1.39 x SaO2 + (0.0031 x PaO2)
CvO2: oxygen content of venous blood (hbg x 1.39 x SvO2 + (0.0031 x PvO2)
What is the typical oxygen delivery and oxygen consumption in healthy individuals?
Oxygen delivery (DO2) is typically ~16 mL/kg/min
Oxygen consumption is ~4 mL/kg/min
Therefore, total body oxygen extraction fraction (OEF) is about 25% and returning oxygen (SvO2) is about 65-80%
*cellular O2 utilization is constant
At what oxygen extraction fraction (OEF) does cellular metabolism become anaerobic?
at OEF of 70% cellular metabolism becomes anaerobic and lactic acidosis
Mitochondria will metabolize aerobically at PaO2 > 2 mmHg
What determines oxygen consumption? What increases and decreases it?
It is determined by basal metabolic rate
Increased by fever, thyrotoxicosis, exercise, stress, shivering
Decreased by hypothermia, hypothyroidism, and ANESTHESIA (GA reduces O2 consumption by 10-15%; hypothermia reduces it to about 50% basal metabolic rate at 31*C)
*Estimated by the Brody Equation
What are the causes of hypoxemia (low PaO2) in the OR? (3)
- Low inspired O2
- Hypoventilation
- V:Q mismatch
What is the normal V/Q?
What does a high and a low V/Q mean?
Normal V/Q = 0.8 (Alveolar ventilation/CO – 4-5 L/min)
Low: V/Q = 0 (Absolute Shunt)
- NO ventilation (ex. atelectasis)
- desaturated blood from right heart returns to left heart without being oxygenated
High: V/Q = infinity (Absolute Dead Space)
- NO perfusion (ex. PE, cardiac arrest)
- CO2 can’t be ventilated off (leads to decreased ETCO2 and increased PaCO2)
What are the types of dead space in the pulmonary system?
Apparatus dead space (mechanical)
Airway dead space (anatomic): breath which is in the mouth, pharynx, and tracheobronchial tree but doesn’t enter the alveoli
*decreased by ETT placement but not clinically relevant
Alveolar dead space: portion of breath that enters alveoli that are ventilated and not perfused (West’s zone 1)
How is V:Q mismatch estimated?
Shunt = difference in PAO2 - PaO2 (alveolar oxygenation minus arterial oxygenation)
*normal breathing RA = 5-15 mmHg difference
Dead Space = difference in PaCO2 - PACO2 (ABG CO2 minus ETCO2)
*normal = 2-10 mmHg difference
What are the normal A-a oxygen gradients (AaDO2)?
Room Air: 5-15 mmHg
- progressively increases with age up to 20-30
- AaDO2 in healthy elderly is ~37.5 mmHg
- PaO2 = 102 - (age/3)
- PaO2 range = 60 - 100
On 100% O2: PAO2 - PaO2 < 100 mmHg
What is the effect of increasing FiO2 for treating hypoxemia?
- increasing FiO2 alone may do little to increase PaO2 if the problem is due to absolute right to left shunt (ex. PDA, atelectasis)
- increasing FiO2 should increase PaO2 if the problem is primarily hypoventilation or increasing dead space (ex. PE)
100% FiO2 – absorption atelectasis
Intra-Pulmonary Pressure vs Intra-Pleural Pressure
Intrapulmonary: pressure within the alveoli
-negative with inspiration – positive with expiration
Intrapleural: pressure in the potential space between the inside of the chest wall and the lungs
- lungs recoil inward and chest recoils outward
- ALWAYS negative during normal tidal breathing
- becomes more negative during inspiration and less negative during expiration
- becomes positive during FORCED expiration or during Valsalva maneuver
Intrapleural pressure is more negative in the _____ lung and less negative in the ____ lung.
Intrapleural pressure is more negative in the NON-DEPENDENT lung and less negative in the DEPENDENT lung.
What portion of the lung has better ventilation? Better perfusion?
Apices of the lung have better ventilation in relation to perfusion — Higher V:Q ratio
Lower levels of the lung have better perfusion than ventilation — Lower V:Q ratio
How is interpretation of pulmonary function tests helpful?
Helps in determining causation and severity of pulmonary dysfunction
Define the following pulmonary function test terms: FRC, RV, and VC
FRC: Functional Residual Capacity (2.5 L) – lung volume at end of normal exhalation
RV: Residual Volume (1.25-2.0 L) – lung volume remaining after max exhalation
VC: Vital Capacity (3.5-5.5 L) – max volume of gas that can be exhaled following max inspiration
*Normal lung volumes in a 70kg individual
What do pulmonary function tests depict?
Depiction of forced exhalation of lung gas from TLC to RV measured as a function of time
What are FEV1, FVC, and FEV1/FVC?
FEV1: forced expiratory volume in one second – volume of gas that can be exhaled within one second of beginning a force expiration
-normal = 4 L/sec
FVC: volume of gas that can be exhaled during a forced expiratory maneuver
-normal = 5 L/sec
FEV1/FVC: ratio useful in distinguishing between obstructive and restrictive diseases
- proportion of a person’s vital capacity that they are able to expire in the first second of expiration
- normal = 0.8 or 80%
What is FEF25-75%?
Midmaximal expiratory flow (MMEF)
- rate of flow occurring in a forced expiratory flow from the point where 25% of the FVC has been exhaled to the point where 75% has been exhaled
- best test for assessing small airway disease
*Independent of respiratory effort