Ventilation Mechanics PDF

VENTILATION Tamethia Perkins MS, RRT-NPS, RRT-ACCS RT 3005/6005  Introduction ◼ VENTILATION ◼ Gas exchange between external environment and alveoli. ◼ O2 vs. CO2  Pressure Differences Across the Lungs ◼ Driving P: P difference between 2...

VENTILATION Tamethia Perkins MS, RRT-NPS, RRT-ACCS RT 3005/6005  Introduction ◼ VENTILATION ◼ Gas exchange between external environment and alveoli. ◼ O2 vs. CO2  Pressure Differences Across the Lungs ◼ Driving P: P difference between 2 points. ◼ PIP= 30 cmH20 ◼ PEEP= 5 cmH20 ◼ Driving P= 25 cmH20  Transairway Pressure (Pta) ◼ P difference between the mouth P and the alveolar P. ◼ Pta = Pm – Palv ◼ Pm= 760 mmHg Palv= 757 mmHg ◼ Pta= 3 mmHg 2-1 PTA 2-1 PTA Transairway P  Transpulmonary Pressure (Ptp) ◼ Ptp = Palv – Ppl ◼ Ppl=755 mmHg Palv= 760 mmHg ◼ Ptp = 5 mmHg 2-2 and 2-3 PTT 2-2 and 2-3 PTT Transpulmonary P  Transthoracic Pressure (Ptt) ◼ Ptt = Palv – Pbs (body surface P) ◼ Palv=757 mmHg Pbs= 760 mmHg ◼ Ptt = -3 mmHg (inspiration) Tranthoracic P 2-5 diaphragm Role of the Diaphragm ◼ P gradient is generated by contraction and relaxation of the diaphragm. ◼ Inspiration: contract = low Ppl and Palv. ◼ End inspiration: equilibrium. No P. ◼ Expiration: upward movement = low thoracic volume = high Ppl and Palv. ◼ End expiration : Ppl = Pm. ◼ Ppl is always negative in normal breathing.  Diaphragmatic 2-5 diaphragm Function 2-5 diaphragm Normal Values ◼ Normal diaphragmatic excursion: ~1.5 cm. ◼ Deep inspiration: 6-10 cm. ◼ N intrapleural P change: 3-6 cmH2O. ◼ Deep inspiration: - 50 cmH2O. ◼ Deep exhalation: + 70-100 cmH2O.  Static Mechanics ◼ STATIC: study of matter at rest. ◼ Lungs: natural tendency to collapse. ◼ Chest: natural tendency to expand. ◼ Lungs at resting volume (FRC). ◼ Recoil force of the lung= distending force of the chest wall. ◼ Static Forces of the Lung: ◼ Elastic properties ◼ Surface tension  Elastic Properties ◼ LUNG COMPLIANCE: change in volume per unit pressure change. ◼ CL= ∆V (L)/ ∆P (cmH2O). ◼ CL= L/cm H2O. ◼ How much air the lungs (L) will accommodate for each cm H2O P change. ◼ Ppl= -5 cm H2O during inspiration. ◼ Lungs accept.75 L of gas … CL = ◼ CL=.75 L/5cm H2O =.15 L/cm H2O.  Compliance Curve CL normal= 0.1 L/cm H2O. Compliance Changes  Overdistension With little or no change in VT Normal Abnormal Volume (ml) Pressure (cm H2O) Paw rises  Dynamic and Static Compliance ◼ Static ◼ Plateau pressure need Normal 70-100 ml/cm H2O ◼ Dynamic ◼ Mix of compliance and resistance Normal 50-80 ml/cm H2O  Resistance ◼ Opposition to the flow of gases through the airways ◼ Normal ◼ 0.5-3.0 cmH2O/L/sec ◼ Factors affecting Raw ◼ Airway length ◼ Radius ◼ Flow rate  Hooke’s law ◼ ELASTANCE: natural tendency to respond to force and to return to its original resting position or shape. ◼ Elastance= ∆ P/ ∆ V. ◼ Elastance is reciprocal to Compliance. ◼ Lungs with High C have low elastance and Vice versa ◼ HOOKE’s LAW: if 1 unit of force (P) is applied to an elastic body it will stretch 1 unit of length (V)… ◼ Normal lung functional range: if more force is applied = rupture. Hooke’s Law Hooke’s Law Application Pneumothorax  Surface Tension ◼ Liquid molecules surrounded by liquid molecules are mutually attracted to each 2-10 surface tension other moving freely in all directions. ◼ In a liquid-gas interface, the liquid molecules at the surface are attracted to the liquid molecules within the liquid mass. ◼ This cohesive force is called surface tension. ◼ Maintains the shape of a water droplet. 2-10 surface tension Surface Tension  Surface Tension ◼ Surface tension is measured in dynes/cm. ◼ 1 dyne/cm: force necessary to cause a tear 1cm long in the surface layer of a liquid. ◼ The liquid alveolar lining can exert forces in excess of 70 dynes/cm = complete alveolar collapse. 2-11 Laplace Laplace’s Law ◼ The distending P of a liquid bubble is influenced by: ◼ Surface tension= directly proportional. ◼ Size of the bubble= inversely proportional. ◼ P= 4ST/r ◼ It takes more P to hold a bubble open if ST increases or r decreases. 2-11 Laplace Laplace’s Law ST  Laplace’s Law 2-12 and 2-13 ST ◼ When 2 different size bubbles with same ST are in direct communication, the greater P in the smaller bubble will cause the smaller bubble to empty into the larger bubble. ◼ Laplace’s law does not state that the surface tension varies with the size of the bubble. ◼ Distending pressure—not the surface tension—that varies inversely with the radius. ◼ Surface tension remains the same until the size of the bubble goes beyond its natural elastic limit and ruptures 2-12 and 2-13 ST ST  Critical Opening P ◼ Laplace’s law does not apply until critical opening P is reached. ◼ High P with little V change. ◼ Similar to the initial P required to blow up a new balloon. ◼ Once critical opening P is reached, distending P progressively decreases as the bubble increases its radius ST  Laplace’s Law and Alveolar Fluid Lining ◼ According to Laplace’s law, a high Ptp must be generated to keep the small alveoli open. ◼ Offset by the pulmonary surfactant. Pulmonary Surfactant ◼ Phospholipid (DPPC). ◼ Type II cell. ◼ Hydrophobic (gas phase) and hydrophilic (liquid phase) ends. ◼ Decreases ST in proportion to the ratio of surfactant to alveolar surface area. ◼ Large alveolus= high ST ◼ Small alveolus= low ST 2-16 surface tension Surface Tension Pulmonary Surfactant ◼ ST of average alveolus= 1-5 dynes/cm (small) to 50 dynes/cm (fully distended). ◼ In the absence of surfactant, the ST in the alveolus reaches 50 dynes/cm, distending P to overcome liquid phase is high. ◼ If distending P is below critical opening= ◼ Liquid walls of the alveolus come in contact with one another resisting re-expansion. ◼ Condition called = atelectasis. Surface Tension PV Surface Tension