RSPT 1060 MODULE C Applied Physics Lesson #1 - Mechanics OBJECTIVES At the end of this module, the student should be able to define the terms and abbreviations used in the module. draw & explain the equation of motion. list the forces that oppose lung inflation & cause the work of breathing. list the different types of compliance, their normal values & equations. OBJECTIVES At the end of this module, the student should be able to differentiate between compliance & elastance. list some pulmonary disorders that could change compliance. explain the relationship between pressure, surface tension & radius. explain how LaPlace s Law relates to surface tension. OBJECTIVES At the end of this module, the student should be able to explain the purpose of surfactant. list the different types of resistance, their normal values and equations. explain how Poiseuille s Law relates to resistance. list some pulmonary diseases that will alters resistance. explain what the equal pressure point is. Ventilation vs. Respiration Ventilation: The bulk movement of gas in and out of the lung. Respiration: The exchange of gas (specifically oxygen and carbon dioxide) at the cellular level. Internal Respiration: The exchange of gas between a peripheral capillary and a cell of the body. External Respiration: the exchange of gas across the alveolar-capillary membrane. EQUATION OF MOTION Pressure to Inflate Volume STRETCH Resistance Flow FRICTION 1
Equation of Motion Mechanical positive pressure to inflate Mechanical Ventilator vs. Spontaneous Breathing Ventilator-Pump Thorax-Diaphragm Pump Resistance (gas flow) Pressure Contraction P MUS Muscle negative pressure to inflate (volume) Inflation pressure Spontaneous Breathing Contraction of muscles generates a negative pressure in lungs & gas is pulled into lungs. Work done by patient. Mechanical Ventilator Positive pressure builds in the ventilator circuit & gas is pushed into the lungs. Work done by machine. WORK OF BREATHING FORCES OPPOSING INFLATION Elastic (stretch) Physical tendency of an object to resist stretching Non-elastic (friction) Occurs only when gas and the system is moving Work of Breathing Resistance Elastic Work (Stretch) Non-elastic Work (Friction) 65% 35% Lungs & Chest Wall Tissue & Airway (20%) (80%) Surfactant Tissue Skeletal Lungs Ribs Airways Lungs Muscular Diaphragm Pleura Skin Abd. organs Gas flow COMPLIANCE ELASTIC OPPOSITION TO INFLATION Elastic and collagen fibers found in lung parenchyma give the lungs elasticity. Inflation - occurs as a result of forcibly stretching lung fibers during inspiration. (work) Deflation or exhalation is normally passive. The resting position of the lung is deflation. 2
Pressure-Volume loop Pressure-Volume loop High High volume (over-inflated) Vol Normal Vol Normal volume (filling) Low compliance Low volume (opening) Pressure Pressure Balloon EXPERIMENT : Distensibility of the lung Initial inflation easy or difficult? Normal inflation easy or difficult? Elastance: Property of resisting deformation or desire to return to original shape Over inflation easy or difficult? 1 Elastance Comparison Example: Tennis ball vs. Balloon Tennis ball High elastance & low compliance High resistance to change in shape Low ability to stretch Balloon Low elastance & high compliance Low resistance to change in shape High ability to stretch Comparison Normal lung vs. Emphysema vs. Pneumonia Normal lung Normal elastance & Normal compliance Returns to original shape easily Easily filled Emphysema Low elastance & high compliance Does not return to original shape easily (floppy) Easily stretched until air-trapping occurs Pneumonia High elastance and low compliance Readily returns to collapsed state Very difficult to inflate 3
Lung Disease states that cause a change in lung compliance? = Volume (liters) Pressure pl (cmh 2 O) Total is composed of: Lung Chest Wall Decrease Fibrosis Adult Respiratory Distress Syndrome Pulmonary Edema Increase Emphysema Lung How will a patient with decreased lung compliance breathe? Rapid Shallow Experiment Wrap belt tightly around chest. Breathe slow & deep Breath rapid & shallow Which feels better? Chest Wall Stiffness of chest wall When thorax is intact, its resting level is FRC With disruption to chest wall lung collapse and chest wall expands (open pneumothorax) Force of movement of chest wall is opposite that of lung Chest Wall has tendency to expand (pull out) Lung has tendency to collapse (pull in) At rest they balance (FRC) Chest Wall Disease states that cause a change in chest wall compliance? Decrease Chest trauma Chest burns Kyphosis Scoliosis Chest wall deformity 4
Total Normal Lung compliance = 0.2 L/cmH 2 O Normal Chest wall compliance = 0.2 L/cmH 2 O Normal Total compliance = 0.1 L/cmH 2 O Calculation of Total Clinically we measure Dynamic and Static Lung Why is total less than the individual compliances? Lungs at rest = collapse Chest wall at rest = expansion They are working in opposite directions C DYN C STAT Tidal Volume {V t } (liters) With air = movement Peak Pressure PEEP (cmh 2 O) No air Tidal Volume {V t } (liters) = movement Plateau Pressure PEEP (cmh 2 O) NOTE: PEEP stands for Positive End Expiratory Pressure. It is the BASELINE or starting pressure. Calculations Calculations Normal compliance = 0.1 Liter/cmH 2 O Low compliance = 0.05 Liter/cmH 2 O Volume Pressure = 0.5 liters = 0.1 L/cmH 2 O 5 cmh 2 O Volume Pressure = 0.5 liters = 0.05 L/cmH 2 O 10 cmh 2 O Stiffer lung needs more inflation pressure. Calculations SURFACE TENSION High compliance = 0.17 Liter/cmH 2 O The alveoli are like bubbles lined with fluid and filled with air. Volume = 0.5 liters = 0. 17 L/cmH 2 O Pressure 3 cmh 2 O Floppier lung needs less inflation pressure. Surface tension is the attractive force exerted by like molecules at the liquid s surface. Surface tension forces cause the bubble to collapse. 5
LaPlace s Law The Force of Surface Tension in a drop of liquid. Cohesive force (arrows) attracts molecules inside the drop to one another. Cohesion can pull the outermost molecules inward only, creating a centrally directed force that tends to contract the liquid into a sphere. Two bubbles of different sizes with the same surface tension. Bubble A, with the smaller radius, has the greater inward or deflating pressure and is more prone to collapse than the larger bubble B. Because the two bubbles are connected, bubble A would tend to deflate and empty into bubble B. Conversely, because of bubble A's greater surface tension, it would be harder to inflate than bubble B. Calculation of Surface Tension LaPlace s formula: Pressure in a bubble = 4ST r ST = surface tension P If surface tension increases, the pressure to inflate the bubble increases. R = radius If the radius decreases, the pressure to open the alveoli increases. P Surfactant Alveoli are lined with a surface-tension lowering agent (surfactant) produced by alveolar type II cells. Surfactant has a low attractive force exerted by its molecules. Surfactant helps stabilize the alveoli so they do not collapse completely on each exhalation. Surface Tension Disorders altering or destroying surfactant: Prematurity Adult respiratory Distress Syndrome Oxygen toxicity Destruction of surfactant will significantly decrease compliance and increase the work of breathing. 6
RESISTANCE Tissue Resistance INELASTIC OPPOSITION TO INFLATION Occurs only when the system is in motion and air is moving. (friction) Tissue Viscous Resistance Airway Resistance Tissue Viscous Resistance (20%) Things that increase tissue resistance: Obesity Fibrosis Abdominal distention Airway Resistance Poiseuille s law Airway Resistance (80%) Mainly in upper airway Only 20% in small airways (less than 2 mm) Things that increase airway resistance: High gas flow Turbulent gas flow Narrow airway Long airway Viscous gases Pressure = ŋ 8 l V r 4 ŋ = gas viscosity V = gas flow Pressure increases with increased tube length and gas viscosity. Pressure increases with decreased radius l = tube length r = tube radius 7
Poiseuille s law Reducing the radius of a tube by ½ requires an increase in pressure 16 fold to maintain the same speed of gas flow through the tube. Poiseuille s law Egan: Rule of Thumb page 215 Pressure = 1 cmh 2 O Pressure = 16 cmh2o Mini Clini page 215 Airway Size If a smaller radius causes increased resistance then why is resistance less in the smaller airways? More cross section Slower flow Laminar flow Large airways have less cross section & higher more turbulent flows thus more resistance. Increased resistance Decreased resistance Calculation of Airway Resistance Calculations Pressure (cmh R 2 O) AW = X 60 Flow (Liters/min.) Normal Resistance = 0.5 2.5 cmh 2 O/L/sec Pressure = 10cmH 2 O = 2 cmh 2 O/L/sec Flow 5 L/sec 8
Calculations Increased Resistance = 4 cmh 2 O/L/sec Pressure = 20 cmh 2 O = 4 cmh 2 O/L/sec Flow 5 L/sec Narrower airways require more pressure. Resistance Diseases that cause an increase in airway resistance Asthma Emphysema Excessive sputum production Tumors Things that decrease airway resistance Bronchodilators Anti-inflammatory agents Resistance How will a patient with increased airway resistance breathe? Experiment Breathe through a straw or coffee stirrer Slow Deep Breathe slow & deep Breath rapid & shallow Which feels better? EQUAL PRESSURE POINT See Egan page 217 Figure 9-9 Point where the pressure within the airway is equal to the pressure outside the airway. The airway will collapse downstream from the EPP. 9
EPP Airway caliber is determined by: Anatomical support from cartilage and traction by tissues Pressure differences across their walls Inside pressure > outside = open Inside pressure < outside = collapse During forced exhalation, outside pleural pressure can become higher than inside airway pressure and airways collapses. EPP & Emphysema Low compliance found in emphysema results in less recoil pressure and less pressure inside the airway. Airways collapse sooner and more gas is trapped as EPP moves upstream toward smaller airways. (increased resistance) Encourage slow deep inspiration and slow exhalation through pursed lips. Air Trapping Assignments READ: Egan Chapter 6 Surface tension - pgs. 99-100 Chapter 10 Mechanics of Ventilation - pgs. 216-224 Do Sibberson Math Book Chapter 13 First Third sample sets Practice exercises pgs. 160 171 FIG 28-3, page 609 Look to objectives 10