MECHANICAL VENTILATION


I. Basic types of ventilators

  1. Positive pressure
  1. Use positive inspiratory airway pressure to inflate lungs
  2. Used with cuffed ET tubes
  3. Volume-cycled most common; reliably delivers given TV (tidal volume) despite fluctuations in resp. mechanics
  4. Pressure-cycled rare these days (insp. stops when preset pressure reached)
  1. Negative pressure (iron lung)

II. Physics

  1. Static compliance (C-st) of lunge = change in vol/change in pressure
  1. For most lungs = 50-100ml/cmH2O
  2. Determines pressure at end-inspiration for given TV
  1. Dynamic compliance
  1. Due to airways resistance
  2. Can get high peak proximal airway pressure if this is low, e.g. in asthma

III. Indications for mechanical ventilation

  1. To improve pulmonary gas exchange in resp. failure of pulmonary or extrapulmonary (chest bellow, central resp. drive) etiology
  1. Raise PAO2
  2. Reduce PaCO2
  1. To Relieve Respiratory distress
  1. Decrease O2 cost of breathing (can be enormous!)
  2. Reverse resp. mm. fatigue
  1. To alter pressure-volume relationships
  1. Prevent and reverse atelectasis
  2. Improve lung compliance
  1. Reduced CNS resp. drive (intoxication, brainstem CVA)
  2. To treat increased intracranial pressure by hyperventilating to reduce cerebral blood flow
  3. Things to remember
  1. Always consider individual pt's characteristics, e.g. PCO2 of 55 in a young asthmatic indicates impending resp. failure, whereas in an elderly COPD'er it may be their baseline, compensated state!
  2. Remember alternative methods of CPAP and nasal CPAP which don't require ETT but are uncomfortable and claustrophobic for many

IV. Choosing a ventilatory mode: the options

  1. Paralysis, sedation, controlled ventilation-no longer popular; get worse resp. mm. atrophy than with other techniques
  2. Assist Control ("AC")
  1. Whenever pt makes a resp. effort, vent is triggered to deliver a breath at a preset TV
  2. If pt doesn't initiate a breath for a given period of time (e.g. 6 sec. if the RR is set at 10/min), the vent delivers a breath at the preset TV
  1. Intermittent mandatory ventilation ("IMV")
  1. Vent delivers breaths at set volume and rate
  2. Pt may take additional breaths but they're not assisted, so they entail more work than pt-initiated breaths in AC mode-provides "workout" of resp. mm, esp. if RR is set below pt's spont. RR (see under IMV wean below)
  3. Better than AC for pts with auto-PEEP (see below) or with high spont. RR, e.g. anxiety (b/c may get severe resp. alkalosis with AC)
  1. Pressure support
  1. Inspiratory effort by the pt triggers the vent to supply a preset level of pressure until inspiration ceases
  2. No breaths are delivered in the absence of spont. pt inspirations
  3. Used for weaning and for pts who have been intubated purely for airway protection
  4. Can use in combination with IMV, giving some (but not full) assistance with pt-initiated breaths
  5. Us. use 5-20 cm H2O

V. Setting and adjusting the settings

  1. Frequent reassessment (2-3x/24h) is the key to good results
  2. Initial post-intubation settings
  1. Mode: both AC and IMV are used; Harrison's recommends AC
  2. FiO2: start with 100%; wean downward to sats around 90%
  3. TV: 10-15ml/kg to start
    1. Lower TV ass'd with sig. lower 180d mortality c/w traditional settings (6mL/kg vs. 12mL/kg; 31% vs. 40% mortality) (NEJM 342:1334, 2000--JW)
  4. RR: 12-15/min
  1. Subsequent refinement
  1. Oxygenation
  1. Sats > 90% don't significantly increase tissue oxygenation
  2. Sat is mostly a fn of FiO2 rather than TV or RR
  3. If you need FiO2 >60% consider second-line approach to improving oxygenation, which is:
  4. PEEP
  1. No affect on ventilation, but improves oxygenation by splinting open collaped alveoli, driving fluid into interstitium, this allowing for reduction of FiO2
  2. May reduce cardiac output by same mechanism as ventilation itself
  3. Usually start at 5cm H2O; avoid going higher
  4. May also help to reduce auto-PEEP
  1. Ventilation, i.e. getting rid of CO2
  1. A function of minute ventilation = RR x TV
  2. "Permissive hypercapnia," aka "controlled hypoventilation is a concept which became popular in the 1980's
  1. Consists of limiting TV to 5-7ml/kg
  2. Accept some hypercapnia in exchange for
  3. Lower PIP and lower risk of barotrauma

VI. What to monitor on ventilated pts

  1. ABG: 30-60 min post any changes in vent settings; and periodically
  2. CXR: daily to r/o PTX and monitor ETT position
  3. O2 sat: at all times except in stable long-term vent pts
  4. RR: is pt overbreathing the vent? Check daily
  5. PIP: should be < 30mm H2O; if breated, consider changes detailed below
  6. Auto-PEEP: most vents can measure this
  7. Tracheal cuff pressures: should allow a little leak; >15mm Hg may be too much
  8. Hemodynamic parameters: blood pressure, u/o, CO and PCWP in selected cases
  9. Lytes and fluid balance b/c mech. ventilation can cause Na and fluid retention

VII. Other things to remember with pts on vents

  1. Prophylactic H2-blockers (better at preventing GIB than sucralfate; NEJM 338:791, 1998--JW)
  2. DVT prophylaxis
  3. Check butt Q2-3d for decubiti
  4. HOB at >30' if poss., to reduce risk of aspiration
  5. Nutritional support
  6. Sedation if needed
    1. Interrupting sedation once daily and waiting to see if pts needed sedation resumed, compared with continuous sedative infusion, was ass'd with sig. shorter median duration of ventilation (4.9 vs. 7.3d), sig. shorted median ICU stay (6.4 vs. 9.9d), and nonsig. lower in-hospital mortality (36% vs. 47%) (NEJM 342:1471, 2000--JW)

VIII. What to do if PIP's (peak inspiratory pressure) are high

  1. High PIP's increase risk of barotrauma
  2. Limiting PIP's may not change outcomes, e.g. mortality or incidence of pneumothorax (NEJM 338:355 and 335:341, 1998--JW)
  3. Causes include
  1. High airways resistance, e.g. severe asthma
  2. Low lung compliance, e.g. pulm. edema
  3. Relatively short exp. time
  4. Use of PEEP
  5. Auto-PEEP
  6. "Fighting the vent"
  1. Treatment consists of:
  1. Sedation if fighting the vent
  2. Increase exp. time by either
  1. Decreasing TV
  2. Decreasing RR
  3. Increasing insp. flow rate
  1. Change from AC to IMV so some breaths are unassisted
  2. Give fluid if BP is low from high PIP

IX. Complications of mechanical ventilation

  1. Hypotension due to decreased pulm venous return and thus decreased preload; with consequent Na and H2O retention
  2. Barotrauma (pneumothorax, pneumomediastinum, sub-Q emphysema)
  1. Monitoring PIP may help avoid (see above)
  1. O2 toxicity
  1. Us. occurs with FiO2 > 60% for > 72h
  2. Avoid by using lowest FiO2 that will give sat > 90
  1. Auto-Peep
  1. Gas-trapping in alveoli, e.g. due to airways disease
  2. Get persistent positive alveolar pressure at end of exhalation (in nl people, end-expiratory alv. pressure = atmospheric pressure)
  3. Makes it harder for pt to trigger the vent b/c pt has to generate more negative pressure (has to generate neg. pressure equivalent to the amount of Auto-PEEP plus the amount required to trigger vent)
  4. Also increases effective dead space
  5. Tx included PEEP, which can make it easier to trigger vent (requires less of a drop in pressure by the pt)-?
  1. Resp. mm. deconditioning
  2. Pneumonia--
    1. Incidence usually 1-4 cases per 1,000 ventilator days
    2. Diagnosis can be difficult; can get "false-positive" cultures from tracheal aspirates
    3. Pts randomized to diagnosis of pneumonia by culture of bronchoscopic lavage specimens had sig. less abx use and sig. lower 14d morality (16% vs. 26%) compared with usual diagnostic measures (including endotracheal aspirate culture) in a study of 413 pts with new pulmonary infiltrates & fever while on mechanical ventilation (Ann. Int. Med. 132:621, 2000--JW)
    4. 8d vs. 15d of antibiotics (specific regimen chosen by treating physician) for culture-confirmed ventilator-associated pneumonia was ass'd with no sig. differences in 28d all-cause mortality or recurrent infection; also no sig. differences in length of ICU stay or 60d all-cause mortality.  The subgroup of pts with nonfermenting gram-negative bacillary infections (e.g. Pseudomonas aeruginosa) had no sig. diffs. in most outcomes but did have higher incidence of recurrent pneumonia over study period than those receiving 15d of tx (40.6% vs. 25.4%) in a randomized trial in 401 pts (JAMA 290:2588, 2003--JW)
    5. Effective preventive strategies (some from a systematic review-Ann. Int. Med. 141:305, 2004):
      1. Orotracheal (as opposed to nasotracheal) intubation
      2. Maintain endotracheal cuff pressure at 20cm H2O or greater
      3. Keep duation of intubaton as short as possible; assess daily for readiness to wean
      4. Maintain head of bed at 30-45' elevation
      5. Avoid gastric overdistension
      6. Subglottic secretion drainage
      7. Use of humidifiers
      8. Kinetic bed therapy
      9. Avoid H2-blockers and proton pump inhibitors unless strong indication exists
      10. Use oral antiseptic washes, e.g. chlorhexidine
      11. Serial endotracheal aspiration culturers to look for ventilator-associated tracheobronchitis (present in 4-10% of intubated ICU patients), and targeted antibiotic therapy if pathogenic organisms are detected.
    6. Prevention with oral decontamination
      1. In a study in 98 pts on mechanical ventilation for severe head trauma randomized to rinsing of nasopharynx and oropharynx Q4h with either normal saline or 10% povidone-iodine or standard care with no wash, incidence of ventilator-associated pneumonia was sig. lower in the povidone-iodine group (8% vs. 39% with saline rinse and 42% with no rinse) (Crit. Care Med. 34:1514, 2006--JW)
  3. Psychological trauma

X. Weaning

  1. Overall clinical assessment is the most important thing: wean when:
  1. CV stable
  2. Original problem that resulted in intubation if better
  3. Ventilatory supply (resp. mm. fn, pulm fn) meets demand (high O2-consumption states, e.g. sepsis)
  4. PaO2 > 60 with FiO2 < 40
  5. "Weaning parameters" are favorable:
Category Parameter Good No good
Resp. mm. function Max. insp. pressure > -30mm Hg < -20mm Hg
Ventilatory demand Spontaneous RR

Minute ventilation

Vd/Vt

< 30/min

< 10 l/min

< 0.4

> 35/min

> 10 l/min

> 0.6

Ventilatory ability Vital capacity

Max. Vol. Vent.

RR/TV over 1min of unassisted breathing

> 15 ml/kg

> 2x resting minute vent.

< 100/min*l

< 10 ml/kg

< 2x resting minute vent.

> 100 min*l

Oxygenation PaO2 with FiO2 < 40

R-L shunt

> 60

< 20%

< 60

> 60%

  1. Weaning trials
  1. T-Piece trial
  1. Pt breathes spontaneously through T-tube connected to humidified air source
  2. Trials of preset duration, us. starting at 5 min., not to fatigue patient, gradually lengthening
  3. Advantages: pt doesn't need to work to open any demand valve
  4. Disadvantages: no alarms, have to stay at bedside in case poops out
  5. In a trial that randomized 300 ventilated pts who passed screening of "weaning parameters" to either 2h t-piece trial or "usual care," the t-piece pts had sig. shorter duration on vent (4.5 vs. 6d) and lower incidence of complications (reintubation, vent x >3wks, etc.) (NEJM 335:1864, 1996-JW)
  1. IMV (intermittent mandatory ventilation)
  1. Ventilations are mandatory, preset tidal volume at preset rate
  2. Delivered at gradually decreasing rate
  3. Final rate usually 0.5-1/minute; timed not to be in synch with pt's own efforts
  4. Pt breathes spontaneously on his/her own between IMV breaths
  5. Can take sev. days or more
  6. RR > 25 indicates failure
  1. Pressure support
  1. Pt breathes spontaneously at their own pace
  2. Each of pt's inspirations augmented by preset posivit pressure from vent
  3. Can result in varying tidal volumes if pt's resp. mechanics are unstable (?)
  4. Advantages: more comfortable for pt
  5. Disadvantages: can get variable tidal volumes
  1. CPAP trial
  1. Like pressure suppot, but with positive pressure during exhalation too
  2. Advantages: alarms, pt is monitored by vent
  3. Disadvantages: pt must work to open vent's demand valve
  1. Extubation
  1. May need trach tube after weaning from vent if upper airway is obstructed, full of secreations, no gag reflex, etc.
  2. Return of swallowing function may take hours-days; don't rush start of oral intake
  3. Prevention of post-extubation laryngeal edema
    1. In a study in 761 adults intubated x > 36h randomized to methylprednisolone 20mg vs. placebo, given 12h before planned extubation and Q4h until extubation, the methylprednisolone recipients had sig. lower incidence of postextubation laryngeal edema in 1st 24h post-extubation (3% vs. 22%) and sig. lower incidence of reintubation (4% vs. 8%) (Lancet 369:1083, 2007--JW) (Lancet 369:1083, 2007--JW)

XI. Alternatives to mechanical ventilation through ET tube

  1. Noninvasive postivie-pressure ventilation
    1. May reduce complications (pneumonia, sepsis, shorter ICU stays) though effect on mortality is unproven as of 1998 (NEJM 339:429, 1998--JW)
    2. May reduce need for intubation in pts with respiratory failure from community-acquired pneumonia, especially those with COPD (Am. J. Resp. Crit. Care Med. 160:1585, 1999--JW)
    3. In a meta-analysis of 15 studies of non-invasive positive pressure ventilation vs. usual care, NPPV was ass'd with absolute risk reduction of 10% for in-hospital mortality. (Ann. Int. Med. 138:861, 2003--JW)

(Sources include Core Content Review of Family Medicine, 2012)