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Chapter 43 Physiology of Ventilatory Support

Chapter 43 Physiology of Ventilatory Support. Learning Objectives. Discuss the pressures and pressure gradients that affect gas delivery during spontaneous breathing and negative and positive pressure ventilation

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Chapter 43 Physiology of Ventilatory Support

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  1. Chapter 43Physiology of Ventilatory Support

  2. Learning Objectives • Discuss the pressures and pressure gradients that affect gas delivery during spontaneous breathing and negative and positive pressure ventilation • Discuss are the effects of mechanical ventilation on ventilation, oxygenation, and lung mechanics. • Discuss the currently available modes of mechanical ventilation • Discuss the indications and physiologic effect of PEEP

  3. Learning Objectives (cont.) • Describe the cardiovascular effects of positive and negative pressure ventilation • Describe the effects of positive pressure ventilation on other body systems. • Describe and list the complications and hazards of providing mechanical ventilatory support. • Discuss how to minimize the adverse effects of mechanical ventilation.

  4. Pressure & Pressure Gradients • Gas flows only when pressure gradient exists • To understand pulmonary gas flow, must understand: • Pressure at airway opening (Pawo) • Alveolar pressure (Palv) • Intrapleural pressure (Ppl) • Atmospheric or pressure at body surface (Pbs) • See Figure 33-1 • Review Chapter 10 on normal ventilation

  5. Pressure, Volumes, & Flows During Negative-Pressure MV Mimics spontaneous ventilation: creates negative pressure at chest wall at beginning of inspiration Pbs transmitted to pleura (⇓Ppl) and alveoli (⇓Palv) As Pawo is atmospheric, a pressure gradient between Pawo and Palv causes gas flow into lungs Expiration is passive during spontaneous, negative pressure, & positive pressure ventilation Lung recoil creates positive Palv compared to Pawo, creating gradient for gas flow out of lungs See Figure 43-4

  6. Pressure, Volumes, & Flows During Negative-Pressure MV

  7. Pressure, Volumes, & Flows During Positive-Pressure MV Inspiration occurs due to positive Pawo creating gradient compared to Palv, causing gas to enter lung Entering gas increases airway pressure and volume, expanding lungs Ppl often becomes positive as compressed between expanding lung and chest wall Positive pressures exist in thorax from beginning of inspiration until near end of exhalation Quite different than normal ventilation See Figure 43-4

  8. Pressure, Volumes, & Flows During Negative-Pressure MV

  9. Ventilation Minute ventilation (VE): VE = VT f Indication for MV is acute hypercapnic respiratory failure, aka ventilatory failure Increased PaCO2 with decreased pH Assumption that MV will improve VE and thus VA by which PaCO2 will be normalized Accomplished by increasing VT, f, or both . . . .

  10. After obtaining an arterial blood gas from a patient who is on MV, the respiratory therapist determines the patient is hypercapnic. What ventilator settings changes can the therapist recommend to normalize the PaCO2? • Decrease VT, respiratory rate or both • Increase VT, respiratory rate or both • Increase Positive End Expiratory Pressure • Increase the FIO2

  11. Effects of Mechanical Ventilation Alveolar ventilation (VA): (VA = VCO2 0.863)/PaCO2 As PaCO2 is inversely related to VA, improved VA will decrease PaCO2 & vice versa ⇓VA or ⇑VCO2 may cause an ⇑PaCO2 and acute ventilatory failure Increased V/Q mismatch PPV increases ventilation to nondependent regions Blood flow greatest to dependent regions Therefore, PPV tends to worsen V/Q and contribute to & increased P(A – a)O2 See Figure 43-5 . . . . . . . . .

  12. Effects of Mechanical Ventilation

  13. Effects of Mechanical Ventilation Effects on Alveolar & arterial carbon dioxide Normally PACO2 & PaCO2 equal 40 mm Hg as equilibration occurs across A/C membrane ⇑VCO2 or ⇓VA may cause ⇑ PaCO2 ⇑VD/VT can also ⇑PaCO2 Occurs due to excessive PEEP, pulmonary embolus, or emphysema Results in high V/Q ratio as all ventilation in excess of perfusion is wasted . . . .

  14. Effects of Mechanical Ventilation Changes in acid-base balance Respiratory acidemia (PaCO2 > 45, pH < 7.35) indicates inadequate VA: increase VE to correct Patient anxiety may cause ventilator asynchrony Can occur with a small VT and ⇑VD/VT Respiratory alkalemia (PaCO2 < 35, pH > 7.45) Indicates VA is too high, decrease VE to correct Caused by anxiety, pain, or inappropriate set VE Changes in acid-base status impact electrolyte balance and O2Hb dissociation curve, which affect loading and unloading of O2 . . . . .

  15. A patient in the ED has experienced an increase in ventilatory dead space. Which of the following can occur with this patient? • Increased PaO2 • Increased PaCO2 • Decreased PaCO2 • Increased FIO2

  16. Effects of Mechanical Ventilation on Oxygenation Increased Inspired Oxygen Mechanical ventilators deliver increased inspired oxygen concentration (Fio2) Increase Fio2 may restore PAO2 and PaO2 to normal with appropriate management

  17. Effects of Mechanical Ventilation on Oxygenation (cont.) FIO2 available: 0.21–1.0 May restore PAO2 and PaO2 to normal, particularly if hypoxemia is caused by V/Q mismatch Hypoxemia caused by diffusion defect or shunt responds best to PEEP and increased FIO2 Hypoventilation induced hypoxemia improves with MV as it increases VA . . .

  18. Effects of Mechanical Ventilation on Oxygenation (cont.) Alveolar oxygen & alveolar air equation Increasing FIO2 increases PAO2 predictably: PAO2 = FIO2 (PB – 47) – PaCO2 (FIO2 + (1 – FIO2/R) Even with changes in PaCO2 and R, PAO2 changes proportionally to FIO2 How PaO2 responds to changes in FIO2 depends on pathology that has caused it to fall

  19. Effects of Mechanical Ventilation on Oxygenation (cont.) Arterial oxygenation & oxygen content (CaO2) Fick’s law of diffusion: Diffusion = [A  D  (P1 – P2)]/T where A = surface area, D = diffusion coefficient, P1 = PAO2, P2 = PcO2 or capillary PO2, T = tissue thickness. If PAO2 is low (P1), decreased gradient and low PaO2 ⇑VA and ⇑FIO2 reestablishes PaO2 Need PEEP and ⇑FIO2 reestablishes PaO2 Diffusion defects: Increased tissue thickness (T) ALI increases T & decreases A

  20. Effects of Mechanical Ventilation on Oxygenation (cont.) Arterial oxygenation & oxygen content (CaO2) CaO2 determined by arterial oxygenation & [Hb] as: CaO2 = (1.34  Hb  SaO2) + (PaO2  0.003) where 1.34 = maximum oxygen a mg of Hb can carry, SaO2 = O2 saturation of Hb, 0.003 = ml of O2 dissolved in plasma for each mm Hg of PaO2 Normal ~19.8 ml O2/dl CaO2 alters with changes in Hb, SaO2, or PaO2

  21. Effects of Mechanical Ventilation on Oxygenation (cont.) PEEP/CPAP is required to decrease shunt Helps maintain, open, and stabilize alveoli, increasing surface area for gas exchange Improves arterial oxygenation Use carefully- may overdistend alveoli, sending blood to poorly ventilated alveoli worsening shunt Tissue oxygen delivery (DO2) DO2 = CaO2 CO (normal 1000 ml/min) Mechanical ventilation may impede venous return, so increased CaO2 can be offset by decreased CO

  22. The physician has informed you that a ventilated patient has a severe shunt due to widespread atelectasis. In an effort to better oxygenate this patient, what should the respiratory therapist recommend? • Decrease PEEP • Increase PEEP • Decrease FIO2 • Increase FIO2

  23. Effects of Positive-Pressure Mechanical Ventilation on Lung Mechanics Increased pressure Peak inspiratory pressure (PIP) is pressure necessary to overcome airway resistance & lung & chest wall compliance Pplat is static Palv at peak inspiration With square flow waveform, difference between Pplat & PIP is due to Raw Raw = (PIP – Pplat)/flow Cstatic = VT/(Pplat – PEEP) common measurement of pulmonary compliance Tracks changing lung conditions Pplat should be kept <30 cm H2O

  24. Effects of Positive-Pressure Mechanical Ventilation on Lung Mechanics (cont.) Mean airway pressure (see Box 43-1) Average pressure across respiratory cycle Any changes in settings, PIP, VT, etc. alter MAP Most closely tied to oxygenation, direct relationship Effect of peak airway pressure on lung recruitment ⇑PIP recruits previously closed alveoli PEEP then maintains or keeps alveoli open Extrinsic PEEP controlled by PEEP control Intrinsic (auto) PEEP results from gas trapping Breath starts during ongoing expiratory flow.

  25. Effects of Positive-Pressure Mechanical Ventilation on Lung Mechanics (cont.) Tidal volume PCV, VT varies inversely with changes in Cstatic & often Raw If inspiratory time is sufficient, changes in Raw will not alter VT but will shorten period of no flow Functional residual capacity (FRC) Varies directly with PEEP level In restrictive disease as PEEP and FRC increases lung compliance increases (⇑Cstatic) until lung overdistention occurs

  26. Effects of Positive-Pressure Mechanical Ventilation on Lung Mechanics (cont.) P/V curve and lung recruitment in ARDS Set PEEP 2 cm H2O above lower Pflex Avoid hitting upper Pflex as indicates overdistention Note that Pflex points are often difficult to identify

  27. Effects of Positive-Pressure Mechanical Ventilation on Lung Mechanics (cont.) P/V curve and lung recruitment in ARDS Lung recruitment (RM) has proved very useful Commonly 40–50 cm H2O applied over <3 minutes Recent approach to setting PEEP Decremental PEEP trial following RM Best PEEP where highest compliance is noted Recruit lung again, then set PEEP at best level Decreased work of breathing (WOB) (see Figure 43-8) Properly set MV should alleviate or lessen WOB Continuum from full support (A/C) to little (CPAP)

  28. Effects of Positive-Pressure Mechanical Ventilation on Lung Mechanics (cont.)

  29. After increasing the PEEP for a mechanically ventilated patient who appears to have a severe shunt, which of the following has been directly affected by this increase? • VT • IRV • IC • FRC

  30. Minimizing Adverse Pulmonary Effects of PPMV Decreasing pressure Attempt to keep pressures as low as possible PIP < 40 cm H2O, Pplat < 30 cm H2O Use of diuretics, bronchodilators, inotropes may help to lower ventilating pressures May need to decrease VT to keep pressures low High MAP can impede venous return, reducing CO. Reduce by decreasing TI, VT, f, PEEP, or PIP

  31. Minimizing Adverse Pulmonary Effects of PPMV (cont.) PEEP/CPAP Used to improve oxygenation, particularly with refractory hypoxemia (PaO2 < 50–60, FIO2 > 0.5) Maintains open alveoli, increasing FRC, reducing shunt, often improving Cstatic & PaO2/FIO2 ratio Allows FIO2 to be reduced while maintaining PaO2

  32. Minimizing Adverse Pulmonary Effects of PPMV (cont.)

  33. Minimizing Adverse Pulmonary Effects of PPMV (cont.) Effects of ventilatory pattern (flow waveform) Square waveform typical of VCV Decelerating waveform typical of PCV Compared to square waveform ⇓PIP, ⇓Pplat, ⇓WOB, ⇓VD/VT, ⇓PaCO2, ⇑PaO2, and ⇑MAP PPV low flows can cause gas trapping and patientventilator asynchrony (especially in COPD, asthma). Minimize risk with higher flows and shorter TI, to allow more time for exhalation

  34. Physiologic Effects of Ventilatory Modes Volume-Control vs. Pressure Control Primary variable we wish to control is patient’s minute ventilation Continuous Mandatory Ventilation-CMV provides all breaths, even patient triggered, as mandatory breaths Mandatory breaths may be Volume Control-VC or Pressure Control-PC

  35. Physiologic Effects of Ventilatory Modes Volume-controlled modes VC-CMV provides all breaths, even patient triggered, as mandatory breaths. Breaths at set VT but pressures will vary High MAP results For true control patients must be sedated Problem is muscle atrophy Must set trigger and flows to meet patients needs otherwise patient-ventilator asynchrony occurs Graphics reflect needed adjustments

  36. Physiologic Effects of Ventilatory Modes (cont.) Volume-controlled modes VC-SIMV used for patients capable of providing some ventilation, the mandatory breaths will be at set VT all other breaths are spontaneous Advantages Inspiratory muscle usage avoids atrophy ⇓MAP Reduced risk of cardiovascular compromise

  37. Physiologic Effects of Ventilatory Modes (cont.) Volume-controlled modes VT delivered to patient is less than set because Pressure compresses gas, occupy less volume Ventilator circuits are compliant so under pressure they expand, which takes volume Not critical unless ventilating with small VT Minimized by using Low compliance tubing, low-volume humidifier Compensating for lost volume Adjust manually or use modern ventilator, which compensates for tubing compliance

  38. Physiologic Effects of Ventilatory Modes (cont.) Leaks in volume ventilation Noted two methods Inspired VT significantly greater than expired VT An abrupt, above baseline end to expired volume as seen on volume-time scalar

  39. Physiologic Effects of Ventilatory Modes (cont. )

  40. The respiratory therapist has intubated and placed a patient in the ICU on MV. Which of the following is the primary variable the therapist wants to initially control? • ICP • Minute ventilation • Arterial pressure • Mean airway pressure

  41. Physiologic Effects of Ventilatory Modes (cont.) Pressure-controlled modes PC-CMV, PC-SIMV, PSV, APRV, BiPAP, etc. All PC modes feature Patient or time triggered to inspiration Pressure limited inspiration Decelerating flow waveform Time or flow cycled to expiration Benefits include limitation of Palv and improved patient ventilator synchrony, possibly improves distribution of ventilation, oxygenation, and gas mixing

  42. Physiologic Effects of Ventilatory Modes (cont.) PC-CMV May reduce PIPs and Palv Each breath has preset pressure, so constant PIP Volume & flow will vary with Cstatic and Raw Commonly used to treat ALI/ARDS patients May include lung protective strategy with permissive hypercapnia CO2 allowed to rise gradually with pH >7.2 Thought to reduce risk of further lung injury

  43. Physiologic Effects of Ventilatory Modes (cont.) PC-IRV PC-CMV may be used by setting I:E >1:1 MAP increases proportional with I:E Suggested to treat severe hypoxemia when high FIO2 and high PEEP fail as in ALI/ARDS Short time constants associated with ALI/ARDS so less expiratory time required Depending on degree of IRV may cause auto-PEEP If applied PEEP in conventional ventilation is equal to total PEEP in IRV, oxygenation will be equivalent

  44. A patient in the ED has been placed on PC-CMV. Which of the following variable is dependent on the patient’s lung compliance and airway resistance? • Pressure • Inspiratory Time • Tidal Volume • FIO2

  45. Physiologic Effects of Ventilatory Modes (cont.) APRV Variation on PC-IRV Patient breaths spontaneously at high and low levels of CPAP, spontaneous effort may be PSV supported Most time is spent at high CPAP level with brief periods of release (<1.5 seconds) to low CPAP During release patient exhales passively When pressure is restored to high CPAP, it causes a mechanical inspiration Often fewer hemodynamic side effects than PC-IRV Less sedation may be required

  46. Physiologic Effects of Ventilatory Modes (cont.)

  47. Physiologic Effects of Ventilatory Modes (cont.) PSV-Pressure Support Ventilation Patient triggered, pressure limited, flow cycled Flow cycle may be factory preset or adjustable Minimal levels set to overcome Raw caused by ETT, secretions, bronchospasm, imposed WOB Patient has control over f, TI, and flow VT is determined by PSV level, patient effort, Cst, Raw May decrease f, WOB, VO2, and increase VT Patients generally prefer to other modes. .

  48. Physiologic Effects of Ventilatory Modes (cont.) VA-PSV (Volume Assured-PSV) Dual control mode Patient initiates all breaths, which most resembles VC with constant flow If patient demand is greater then set flow, breath becomes a pressure breath Breath ends when (1) volume is reached or (2) if volume is exceeded, when patient’s flow falls back to set flow

  49. Physiologic Effects of Ventilatory Modes (cont.) CPAP Spontaneous breathing at elevated baseline pressure Positive pressure maintains higher alveolar inflation, used to reestablish FRC in low FRC states May reduce need of high FIO2, reduce shunt, ⇑V/Q Adaptive Support Ventilation (ASV) Automated dual-control mode, where support varies based on patient effort Input: IBW, high pressure alarm, PEEP, FIO2, rise time, flow cycle %, and % predicted VE desired Ventilator makes adjustments based on Otis “ideal” ventilatory pattern . .

  50. A post operative patient has developed ARDS. Which of the following ventilator mode should the respiratory therapist recommend? • PC-IMV • PC-CSV • PC-CMV • VC-CSV

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