Respiratory Failure Flashcards
Definitions of respiratory failure
Definition based on arterial blood gas analysis
Hypoxemic respiratory failure: PaO2 < 60 mmHg on ambient air
• Ex: ARDS, interstitial fibrosis, pneumonia, pneumothorax, pulmonary edema, pulmonary embolism
Hypercapnic respiratory failure: PaCO2 > 45-50 mmHg
• Ex: drug overdose (sedatives, opioid analgesics), neurologic disease (brainstem tumor or stroke, neuropathy, NMJ), muscle disease (MS, tetanus)
- Two types often coexist (asthma, COPD, CF)
- Both contribute to increased work of breathing
Definition based on time of development
• Acute respiratory failure = rapidly develop (minutes to hours)
• Chronic respiratory failure = slowly develop (days to months)
Etiologic definition = syndrome of inadequate gas exchange due to dysfunction of one or more essential components of the respiratory system (chain of command):
Describe the spectrum of etiologies that can lead to Respiratory Failure
CNS depression
• Drug overdose – hypnotic, narcotic, sedative
• Brainstem lesion
• Bulbar poliomyelitis
Neuromuscular • Malnutrition and electrolyte disorders • Spinal cord injury • Multiple sclerosis • Guillain-Barre syndrome • Myasthenia gravis • Tetanus
Chest wall and pleural space abnormality • Pleural effusion • Postoperative • Obesity • COPD hyperinflation • Kyphoscoliosis • Flail chest • Restrictive chest wall burn
Airway abnormality
• Chronic bronchitis/emphysema – secretions
• Asthma
• Upper airway obstruction
Parenchymal disease • Pulmonary edema • Pneumonia • COPD – V/Q mismatch • ARDS • Interstitial fibrosis • Pneumothorax
Vascular disorders
• Pulmonary embolism
• Pulmonary HT
Mechanisms of acute respiratory failure: Hypoxemic respiratory failure
• Typically occurs in diseased of lung parenchyma and/or airways (ex: pneumonia and ARDS)
• Characterized:
Low V/Q mismatch
o A regional problem (not all lung units are the same):
o Ventilation: regional differences in elastic fiber recoil, airway obstruction, expansion
o Perfusion: segmental emboli, regional destruction of capillaries, regional compression
Shunt
o Doesn’t respond to 100% O2
o 2-4% shunt is normal
o O2 content is proportional to Hgb and saturation
o Flattening of oxyhemoglobin saturation curve prevents compensation for increased proportion of deoxygenated blood leaving lung
o Not take much shunt to cause relatively severe hypoxemia
• Can be exacerbated by low mixed venous O2 (SVO2)
• From hypotension (shock)
• An extreme example of a shunt:
o Normal mixed venous PvO2 = 40 (saturation 75%)
o Peripheral tissue ischemia and acidosis → more O2 off-loading (more extraction)
o Extreme cases may have a mixed venous saturation of <40%
• Correct the underlying problem: IVF, Hgb, +/- inotropes
• Treat: supplemental O2
- Typically = patients hyperventilate but have increased work of breathing → may lead to Hypercapnic respiratory failure (depends on underlying cardiopulmonary status)
- May require assisted ventilation
Mechanisms of acute respiratory failure: Hypercapnic respiratory failure
Characterized:
Decreased minute ventilation (VE) relative to demand
o Decreased supply (ventilation)
o Increased demand: from increased VCO2 or VO2; increased drive to breathe
Decreased alveolar ventilation:
• VA = VE – (VE x Vd/Vt)
• VE = Vt x RR
o Increased dead space ventilation (VD or Vd/Vt)
o Decreased Vt (could be from abnormal respiratory or muscle mechanics)
o Decreased respiratory rate
o Increased dead space
Examples of Hypercapnic respiratory failure:
• CNS depression (decreased RR, decreased Vt, increased Vd/Vt)
• Neuromuscular diseases (decreased Vt, increased Vd/Vt)
Since underlying lung may be normal → (A-a)O2 gradient may be normal
• Even if hypoventilation leads to hypoxemia
Treatment: assist ventilation
Overall treatment goals for respiratory failure
o Treat underlying cause
o For chronic causes: supplemental O2 increased quality of life; prolongs survival in some cases
o Aim to achieve a oxyhemoglobin saturation at least 90% (to ensure optimum O2-carrying capacity)
• Based of Hgb curve = a PaO2 about 60 mmHg (to avoid pulmonary HT)
• Amount needed:
o (Baseline PaO2/Baseline FiO2) = (Desired PaO2/New FiO2)
Possible side effects of oxygen treatment
• Careful: supplemental O2 may induce hypoventilation → paradoxical increase in PaCO2:
o Causes:
• Central depression decreases drive to breathe
• Muscle weakness
o Leads to decreased Vt → hypoventilation
o Patients at higher risk = chronic hypoventilation + muscle weakness
• Ex: COPD patients experiencing exacerbation
• Also beware of O2 toxicity due to free radical production
List different modes of mechanical ventilation.
Nasal canula Face mask CPAP BiPAP/BPAP Invasive mechanical ventilation
Nasal canula
o Delivers up to 6 L/min
o Each L/mi 2-3% FiO2
o Max FiO2 around 35%
o Easy to use = can be used in ambulatory settings
o Can be limited by side effects (dry nose)
Face Mask
o Delivers up to 15 L/min
o Limits entrainment of room air FiO2 around 40%
o Can increase FiO2 closer to 100%
o Used in acute care settings: hospital patients, hospice
CPAP
(continuous positive airway pressure)
Tight mask delivers high flow of gas = airway and alveolar pressures are always positive
Useful for hypoxemia
• Recruits and stabilizes alveoli during expiration → Improved gas exchange (decreased venous admixture)
• Increased lung volume → improves compliance → can decrease the work of breathing
Advantages:
• Patient not intubated
• Minimal cardiovascular effects (minimal reduction in preload)
• May prevent need for intubation in patients without pneumonia
Disadvantages:
• Does not improve ventilation; not helpful for hypercarbia
• Patient must be alert and have intact drive to breathe
• Relatively small impact on work of breathing
• Prolonged used may dry respiratory secretions = risk for acute upper airway obstruction
o Also effective therapy for obstructive sleep apnea → stents upper airway
BPAP/BiPAP
(Bi-level positive airway pressure)
o During inspiration = use a higher pressure setting
o Decreases work of breathing and improves ventilation
• So can treat hypoxemia and hypercarbia
o Newer machines have back-up respiratory rate
• Prevents apnea in patients with impaired respiratory drive
o Can measure Vt = allows for adjustment of gradient between inspiration and exhalation
o Nocturnal ambulatory use: neuromuscular disease, occasionally COPD
Invasive Mechanical Ventilation
(via endotracheal tube)
Modes defined by trigger and cycle (by volume or pressure)
Often add Positive end expiratory pressure (PEEP)
• Helps hold non-compliant alveoli open at end expiration
• Improves FRC, lung compliance, and oxygenation
Advantages → Allows for: • Precise RR, Vt, FiO2 • Patient triggers for extra breaths • Airway pressure monitoring • Secretion management • Airway protection (ex: in people with decreased gag reflex) • Measurement of lung compliance
Disadvantages: • Invasive (potential trauma) • Infection • Interferes with communication • Negative cardiovascular effects (Decreased CO)
Name the oxygen treatment for the disorder
Decreased FiO2
Diffusion impairment
V/Q mismatch
–>Nasal canula, Face mask
Shunt
–>CPAP, Invasive mechanical ventilation with PEEP
Acute hypoventilation
–> BiPAP, Invasive mechanical ventilation
Decreased Mixed Venous Saturation
–> Invasive mechanical ventilation
Explain the relative utility of PEEP in relation to lung compliance and its disadvantages
PEEP = amount of pressure (set or intrinsic) left in system at the end of expiration
o Requires a closed system
o Set PEEP = useful tool for hypoxemia (especially due to shunt)
o PaO2 increases in proportion to PEEP
o Improves lung compliance of units not participating in gas exchange due to pulmonary edema or secretions
• Compliance = change in volume / change in pressure = Tidal volume/ (plateau pressure – PEEP)
• Plateau pressure = average pressure throughout system at Vt
o With diffuse lung injury = best PEEP associated with best lung compliance
Disadvantages:
o Too much PEEP limits Cardiac Output (by decreasing preload)
o May increase intrathoracic pressure → Over distend some alveoli → increases dead space ventilation because alveoli start to compress capillaries = impair blood flow
• Counteract by decreasing Vt (and increasing RR for adequate VE)
o Alveolar distension can also result in rupture (pneumothorax) or amplified inflammatory response in lung
Ventilator Strategies
Increased compliance at baseline
- normal TV
- prolong expiratory time
- avoid autpPEEP
- normal goals, including PaCO2 within 5 of baseline
Decreased compliance at baseline
- advanced directives
- low TV (ex: 6 cc/kg to keep pressures low)
- avoid secondary pulmonary edema
- adjusted physiologic goals, early tracheostomy
