Acute Respiratory Failure

Introduction

• Brian Sheehan, a hospitalist with experience in critical care, will discuss respiratory failure.
• The presentation will cover high flow nasal oxygen, non-invasive ventilation, and ARDS.

Respiratory Physiology Basics

• Simplified model: a straw and two balloons.
• Inhalation: Diaphragm contracts, creating negative pressure, which pulls air in.
• Interventions often use positive pressure, altering physiology.
• Lung function: Exchange carbon dioxide out for oxygen in.
• Failure to oxygenate leads to hypoxemia and hypoxia.
• Failure to remove carbon dioxide leads to hypercapnia.

Clinical scenarios

• Scenario: A 19-year-old defensive lineman demands oxygen after running off the field.
  • Physiologically, oxygen is not helpful in this situation.
  • To oxygenate, increase FiO2 or mean airway pressure (PEEP).
  • To remove carbon dioxide, breathe deeply and rapidly.
  • Anaerobic activity leads to carbon dioxide and lactic acid production; only way to recover is to breathe big.
  • Giving the player oxygen may be helpful if at high altitude.

Types of Respiratory Failure

• Type 1 (Hypoxemic): Low oxygen in the blood ($PaO_2$ < 60 mmHg).
  • $PaCO_2$ is generally normal but may be low if the patient is dyspneic or anxious.
• Type 2: Low oxygen due to carbon dioxide displacing oxygen.
  • High carbon dioxide ($PaCO_2$ > 45 mmHg) leads to hypercapnia.
  • Low pH indicates acute respiratory acidosis.
  • In acute respiratory acidosis, bicarbonate increases slightly as the kidney releases bicarbonate to buffer the pH.
  • Bicarbonate increases by approximately one for every 10 of CO2.
  • Normal pH with high CO2 indicates a chronic condition, with potentially very high bicarbonate levels.

Hypoxemia vs. Hypoxia

• Hypoxemia: Low oxygen in the blood.
  • Describes a symptom (e.g., low pulse ox or $PaO_2$).
• Hypoxia: General state of low oxygen, leading to end-organ damage.
  • Indicates tissue-level oxygen deficiency causing damage (e.g., hypoxic brain injury).

Clinical assessment with a venous blood gas

• Scenario: A 14-year-old female with asthma presents in respiratory distress.
  • Venous Blood Gas (VBG) can be used if an ABG is not readily available.

Venous Blood Gas (VBG) Interpretation

• Arterial Blood Gas (ABG) normal values (Rule of Fours):
  • pH: 7.4
  • $PCO_2$: 40
  • Bicarbonate: 24
• Venous Blood Gas (VBG) normal values (Rule of Fours):
  • pH: Subtract 0.04 (Normal Venous pH)
  • $PCO<em>2$: Add 4 to 8 (Normal Venous $PCO</em>2$)
• Never use $PaO_2$ from a venous gas.
• Bicarbonate is the same in VBG and ABG; VBG might be more accurate for bicarbonate.
• Base excess is equivalent in VBG.

Pulse Oximetry

• Pulse oximetry uses red and infrared light to detect oxygen bound to hemoglobin.
• It identifies arterial oxygen saturation by detecting the pulse.
• A good waveform on the pulse oximeter indicates an accurate reading.
  • If the waveform is poor, the reading is unreliable.
• Pulse oximetry estimates $PaO_2$:
  • SpO2 of 90% corresponds to a $PaO_2$ of approximately 60.
  • SpO2 of 88% corresponds to a $PaO_2$ of approximately 55, which is acceptable for COPD patients.
• Danger of 100% SpO2: the curve plateaus, so you don't know if the $PaO_2$ is much higher.

Advantages of Pulse Oximetry over ABG

• ABG is painful and expensive.
• ABG results can be variable, even when repeated quickly.
• Pulse oximetry provides continuous monitoring.

Limitations of Pulse Oximetry

• Poor waveform.
• Nail polish.
• Carboxyhemoglobin (carbon monoxide poisoning): Pulse ox reads 100% despite hypoxemia.
• Methemoglobinemia: Can cause inaccurate readings (example with topical lidocaine).

Initial Approach to Hypoxemia

• Scenario: A nurse is distressed because Mr. Jones has a pulse ox of 77%.
  • First, assess the patient's airway and mentation by asking their name.

Management of Hypoxemia

• Scenario: Mr. Jones is short of breath with a pulse ox of 77%.
  • Use a non-rebreather mask to deliver high-flow oxygen immediately.
• Nasal Cannula:
  • Easy to access and comfortable but provides limited FiO2 (up to 44%).
• Non-Rebreather:
  • Provides higher FiO2 but requires a good fit.
  • Must ensure adequate oxygen flow (15 liters/minute or higher) to avoid suffocation.

Escalating Treatment

• Scenario: Mr. Jones's pulse ox improves to 84% on a non-rebreather, but he still struggles to breathe.
  • Options: Continue non-rebreather, intubate, check VBG, start BiPAP, or start high-flow nasal oxygen.
• Recent literature supports non-invasive ventilation and high-flow nasal cannula.
• The priority is to increase FiO2 and add PEEP to improve oxygenation.

Positive Pressure Ventilation

• PEEP expands alveoli, increasing the surface area for oxygen exchange.

CPAP (Continuous Positive Airway Pressure)

• Provides continuous positive pressure.
• Increases alveolar surface area.
• Used in obstructive sleep apnea, neonatal respiratory distress, cardiogenic pulmonary edema, and post-operative anaphylaxis.

BiLevel Ventilation (BiPAP)

• Provides positive pressure with an extra burst of air upon inhalation.
• Recruits more alveoli, decreases work of breathing, and can increase tidal volume.
• Settings: e.g., 10/5 means CPAP is at 5 (EPAP), and inspiratory pressure (IPAP) is at 10.

Indications for BiPAP

• COPD exacerbation: Prevents intubation by reversing acute respiratory acidosis.
• Cardiogenic pulmonary edema: Prevents further respiratory decline.
• High-risk extubation: Decreases the chance of reintubation.
• Insufficient evidence for immunocompromised, trauma, asthma, or pneumonia patients.
• General rule of thumb: if recovery will take longer than 24 hours, skip BiPAP.
• Non-invasive positive pressure ventilation can be used to pre-oxygenate for intubation.

Contraindications for BiPAP

• Critically ill patients who cannot protect their airway.
• Excessive secretions or vomiting.
• Obtunded patients who cannot remove the mask themselves.

Rules of Thumb for Non-Invasive Ventilation

• Use for no longer than 24 hours.
• Patient must improve immediately; otherwise, intubate.

High Flow Nasal Cannula

• Delivers humidified air and provides a small amount of PEEP (3-4 cm H2O).
• Decreases dead space, allowing for higher FiO2 delivery.
• More comfortable than CPAP/BiPAP and allows patients to eat and speak.
• Decreases work of breathing and can increase tidal volumes.
• Decreases intubation rate in some cases.
• Internal medicine colleagues favor high flow nasal oxygen over non-invasive ventilation in undifferentiated acute hypoxemic respiratory failure.

Indications for Intubation

• GCS < 8 (airway protection).
• Refractory hypercapnia.
• Refractory hypoxemia.
• Exhaustion.

Ventilator Management Post-Intubation

• Scenario: Mr. Jones is intubated, and an ABG shows pH 7.55, PaCO2 22, PaO2 83, and bicarbonate 20.
  • Lower respiratory rate to address over-ventilation and alkalosis.
  • No need to adjust oxygenation settings.

Basic Mechanical Ventilation

• Volume cycled/volume control is commonly used.
• Ventilator interface example:
  • Pressure loop shows pressure over time (e.g., PEEP of 10, pressure rises to 30).
  • Flow loop shows air movement in and out of the patient's lungs.
  • Volume loop shows tidal volume and pressure.
  • Ventilators typically group oxygen settings (FiO2 and PEEP) and carbon dioxide settings (respiratory rate and pressure).

Acute Respiratory Distress Syndrome (ARDS)

• Scenario: Mrs. Jones presents with respiratory distress, pulse ox of 72%, intubated after high flow nasal cannula failure, chest x-ray shows diffuse infiltrates, influenza B positive.
• ARDS Berlin Criteria:
  • Onset within 7 days of a pulmonary insult.
  • Bilateral chest infiltrates not due to heart failure.
  • $PaO<em>2/FiO</em>2$ ratio (P/F ratio) to determine severity.

ARDS Treatment

• Supportive care: Treat underlying disorder and allow the body to heal.
• Mechanical ventilation is used to rest respiratory muscles while providing gas exchange.
• Positive evidence:
  • Early paralysis can work.
  • Proning can work.
  • Low tidal volumes work.
  • High PEEP works.
  • ECMO works.
• Negative evidence:
  • Early paralysis negative.
  • Routine use of ECMO negative.
• Oscillator ventilators are generally ineffective.
• Strategy: High PEEP, low tidal volumes.

ARDSNet Protocol

• Use a PEEP/FiO2 table to adjust ventilator settings.
• Prioritize increasing PEEP over FiO2.
• Inspiratory Hold to Measure Plateau Pressure:
  • Plateau pressure should be < 30 cm H2O to avoid lung injury.
  • Plateau pressure helps assess the safety of PEEP and tidal volume.

ARDS Key Management Points

• Low tidal volumes, high PEEP.
• Conservative fluid management.
• Prone positioning.
• Paralysis only if needed.
• Steroids to treat the underlying cause, not the ARDS itself.
• ECMO, if severe hypoxemia persists despite optimal ventilation strategies.

Summary of Respiratory Support Options

• Nasal cannula and oxygen masks primarily increase FiO2.
• High flow nasal cannula increases FiO2, provides some PEEP, and decreases the work of breathing.
• CPAP provides higher FiO2 and more PEEP than high flow nasal cannula.
• BiLevel offers even higher PEEP, decreased work of breathing, and potentially increased tidal volume.
• Mechanical ventilation takes over all respiratory functions.

Clinical scenarios (Post-Lecture Questions)

• Scenario: Post-gastric bypass patient with 84% SpO2.
  • CPAP is a good choice to address atelectasis.
• Scenario: 14-year-old female with severe asthma and normal ABG.
  • Impending respiratory failure; consider intubation.
  • Normal ABG in an asthmatic indicates fatigue.
  • Be cautious with non-invasive ventilation in asthmatics due to increased mortality.
• Scenario: Asthmatic patient crashing after intubation due to breath-stacking.
  • Disconnect from the ventilator and manually compress the chest.
  • Reduce respiratory rate to allow for full exhalation.
• Scenario: COPD patient not improving with oxygen.
  • Consider pulmonary embolism.
  • Hypoxemia in COPD usually responds well to oxygen; if not, consider other diagnoses.