MD 2026 Internal Medicine II Respiratory Function PDF

INTERNAL MEDICINE II Measurements of ventilatory function consist DATE: NOVERMBER 8 2024 of: LECTURER: DR. FLORES o Quantification of the gas volume...

INTERNAL MEDICINE II Measurements of ventilatory function consist DATE: NOVERMBER 8 2024 of: LECTURER: DR. FLORES o Quantification of the gas volume contained in the lungs under certain Disturbances Of Respiratory circumstances. o Rate at which gas can be expelled Function from the lungs. Learning Outcomes: Ventilation o Lung Volumes and Capacities o Pulmonary Function Test or Spirometry o Gas Exchange o Arterial Blood Gas o Pulse Oximetry o DLCO o Diagnostic approach to hypoxemia TECHNIQUES TO MEASURE LUNG VOLUMES RESPIRATORY FUNCTION SPIROMETER The primary function of the respiratory system: Measures the volume of air that goes in & out o to oxygenate blood of our lungs. o eliminate carbon dioxide. A machine that measures the volume of air à Require virtual contact between blood that goes in and out of the lungs in known as and fresh air, which facilitates diffusion of spirometer. respiratory gases between blood and gas. This process occurs in the lung alveoli, where HELIUM DILUTION METHOD blood flowing through alveolar wall capillaries A subject repeatedly breathes in and out from is separated from alveolar gas by an extremely a reservoir with known volume of gas thin membrane of flattened endothelial and containing trace amount of helium. epithelial cells, across which respiratory gases Helium is diluted by the gas previously present diffuse and equilibrate. in the lungs and very little is absorbed into the Blood flow through the lung is unidirectional pulmonary circulation. via a continuous vascular path, along which Knowing reservoir volume & initial & final venous blood absorbs oxygen from and loses helium concentration, the volume of gas CO2 to inspired gas. present in the lungs can be calculated. For the respiratory system to succeed in ** The helium dilution method may oxygenating blood and eliminating CO2, it underestimate the volume of gas in the lungs if must: there are bullae in the lungs o ventilate the lung tidally and thus to freshen alveolar gas. o provide for perfusion of the individual alveolus in a manner proportional to its ventilation; and o allow adequate diffusion of respiratory gases between alveolar gas and capillary blood. VENTILATION process whereby the lungs replenish the gas in the alveoli HALOG 1  BODY PLETHYSMOGRAPH SPIROMETER patient sits in a sealed box while panting (records ventilatory event) against closed mouthpiece. Because there is no airflow into or out of the plethysmograph, the pressure changes in the thorax during panting cause compression and rarefaction of gas in the lungs & simultaneous rarefaction & compression of gas in the plethysmograph By measuring the pressure changes in the plethysmograph and at the mouthpiece, the volume of gas in the thorax can be calculated using Boyle’s law BOYLE'S LAW It records the ventilatory event wherein the x Gas law that describes how the pressure of axis is time while the Y axis is volume. a gas tends to increase as the volume of the Inspiration is recorded as upward deflection container decreases. while expiration is recorded as a downward A modern statement of Boyle's law is deflection. The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the LUNG VOLUMES volume it occupies if the temperature and amount of gas remain unchanged within a closed system. BODY PLETHYSMOGRAPHY The amount of air that goes in and out of our lungs per breath is TIDAL VOLUME (NORMAL QUITE BREATHING). o It has 2 reference points namely end inspiration and end expiration. LUNG VOLUMES At end inspiration if we inspire maximally the additional amount of air that goes inside our Tidal Volume (TV): The amount of air that lungs is known as INSPIRATORY RESERVE enters and exits the lungs during normal quiet VOLUME. breathing. Inspiratory Reserve Volume (IRV): The At end expiration, if we expire maximally the additional amount of air that can be inspired additional amount of air that goes out of our after normal tidal inspiration. lungs is known as EXPIRATORY RESERVE Expiratory Reserve Volume (ERV): The VOLUME. additional amount of air that can be expelled The amount of air that remains in our lungs after normal tidal expiration. after maximal expiration is known as RESIDUAL Residual Volume (RV): The air remaining in the lungs after maximal expiration (about 20% of VOLUME. ~20% of TLC Total Lung Capacity - TLC). If we combine 2 or more lung volumes, we come out with CAPACITIES. HALOG 2  LUNG CAPACITIES: BREATHING MANEUVER Inspiratory Capacity (IC): Combination of IRV The Forced Vital Capacity Maneuver is done and TV. during spirometry: Functional Residual Capacity (FRC): o After tidal breathing, the patient is Combination of ERV and RV — the amount of instructed to forcefully inspire to TLC, gas remaining in the lungs after a tidal then forcefully exhale down to RV. expiration. o The maneuver allows measurement of Vital Capacity (VC): Combination of IRV, TV, the Forced Vital Capacity (FVC), and ERV — the amount of gas that can be Forced Expiratory Volume in 1 second exhaled after a maximal inspiration. (FEV1), and the FEV1/FVC ratio. Total Lung Capacity (TLC): Total volume of gas in the lungs after a maximal inspiration (6L for men, 4.2L for women). IRV and TV are known as INSPIRATORY CAPACITY. ERV and RV is known as FUNCTIONAL RESIDUAL CAPACITY. – amount of gas remaining in the lungs after a tidal expiration IRV, TV and ERV is known as VITAL CAPACITY VC – amount of gas that can be exhaled after After tidal breathing we ask the patient to a maximal expiration forcefully inspire to TLC then forcefully expire The combination of all lung volume is known down to RV. Once we the patient is at RV, we as total lung capacity TLC. – 6L M, 4.2L F - ask him to forcefully inspire back to TLC then amount of gas that after a maximal inspiration do several tidal breathing. This maneuver is known as the FORCED VITAL CAPACITY MANEUVER. Thus, if we inspire maximally our lungs contain all the lung volumes which is collectively Basically, from the tracing made during the known as TLC. test we can measure the volume expired from If we expire maximally down to RV, we expire TLC till RV which is known as forced vital our IRV, TV and ERV which is collectively known capacity. as VC. In this tracing the volume expired by the patient is 6 liters. In spirometry, the measured parameter can be classified as a VOLUME or a FLOW PARAMETER. FVC is classified as a volume parameter. HALOG 3  Likewise, since we are recording time, we can measure the volume expired in one second. There are two ways of graphically representing This is known as the FEV1. the forced vital capacity maneuver namely: FEV1 is considered as a flow rate parameter. The time volume curve and the flow volume loop. Since we can directly measure FEV1 & FVC, we can compute for the V1/VC ratio. V1/VC is classified as a flow rate parameter. In the flow volume loop, the breathing PARAMETERS MEASURED IN SPIROMETRY maneuver is recorded in a piece of graphic FVC (Forced Vital Capacity): The total volume paper wherein the X axis is volume and Y axis is exhaled after a maximal inspiration. This is a flow. volume parameter. The expiratory events are recorded in the o Total Volume Expired upper half of the graph while the inspiratory o Volume Parameter events are recorded lower half of the graph. FEV1 (Forced Expiratory Volume in 1 second): The volume exhaled in the first second of FLOW VOLUME LOOP forced exhalation. This is a flow parameter. The Flow Volume Loop records the forced vital o Volume Expired in One Second capacity maneuver with: o Flow Parameter X-axis: Volume FEV1 /FVC Y-axis: Flow o Ratio of FEV1/FVC The expiratory events are recorded in the o Flow Parameter upper half, and the inspiratory events in the lower half of the graph. HALOG 4  If the loop is abnormal, it could either be an obstructive or restrictive ventilatory defect. Question: o In a restrictive ventilatory defect wherein the main manifestation is diminution of lung volume. Which part of the flow volume loop will be affected, the y axis which is flow rate or x axis which is volume. Since the X axis will be affected, the flow volume loop in a restrictive lung defect tends to be thin. In a restrictive lung defect cause by a parenchymal problem or problems in the alveoli or interstium, the loop will tend to be It is quite important that you memorize the thin and tall. normal configuration of a flow volume loop as If the problem is extraparenchymal or you are going to see later, it plays a pivotal pathology in the pleura, neuromuscular or role in the interpretation of the spirometry. thoracic cage, the loop will tend to be narrow Initially, at TLC just before forced expiration, and short. the flow rate is zero. As the subject forcefully expires, a peak flow rate is readily achieved. INTERPRETATION OF FLOW VOLUME LOOP: As the subject continually expires to RV, the Restrictive Lung Defects: flow rate decreases linearly. o Primarily affect lung volume, so the As the subject forcefully inhales back to TLC, loop will appear narrow and tall. the flow rate is most rapid during the mid- o May be due to parenchymal inspiration that is why you have this u problems (e.g., interstitial lung disease) configuration of the inspiratory loop. or extraparenchymal problems (e.g., pleural, thoracic cage, or Normal Loop: neuromuscular issues). At TLC, flow rate is zero. Obstructive Lung Defects: As expiration begins, the flow rate increases to o Primarily affect flow rate, so the loop a peak and decreases as the subject will be shortened. continues to exhale to RV. o Lower obstruction (distal to mainstem During inspiration, the flow rate is most rapid bronchus) will show a scooped-out during mid-inspiration, creating a U-shaped pattern on the expiratory part of the inspiratory loop. loop. o Upper obstruction (proximal to mainstem bronchus) will show flattening of the inspiratory and/or expiratory portions of the loop HALOG 5  If the loop is abnormal, it could either be an obstructive or restrictive ventilatory defect. Question: o In an obstructive ventilatory defect wherein the main manifestation is diminution of flow rate. Which part of the flow volume will be affected, the y axis which is flow rate or x axis which is volume. Since the y axis will be affected, the flow volume loop in an obstructive lung defect tends to be short. If the obstruction is distal to the mainstem bronchus, it is classified as a lower type of obstruction and usually they manifest with a scooped-out pattern of expiratory part of the loop. SPIROMETRY REPORT COMPONENTS: If the obstruction is proximal to the mainstem 1. Demographic Data bronchus, it is classified as an upper type of 2. Measured Parameters: Includes FVC, FEV1, obstruction and usually they manifest with and FEV1/FVC ratio. flattening of inspiratory and or expiratory part 3. Predicted Values: Values a patient is expected of the loop. to generate based on their demographic If the obstruction is permanent, the flattening data. occurs on both part of the loop. 4. Actual Results: Values generated by the If the obstruction is variable, the flattening will patient during the test. depend on whether the obstruction is intra or 5. % Predicted: The actual value divided by the extra thoracic. predicted value, multiplied by 100, indicating If the obstruction is variable intrathoracic, the how the patient’s results compare to flattening will occur during expiration. expected values. If the obstruction is variable extra thoracic, the flattening will occur during inspiration. What are the components of a spirometry report? Basically, there are 2 parts namely: the demographic data and the measured parameters. In the measured parameters portion of the spirometry results, there are usually at least 4 columns. Classification of the parameters. o The first column are the measured parameters. There are several HALOG 6  parameters measured during a test, With regards the FEV1 and FVC, if the % however it is very practical just to predicted is 80% and above, it is considered as consider the 3 parameters we just normal. discussed. With regards the FEV1 over FVC, if the actual o The second column is predicted value is 70% and above, it is considered as values. It is the predicted values that normal. the patient should generate based on In this report, is the FEV1 normal? Yes, because his or her demographic data. the % predicted is 80% and above. o For example for this patient with this In this report, is the FVC normal? Yes, because demographic data, he is expected to the % predicted is 80% and above. generate a FVC of 5 L, FEV1 of 4 L and In this report, is the FEV1 over FVC normal? Yes, a ratio of 80. because the actual value is above 70. o The third column is the actual results. It is the values that the patient generated during the test. Thus, during the test, the patient was able to generate 4.5 L of FVC …. o The fourth column is the % predicted. It just the computed value of the actual over the predicted multiplied by 100. It just tells you that the actual numbers produced by the patient is just blank percentage of the predicted. NORMAL SPIROMETRY CLASSIFICATION: FVC and FEV1: Considered normal if the % predicted is 80% or above. FEV1/FVC ratio: Considered normal if it’s 70% or above. There are several algorithms that can be used in interpreting a spirometry result. This is just one When do we classify the parameter as normal? of them. There are several ways, however the simplest is the percentage system. In this system you just to memorize the numbers 80 and 70. HALOG 7  conducting airways of the lung à Anatomic dead space component 70% : reaches the alveolar zone, mixes rapidly with the gas already there, and can participate in gas exchange DISTURBANCES IN GAS EXCHANGE Respiratory system (lungs and chest wall) MOVEMENT OF GAS ACROSS THE BLOOD-GAS supply oxygen to and remove carbon dioxide INTERFACE from the mixed venous blood entering the is by simple passive diffusion. lungs. The gases travel from an area of high to an area of low partial pressure. RESPIRATION The lungs is well designed for this, having an extremely thin blood gas barrier and a very VENTILATION AND BLOOD FLOW wide area for diffusion. The repetitive movement of gas into and out of the lungs. MEASUREMENT OF GAS EXCHANGE Ventilation delivers the O2 and removes the CO2 that is exchanged across the alveolar- Arterial Blood Gases capillary membrane. o most used measures of gas exchange: PaO2 and PaCO2 GAS EXCHANGE o these partial pressures do not directly The transfer of O2 and CO2 between alveolar measure the quantity of O2 and CO2 gas and pulmonary capillary blood in the blood but the driving pressure of gas in the blood. NORMAL RESPIRATION: o the actual quantity or content of a At rest, a normal individual breathes about 12– gas in blood also depends on 16 times per minute, with each breath having § the solubility of gas in plasma a tidal volume of ~500 mL. § and ability of any component Approximately 30% of the inspired air remains of blood to react with or bind in the conducting airways (anatomic dead the gas of interest. space), while 70% reaches the alveolar zone and mixes with existing alveolar gas. Oxygen Transport in Blood o since hemoglobin is capable of binding large amts of O2, oxygenated A normal individual at rest inspires ~12–16 times Hgb is the primary form in w/c O2 is per minute -à each breath having a tidal transported in blood. volume of ~500 mL o actual content of O2 in blood ~30% of the fresh air inspired with each breath depends on does not reach the alveoli à remains in the § hemoglobin concentration § PaO2. HALOG 8  - PaO2 determines ALVEOLAR-ARTERIAL O2 DIFFERENCE what percentage of to determine the A-a gradient, the Alveolar Hgb is saturated with PO2 (PAO2) must be calculated: O2 based on the PAO2 = [FiO2 x (PB – P H2O)] – (PaCO2/ R) position on the o FiO2: Fraction of inspired O2 (0.21 at oxyHgb dissociation room air) curve o PB: Barometric pressure (approx 760 mmHg at sea level) o P H2O: water vapor pressure (47 mmHg when air is fully saturated at 37 C o R: Respiratory Quotient (ratio of CO2 production to O2 consumption, assumed to be 0.8) ALVEOLAR-ARTERIAL GRADIENT calculated by subtracting measured PaO2 from calculated PAO2. A-aDO2 = PAO2 – PaO2 o In healthy young person breathing room air, the A-A gradient is normally less than 15 mmHg; this value increases with age and may be as high as 30 mm Hg in elderly patients. Elevated Aa gradient = V/Q mismatch, impaired gas diffusion R-to-L shunt Normal gradient = hypoventilation, low FiO2 (altitude) PULSE OXIMETRY Using a probe clipped over a pts finger, it PaO2 calculates oxygen saturation (rather than is the measurement of used to assess the PaO2) based on measurement of absorption effect of respiratory disease on the of 2 wavelengths of light by Hgb in pulsatile oxygenation of arterial blood. cutaneous blood. a useful calculation in the assessment of oxygenation is the alveolar-arterial O2 Problems with use of Pulse oximeter: difference (PAO2-PaO2), commonly called Because oxyHgb dissociation curve becomes the alveolar-arterial O2 gradient or A-a flat above PaO2 of 60 mmHg (SaO2 of 90%), gradient. the oximeter is relatively insensitive to changes A-a gradient - considers the fact that alveolar in PaO2 above this level. and arterial PO2 can change depending on When cutaneous perfusion is decreased ( low the level of alveolar ventilation reflected by cardiac output, or use of vasoconstrictors) the arterial PCO2. signal from the oximeter maybe less reliable or Example: when a patient hyperventilates and even unobtainable has a low PaCO2, PAO2 and PaO2 will rise Other forms of Hgb (carboxyHgb, metHgb) are not distinguishable from oxyHgb when only 2 wavelengths of light are used, thus unreliable in the presence of significant amounts of carboxyHgb and metHgb HALOG 9  1.Insensitivity Above PaO2 of 60 mmHg (SaO2 = 90%): DISEASES Associated WITH DECREASED DLCO: The oxyhemoglobin dissociation curve Interstitial lung disease becomes flat above this PaO2 level, making o scarring of alveolar capillary units the oximeter less sensitive to changes in PaO2 diminishes the area of the alveolar- above this threshold. capillary bed and pulmonary blood volume 2.Decreased Cutaneous Perfusion: In conditions like low cardiac output or use of Emphysema vasoconstrictors, the signal from the oximeter o alveolar walls are destroyed, surface may become unreliable or even area of alveolar-capillary bed is unobtainable. diminished. 3.Other Forms of Hemoglobin: Pulmo vascular disease (recurrent pulmo Hemoglobin derivatives such as emboli, primary pulmonary HPN) carboxyhemoglobin and methemoglobin are o decrease in x-sectional area & volume not distinguishable from oxyhemoglobin when of the pulmonary vascular bed only two wavelengths of light are used, making the oximeter unreliable in their DLCO MAY BE ELEVATED IN: presence. increased pulmonary blood volume (CHF) alveolar hemorrhage (Goodpasture’s DIFFUSING CAPACITY of Lung for Carbon monoxide syndrome) (DLCO) assess the ability of gas to diffuse across the PROCESSES THAT IMPAIR GAS EXCHANGE alveolar-capillary membrane. Hypoventilation DLCO PROCEDURE Diffusion limitation a small concentration of CO (0.3%) is inhaled, Shunt in a single breath that is held approx 10 sec. Ventilation-perfusion inequality the CO is diluted by the gas present in the alveoli & is taken up by Hgb as the RBC course DIAGNOSTIC APPROACH TO PATIENT WITH HYPOXEMIA thru the pulmonary capillary system. concentration of CO in exhaled gas is measured & DLCO is calculated as the qty of CO absorbed per min per mmHg pressure gradient from alveoli to the pulmonary capillaries. FACTORS AFFECTING DLCO Alveolar-capillary surface area available for gas exchange Thickness of alveolar-capillary membrane Degree of V/Q mismatching Patient’s Hemoglobin level HALOG 10