Indication for mechanical ventilation

• Hypoxia = Type I respiratory failure = PaO₂ < 55 (normal 55-80)
• PaO₂ of 55 = SaO₂ of 90%
• PaO₂ of 45 = SaO₂ of 75%
• A-a gradient = [FiO₂ * (760-47)] - PaCO₂/0.8 - PaO₂ (ABG)
• Normal = Age/4 + 4 = 5 to 20
• Normal gradient = low oxygen supply (hypoventilation, low FiO₂)
• Increased gradient = V/Q mismatch (COPD/asthma, cardiac R-L shunt - PFO, alveolar collapse, pneumonia, pulmonary edema)
• Hypercapnea = Type II respiratory failure = PaCO₂ > 55
• Increased CO₂ production: fever, sepsis, injury
• Decreased CO₂ perfusion (dead space): atelectasis, ARDS, lung injury, PE
• Airway protection

**hide**Other

• **hide**P:F ratio = PaO2 / FiO2 on an ABG, e.g. 100 mmHg/0.21 = 500
• **hide**Normal: ~ 500
• **hide**ARDS: < 300 (200-300 = mild, 100-200 = mod, < 100 = severe)

Vent settings

• Format: RR/TV (or drive pressure)/FiO₂/PEEP, e.g. 24/350/30/5 (24 breaths/min, TV = 350 ml, 30% FiO2, PEEP = 5)

Vent modes

Volume vs. pressure control

• VC = volume control
• Tidal volume set, resulting in variable peak pressures (depending on compliance)
• PC = pressure control
• Drive pressure set (typically at least 5), resulting in variable tidal volumes (depending on compliance)
• VC+/PRVC (pressure regulated volume control)
• Dynamically changes flow/drive pressure as patient compliance changes, to achieve a goal TV with minimal P_plateau and minimize dyssynchrony
• Downside - negative feedback cycle with anxious patient taking heavier breaths → machine gives less pressure → more anxiety

Assist vs. control

• AC = assist control (most common) (e.g. AC/VC, AC/PC)
• Breaths are triggered by pre-set RR (control), and by patient-initiated breaths (assist), guaranteeing at least pre-set RR
• Every breath is assisted exactly the same (pressure AC/PC or volume AC/VC), even if patient breaths faster than pre-set RR
• Caveats: Make sure to match pt demand; patient can "overbreath" the vent → respiratory alkalosis, hyperinflation (breath stacking/auto-PEEP)
• SIMV = synchronized intermittent mandatory ventilation
• Every breath is not assisted the same (compared to AC)
• Ventilator-triggered breaths are full volume/pressure control
• Pt-triggered breaths are assisted, e.g. PSV
• Vent breaths are synchronized to be intermittent/not stack with patient breaths
• Reduces risk of hyperinflation, and pt does extra work compared to AC (attendings differ in opinion as to whether this "workout" is beneficial)
• PSV = pressure support ventilation
• Assists spontaneous patient respirations with a pre-set drive pressure (has safety back-up RR if patient becomes apneic)
• Drive pressure should be < 15 (typically P_peak =10, PEEP = 5 = Drive pressure of 5)
• Pros: reduces work of breathing; enhances patient-ventilator synchrony; patient comfort

Other

• Non-invasive positive pressure ventilation (NIPPV)
• Patients must be awake to cooperate with mask and must be able to protect airway (↑ risk of aspiration with positive pressure)
• BiPAP = PSV without intubation (bilevel positive airway pressure)
• Indicated for ↑PCO₂ or work of breathing; if requires for more than a couple of hours should probably be transferred to ICU (high risk of resp failure)
• Set drive pressure IPAP/EPAP, e.g. 20/12 = drive pressure = 8
• Start at 10/5
• CPAP = PEEP without intubation (continuous positive airway pressure)
• Indicated for hypoxia
• Start at pressure = 5
• AVAPS = newer, probably don't need to know (like BiPAP, but IPAP is dynamically adjusted to deliver a guaranteed TV); useful in chronic muscle weakness
• APRV - newer vent setting, probably don't need to know
• ECMO = ultimate 'low' (nonexistent) tidal volume ventilation (takes lungs out of equation)

ABG (arterial blood gas)

• Format: pH/PaCO₂/PaO₂/bicarb, e.g. 7.38/40/106/25
• Acid-base interpretation
• Use to assess ventilation/oxygenation (see below)
• Get ABG ~ 1 hr after vent changes

Vent setting changes

• Ventilation (pH, PaCO₂)
• Respiratory acidosis (pH < 7.35, PaCO₂ > 40) - increase minute ventilation (increase RR or TV)
• Respiratory alkalosis (pH > 7.45, low PaCO₂ < 40) - decrease minute ventilation
• Oxygenation
• Hypoxemia (PaO₂ < 55) - increase FiO₂ or PEEP
• Possibly lower V_E for permissive hypercapnia (up to pH 7.1 = PaCO₂ of 85)
• May invert I:E ratio (e.g. 2:1 instead of 1:2) to increase inspiratory time (patient must be heavily sedated)

Recruitment maneuvers

• Proning
• Patient is positioned prone instead of supine, reducing compression pressure from heart/diaphragm and improving V/Q matching
• May improve oxygenation in severe resp. failure (e.g. severe ARDS)
• Contraindicated in increased ICP/abdominal pressure, abdominal/chest wounds, C-spine instability; HD instability
• PEEP
• Can gradually cycle PEEP up to as high as 30 (over course of minute or two) to 'pop open'/recruit collapsed alveoli, then return to 'normal' PEEP

Vent weaning protocols

• Daily BEST (breathing spontaneous trial) = decrease sedation and do 30 min breathing trial on PSV
• RSBI (rapid shallow breathing index) = RR/TV
• Fail if > 100
• If pass, and patient can follow commands (e.g. hold head off the bed), can trial extubation
• Cuff leak - assess if concern for airway edema
• Deflate ETT cuff -> should hear air leak around cuff if OK to extubate
• Cuff leak of < 10-15% a/w increased risk for post-extubation stridor
• Trach vent weaning protocol
• STAR (short-term trach assessment of respiration) = trach collar for 4 hours with goal 24 hrs continuous trach collar. Try up to 3 days before 'failing' and switching to SWAT
• SWAT (slow wean after trach) = protocol if failed STAR 3 days in a row.
• CPAP 5/PSV 10 x 30 minutes; gradually increase length of SWAT by up to 100%/day until tolerating CPAP for 12 hours, then transition to trach collar

The alarm is going off, help

• Check vitals, oxygenation, monitor, etc.