15Implication VT falls because FRC encroaches on TLC Limited ability to VT with MV/IPAPBest way to PaCO2 is to VCO2 WOB (frictional and/or elastic) PaCO2 even if VT, VE and VA are constant
16Work of Breathing Total Work Elastic Work Frictional Work RVFRCTLC
24NIPPV Pathophysiology of AECOPD & Asthma is amenable to Rx with NIPPV · EPAP for auto-PEEP· IPAP for inspiratory RawWill work of breathing· VCO2· At constant VA, PaCO2 and pHMay VAMay mortality and intubation rate
25Which Patients with COPD benefit from NIV ? Hospital Mortality 12%NNT 82%
26NIV in Severe Asthma 17 Episodes of ARF due to asthma 2 patients required intubation for worsening PaC02Duration of NPPV was 16±21 h.All patients survived. Length of hospital stay was 5±4 days
27Ventilation in COPD/Asthma Non Invasive VentilationAssessment of mechanics (resistance, auto-PEEP)Pressure or volume modes?Role of PEEPAdministering Bronchodilators
34After the third breath, the airway was occluded at end-expiration using the end-expiratory hold function on the ventilator. During the period of zero flow, pressure in the alveoli and ventilator circuit equilibrate, and the plateau pressure reflects auto or intrinsic positive end-expiratory pressure (PEEPi), indicated by the arrow.
35Giving CPAP to a patient who has auto-PEEP The increased work of breathing associated with auto-PEEP can be offloaded by applying CPAP to the trachea/mouth, and splinting open the connecting airways.
36The use of external PEEP in the setting of auto-PEEP may be conceptualized by the "waterfall over a dam" analogy. In this analogy, the presence of dynamic hyperinflation and 10 cmH20 of auto-PEEP is represented in the top panel by the reservoir of water trickling over the dam represented by the solid block. In the middle panel, as long as the external PEEP is less than or equal to the amount of auto-PEEP, the amount of water in the upstream reservoir, representing dynamic hyperinflation, does not increase. However, once the amount of water in the reservoir does increase (bottom panel), dynamic hyperinflation worsens.
39Excessive Inspiratory Time InspirationNormalPatientIncrease WOB and “Fighting” of the ventilatorTime (sec)Flow (L/min)}Air TrappingAuto-PEEPExpiration
40Ventilation in COPD/Asthma Non Invasive VentilationAssessment of mechanics (resistance, auto-PEEP)Pressure or volume modes?Role of PEEPAdministering Bronchodilators
41Pressure or Volume Mode? Predictable TVPeak-Plat gradientMonitor PlatBetter acidosis controlPressureMinimise over-distensionMonitor Tidal volumeExcess volumes as airway resistance improves
42Mechanical Ventilation of COPD & Asthma Exacerbations Mode: AC vs IMV PS ? rest respiratory muscles: CMV Better sleep with AC vs. IMV-PS Ventilator-induced diaphragm changes (?)Triggering: key issue with either mode PEEP to counter auto-PEEP Major cause of patient-ventilatordissynchrony
43Initial Ventilator Settings Inspiratory time 0.8 – 1.2 secsRR 10-12TV 6-8 ml/KgPplat < 30 cm H2OPEEP ??
44Assessment of Hyperinflation CVS effects disconnectPplatPEEPi measurement
46COPD flow and frequency As flow increased from 30 to 60 and 90 L/min (from right to left), frequency increased from (18 to 23 and 26 breaths/min, respectively), Auto-PEEP decreased (from 15.6 to 14.4 and 13.3 cm H2O, respectively) and end-expiratory chest volume also fell. Increases in flow from 30 L/min to 60 and 90 L/min also led to decreases in the swings in Pes from 21.5 to 19.5 and 16.8 cm H2O.
47Hypercapnia: How permissive? Defence of intracellular pHApnoeic oxygenation in dogs to pH 6.5 and PaCO2 of 55kPaAnaesthetic mishap with PaCO2 of > 300 mmHg (40 kPa) and pH of 6.6 survived without sequelaeAm J Respir Crit Care Med 1994; 150:
48Ventilation in COPD/Asthma Non Invasive VentilationAssessment of mechanics (resistance, auto-PEEP)Pressure or volume modes?Role of PEEPAdministering Bronchodilators
49External & Internal PEEP Waterfall ConceptPaoPao1010EPPEPP10+1010PalvPalv101010PelPplPpl
50Effect of Auto-PEEP Normal airway resistance (end-exhalation) Ptp = 5- 5PA = 0Patm = 0Ppl =- 5Pel = 5- 5D Ppl needed to initiate inhalation: - 1PA drops to - 1 relative to Patm
52Treatment of Auto-PEEP with PEEP or CPAP Airway narrowing with auto-PEEP: Treatmentwith PEEPPtp = 82PA = 10Ppl 2PEEP = 102Pel = 8D Ppl needed to initiate inhalation: - 1The only thing PEEP does is ¯ work of breathing
53Implication PEEP, EPAP, CPAP No effect on VE, VT or VA WOB (elastic)- VCO2 (on next breath)- PaCO2 (on next breath)
54Treatment of Auto-PEEP with Vinsp Longer time for exhalation, PA fallsPtp = 51PA = 6Patm = 0Ppl = 11Pel = 6D Ppl needed to initiate inhalation: - 7
55External PEEP Reduce inspiratory muscle load Improve ventilator triggering80% of PEEPi can be matched without increase PEEP totReduce hyperinflation by improving expiration
62Administration of Bronchodilators Nebuliser or MDI?Lung deposition of radiolabelled drug*MDI 5.6% v Nebuliser 1.2%Urinary excretion**MDI with spacer 38%MDI in line 9%Nebuliser 16%4-10 puffs MDI effective in reducing RAW* Chest 1999; 115:**Am Rev Respir Dis 1990; 141:440–444
65Myopathy in Asthma Steroid myopathy Muscle relaxants Polyneuropathy of the critically ill
66Polyneuropathy of the critically ill Myopathy in AsthmaProximal muscleSubacute (3 weeks)Normal CPKSteroid myopathyDistal & proximalHigh myoglobinHigh CPKMuscle relaxantsSensorimotorPolyneuropathy of the critically ill
67Principles of managing the ventilated patient with obstructive lung disease Provide adequate support for muscle rest with adequate pH and PO2Do not over ventilateMinimize minute volume requirementsMinimize the risk of barotraumaMaintain adequate bronchial hygieneMaintain appropriate nutrition