Respiratory Failure

RESPIRATORY FAILURE

  • Notes about respiratory monitoring
    • O2 goal may be lower for chronic retainer
    • Beware of CO, methemoglobin, abnormal hemoglobin, which may falsely elevate O2 measurement
    • Pulse Ox is less reliable at values below 80
    • Nail polish, dyes, (methylene blue), tremor may cause artifact affecting pulse ox measurement; look for good waveform
  • Rough VBG-to-ABG conversion
    • Subtract 8 from PaCO2
    • Add 0.04 to pH
  • Rough SaO2 to PaO2 conversion:
SaO2 (%) 80 84 88 92 96 100
PaO2 (mmHg) 44 49 55 65 86 145
  • Consider intubation when:
    • Respiratory failure (hypoxemic and hypercapneic)
    • Inability to protect airway (GCS < 8)

 

HYPOXEMIC RESPIRATORY FAILURE

A failure of OXYGENATION

  • Notes: alveoli are more compliant at lung bases, and V and Q are higher at bases
  • Hypoxemia symptoms: for previously healthy patient:
    • PaO2 < 50mm Hg: malaise, light-headedness, nausea, vertigo, incoordination, confusion
    • PaO2 < 35mm Hg: decreased renal blood flow, decreased UOP, bradycardia, conduction blocks, lactic acidosis
    • PaO2 < 25mm Hg: LOC, respiratory depression
  • Hyperoxia symptoms:
    • O2 replacing Nitrogen à atelectasis, poor lung compliance, ROS

HYPERCAPNEIC RESPIRATORY FAILURE

A failure of VENTILATION

  • Mechanisms: airflow obstruction, muscular weakness/ineffective musculature, inadequate ventilatory drive, increased ventilation requirement
    • Note: can follow minute ventilation (f x VT in liters) to trend PaCO2. If trending ABG’s, goal should be normal pH, not normal PaCO2 or PaO2
  • Hypercapnea symptoms:
    • Cerebral vasodilation à increased ICP
    • Adrenergic stimulation à increased CO, PVR
    • Decreased tissue metabolism
    • Improved surfactant function
    • Prevents protein nitration
  • Oxygen can worsen hypercapnia by:
    • Increased V/Q mismatch
    • Suppressed central hypoxemic drive
    • Haldane effect:
      • In RBC’s: CO2 + H2O ó H2CO3 ó H+ + HCO3–.
      • Oxygenation of Hb promotes dissociated of H+ from Hb, which shifts the bicarbonate buffer equilibrium toward CO2 formation, so CO2 is released from RBC’s.
        • Therefore, high plasma [O2] causes Hb to release CO2, and diseased lungs may not be able to adequately increase alveolar ventilation to breathe off this CO2.

Supplemental Oxygen

Escalation of oxygen therapy

Device Flow rate (L/min) FiO2 (%) approx. PEEP?
Nasal cannula 1 24 No
2 28
3 32
4 36
5 40
Face mask 5 40 No
6-7 50
7-8 60
Venturi mask 3-15 24 – 50 No
Non-rebreater 12 – 15 > 90 No
High flow nasal cannula Yes
CPAP Yes
Endotracheal tube 21 – 100 Yes

Non-invasive positive pressure ventilation (NIPPV)

  • CPAP: continuous positive airway pressure
    • CHF (creates pressure gradient from chest to periphery, allowing for decreased afterload, does not push fluid out of lungs)
  • BPAP: bi-level positive airway pressure (IPAP/EPAP)
    • COPD, pressure difference allows for ventilation (think as external diaphragm)
  • Contraindications to NIPPV:
    • Inability to protect airway
    • AMS (GCS </= 8), poor compliance, seizure, stroke
    • Fluid that could be forced into airway: vomiting, epistaxis, upper GI bleed, secretions (avoid in pneumonia unless patient is immunocompromised)
    • Severe facial trauma or burn
    • Pneumothorax, penetrating chest trauma
    • CSF leak

Respiratory Glossary

  • Positive end-expiratory pressure (PEEP): pressure applied to alveoli at end-expiration
  • Plateau pressure (Pplat): pressure applied to small airways/alveoli at end-inspiration during a period of no airflow (inspiratory hold). Reflects lung compliance.
    • Note: Pplat more accurately measured when flow rate is constant, so less reliable in PRVC—if measuring in PRVC, check multiple times and average
  • Peak pressure (Ppeak): the highest pressure needed to move a volume of gas through major airways into the lung. Reflects airway resistance (but is a combination of compliance and resistance).
    • Airway resistance is proportional to viscosity of inspired gas and length of airway, and inversely proportional to radius of airway (Poiseuile’s law)
    • Airway resistance = (pressure in mouth – pressure in alveoli)/airflow (Ohm’s law)
  • Mean airway pressure: average airway (large and small) pressure throughout ventilatory cycle
  • Inspiratory to expiratory (I:E) Ratio: ratio of inspiratory to expiratory time in the ventilatory cycle
  • Lung compliance (C) = tidal volume DV / transpulmonary pressure DP

Mechanical Ventilation

Peri-intubation check-list:

  • Induction agent, Consideration of anxiolytic and analgesic, Paralytic (MUST HAVE ATTENDING PRESENT)
  • pre and peri-intubation oxygenation with high-flow NC in hypoxemic patients
  • Calculate ideal body weight to determine VT; lung protective strategy preferred even in non-ARDS patients (VT < 8 mL/kg IBW)
  • Check for bilateral breath sounds, chest rise, end-tidal CO2, vitals immediately post-intubation
  • Order stat CXR to check position of tube (should be 4cm above carina)
  • No need for daily CXR in intubated patients; on-demand strategy associated with fewer CXR’s and no change in patient outcomes: ventilator days, length of stay, or mortality5
  • Order ABG 30 minutes post-intubation (you do not need to use PO2 to wean FiO2, simply use pulse ox)

Modes:

  • Variables
    • Control: pressure (P), volume (TV), or both
    • Trigger: elapsed time, decrease in intrathoracic pressure, negative inspiratory flow (either time-triggered or patient-triggered)
    • Cycle: respiratory rate (f), inspiratory to expiratory (I:E) ratio
    • FiO2 (0.21 – 1.0)
    • PEEP
Mode Ventilator controls… Patient controls… Notes:
(AC) Volume Control VT, f, PEEP, FiO2, f above vent Airway pressures vary depending on lung and chest wall compliance, airway resistance
(AC) Pressure Control Pinsp, I:E, f, PEEP, FiO2, f above vent VT varies depending on lung compliance, airway resistance
Set MV cannot be guaranteed
PRVC VT, Ppeak, f, FiO2, PEEP f above vent Delivered VT will be lower than set VT if Ppeak exceeded, and may be higher than set VT.
(pressure-regulated volume control)
Pressure Support Pinsp FiO2, PEEP VT, f, flow rate Used for spontaneous breathing trials (SBT’s)
SIMV (synchronous intermittent mechanical ventilation) VT of vent-triggered breaths, f, PEEP, FiO2 VT of extra breaths, f above vent Used commonly in surgical ICU’s
APRV Phigh, Plow, Thigh, Tlow, FiO2 Spontaneous breathing allowed throughout cycle Intentional auto-PEEP to avoid derecruitment
(airway pressure release ventilation) Allows alveolar recruitment while minimizing ventilator-induced lung injury. May be useful in ARDS6

 

 

Weaning

  • Reason for intubation improved
  • Hemodynamically stable
  • FiO2 </= 40%, PEEP </= 5
  • Good cough and gag
  • Discontinue sedation, hold tube feeds in advance (or suction out)
  • SBT (spontaneous breathing trial): PS 10/5, 5/5, 0/5, or t-piece trial
    • Daily spontaneous awakening trial (SAT) + spontaneous breathing trial (SBT) improve outcomes: more ventilator-free days, earlier discharge, decreased 1-year mortality
    • 30 min – 2 hours sufficient
    • RSBI (rapid shallow breathing index) = f/VT
      • RSBI >/= 105 portends likelihood of unsuccessful extubation
    • NIF (negative inspiratory force) less than negative 20
    • FVC >/= 10 mL/kg IBW
    • Cuff leak: deflate ETT cuff; VT should decrease by 20% (because volume is being lost around the tube. No cuff leak indicates laryngeal edema
  • Consider extubating to BiPAP
  • Once extubated
    • Swallow study (bedside is fine unless they fail that)
    • If stridor, give 10mg dexamethasone IV STAT, reintubate earlier rather than later
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