Mastering Mechanical Ventilation Basics

Respiratory failure:

Type 1: hypoxic
- #1 cause V/Q mismatch
  - Decreased ventilation (airway, ILD)
  - Increased perfusion to normally ventilated lung (PE)
- Shunt
  - ARDS, PNA, edema
Type 2: hypercarbic
- Need to correct decreased alveolar hypoventilation
  - Increase minute ventilation +/- decrease dead space (if hypoxic, will correct with oxygenation)

Treatment:
- Reverse cause + positive pressure
  - Decrease work of breathing
  - Restore adequate gas exchange
- Noninvasive (NIV)
  - CPAP, BiPAP, cuirass
- Invasive (ETT or tracheostomy)
  - volume, pressure, hybrid, or “novel” modes (i.e., APRV, NAVA)

Ventilation
- Acceptable pCO2 + pH
- Goal Pplat <30
- Decrease auto-PEEP(breath stacking)
Oxygenation
- Goal SpO2 >88% on FiO2 <60%
- Optimal delivery of O2: (DO2) = CO x (1.34 × Hb × SaO2) + (0.003 × PaO2)
ARDS
- tidal volume ≤ 6 cc/kg IBW (based on height)
- Conservative fluids
Avoid VILI

Control
- Howa breath is delivered (V vs. P vs. dual)
Triggering
- Wheninspiration starts (flow or pressure)
Cycling
- Whatdetermines switch from insp to exp
- Time or flow sensed
Breaths
- Mandatory, assisted, spontaneous
Flow pattern
- Often set by the method of control
- Sinusoidal, accelerating, constant (square), decelerating
Mode or breath pattern:
- Spontaneous, assist control (AC), intermittent mandatory ventilation (IMV)
- Pressure supported (PS) or volume supported (VS)
Scalar
- Waveforms that plot pressure, flow, or volume vs timeon the ventilator screen
Loops
- Pressure or flow vs. volume(look like PFTs upside down)

Allows provider to:
1. Assess real time changes in patient condition
2. Optimize vent settings and treatment
3. Determine effectiveness of vent
4. Detect adverse events
5. Decrease risk of mechanical complications
Pressure waveforms
1. Diagnose: air trapping, obstruction, dyssynchrony, pressures (i.e., plateau or end inspiratory hold), triggering, bronchodilator response
2. Area under the pressure curve = Alveolar distending pressure
Flow waveforms
1. Detect air trapping, obstruction, bronchodilator response, triggering, dyssynchrony
2. Square
  - Pressure rises (higher peak inspiratory pressure)
3. Decelerating
  - Pressure constant
4. Plateau pressure is greaterfor Square vs. Decelerating with same volume
Volume waveform
- Can detect air trapping, leak, tidal volume, dyssynchrony

I (inspiratory) time
- Set for PRVC and PC, VC on some vents (otherwise set flow)
  - PS: cycles once falls below set % of peak flow (decelerating flow)
- Decrease Ti: less autoPEEP, decrease CV effects
- Increase Ti:improves oxygenation
Trigger sensitivity
- Increased sensitivity can lead to autotrigger/false triggering (water in circuit, heartbeat)
Rise time
- Rate of rise of pressure (PC) or flow (VC)
  - Short: may be uncomfortable, but can lead to decreased inspiratory workload
    - Reduced VILI (biotrauma)
  - Long: decreased Tv in pressure mode (less time at set pressure) or increased pressure in volume mode (to reach target volume)

Dx increased peak without change in Pplat
- Increased resistance→ obstruction, bronchospasm, biting, foreign body
Dx increased peak and Pplat
- Decreased lung compliance(or increased Vt)
  - PTX, abdominal HTN, ARDS, edema
- How to detect AutoPEEP
  - Flownot returning to 0 before next breath
  - Area under inspiratory curvenot equal to area under expiratory curve
  - Dyssynchrony(double triggering)
- Treating airway obstruction:
  - Bronchodilators
  - Suction
  - Prolong expiration
    - Decrease I time
    - Increase flow
  - Sedate if necessary
  - Increase extrinsic PEEP (2/3 intrinsic)