Principles of Mechanical Ventilation

1
Principles of Mechanical Ventilation

• RET 2284
• Module 1.0
• Respiratory Physiology and Pathophysiology Review

2
Physiology and Pathophysiology

• Respiratory Failure - 1994 Study
• Forty-four percent of patients diagnosed with
  respiratory failure died in the hospital
• 1999 review showed only a marginal improvement,
  with a 36 hospital mortality

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Physiology and Pathophysiology

• Respiratory Failure
• Inability to maintain either the normal
  delivery of oxygen to the tissues or the normal
  removal of carbon dioxide from the tissues
• PaO2 lt60 mm Hg and /or a PaCO2 gt50 mm Hg in
  otherwise healthy individuals breathing room air
  at sea level

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Physiology and Pathophysiology

• Respiratory Failure
• Two categories
• Hypoxemic (Type I) inadequate O2 delivery
• Hypercapnic (Type II) ventilatory failure
  resulting in elevated CO2 levels
• pump failure

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Primary causes of hypoxemia
• Ventilation/perfusion mismatch
• Shunt
• Alveolar hypoventilation
• Diffusion impairment
• Perfusion/diffusion impairment
• Decreased inspired oxygen

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Ventilation/perfusion Mismatch
• Pathologic V/Q mismatch occurs when disease
  disrupts the balance between ventilation and
  perfusion and hypoxemia results
• Most commonly, ventilation is compromised despite
  adequate blood flow resulting in a low V/Q ratio
• V/Q mismatch responds well to O2

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Ventilation/perfusion mismatch
• Obstructive lung disease
• Bronchospasm
• Mucus plugging
• Inflammation
• Infection
• Heart failure
• Inhalation injury

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Ventilation/perfusion mismatch
• Clinical Presentation of Hypoxemia
• Tachycardia
• Dyspnea
• Tachypnea
• Use of accessory muscles
• Body position (tripod)
• Nasal flaring
• Central cyanosis

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Ventilation/perfusion mismatch
• Clinical Presentation of Hypoxemia
• Irritability, confusion (when severe)
• Bilateral wheezing
• Bilaterally diminished breath sounds in patients
  with emphysema
• Absent breath sounds on one side (pneumothorax,
  pneumonia, effusion)
• Unilateral crackles (alveolar filling due to
  mass, infection, fluid)

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Ventilation/perfusion mismatch
• Clinical Presentation of Hypoxemia
• Radiographic Findings
• Black chest radiograph indicates hyperinflated
  lungs
• White chest radiograph indicate that alveoli are
  partially occluded

Emphysema
RUL Pneumonia
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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Shunt
• No ventilation to match perfusion
• Pathologic Anatomical Shunt
• Right-to-left blood flow through cardiac openings
  (e.g., atrial or ventricular septal defects)
• Physiologic Shunt (leading to hypoxemia)
• Atelectasis
• Pulmonary edema
• Pneumonia
• Shunt shows little to no improvement with O2

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Shunt
• Clinical Presentation of Shunt
• Very similar to V/Q mismatch
• Bilateral or unilateral crackles are common due
  to alveolar filling process
• Unilateral absence of breath sounds may indicate
• Pneumothorax
• Mass
• Effusion

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Shunt
• Clinical Presentation of Shunt
• Radiographic Findings
• White chest radiograph (e.g., diffuse bilateral
  haziness in ARDS)

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Alveolar Hypoventilation
• Will be discussed with Acute Hypercapnic
  Respiratory Failure (Type II)

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Diffusion Impairment
• Reduction in the movement of gas across the
  alveolar-capillary
• Interstitial lung disease (thickening and
  scarring of the interstitium)
• Pulmonary fibrosis
• Asbestosis
• Sarcoidosis

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Diffusion Impairment
• Reduction in the movement of gas across the
  alveolar-capillary
• Emphysema
• Pulmonary vascular abnormalities
• Anemia
• Pulmonary hypertension
• Pulmonary embolus

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Diffusion Impairment
• Clinical Presentation (rarely present as an acute
  hypoxemia)
• Interstitial lung disease
• Dyspneic
• Dry cough
• Fine, basilar crackles
• May have clubbing of nailbeds
• Joint abnormalities (rheumatological)

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Diffusion Impairment
• Clinical Presentation (rarely present as an acute
  hypoxemia)
• Anemia
• Pallor
• Pulmonary hypertension
• Peripheral edema
• Jugular vein distension

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Diffusion Impairment
• Clinical Presentation (rarely present as an acute
  hypoxemia)
• Radiographic findings
• Black chest radiograph (emphysema)
• Reduced lung volumes with interstitial markings
  (interstitial disease)
• Enlarged right ventricle and pulmonary arteries
  (pulmonary hypertension)

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Perfusion/Diffusion Impairment
• Found in individuals with liver disease
• Right-to-left intracardiac shunt combines with
  dilated pulmonary capillaries resulting in
  impaired gas exchange
• Cirrhosis is the most common liver disease
• Though shunt is a component of the syndrome,
  significant supplemental oxygen can overcome the
  gas transfer reduction dir to the dilated vessels

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Physiology and Pathophysiology

• Acute Hypoxemic Respiratory Failure (Type I)
• Decreased Inspired Oxygen
• Inspired oxygen fall below body requirements
• Most commonly occurs at high altitudes where
  barometric pressure decreases, resulting in a
  decrease in the partial pressure of inspired O2
• Air travel (offset by pressurizing cabins,
  travelers with chronic hypoxemia may still need
  supplemental oxygen)
• Mountain climbing

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Physiology and Pathophysiology

• Acute Hypercapnic Respiratory Failure (Type II)
• Ventilatory Failure (pump failure)
• Characterized by an elevated PaCO2 creating an
  uncompensated respiratory acidosis, whether acute
  or acute-on-chronic
• Hypoxemia may often accompany ventilatory failure
  due simply to the displacement of alveolar PO2
  (PAO2) by an increased PACO2 (alveolar
  hypoventilation)
• Identified with a room air ABG by a normal
  P(A-a)O2 gradient

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Physiology and Pathophysiology

• Acute Hypercapnic Respiratory Failure (Type II)
• Ventilatory Failure (pump failure)
• Decreased Ventilatory Drive
• Respiratory Muscle Fatigue or Failure
• Increased Work of Breathing

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Physiology and Pathophysiology

• Acute Hypercapnic Respiratory Failure (Type II)
• Ventilatory Failure (pump failure)
• Decreased Ventilatory Drive
• Dysfunction of the central (medullary) and
  peripheral (aortic and carotid bodies)
  chemoreceptors responding to CO2 tension and O2
  tension that stimulate the drive to breathe. The
  ventilatory drive can be diminished by the
  following
• Drugs (overdose/sedation)
• Brainstem lesions
• Hypothyroidism
• Morbid obesity (obesity hypoventilation)
• Sleep apnea

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Physiology and Pathophysiology

• Acute Hypercapnic Respiratory Failure (Type II)
• Ventilatory Failure (pump failure)
• Decreased Ventilatory Drive
• Clinical Presentation
• Bradypnea (respiratory rate lt12, ultimately
  apnea)
• Drug overdose and brain disorder (altered level
  of consciousness, lethargic, obtunded, comatose)
• Hypothyroidism (fatigue)
• Obesity-hypoventilation (rapid, shallow
  breathing)
• Sleep Apnea (hypersomnolence)

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Physiology and Pathophysiology

• Acute Hypercapnic Respiratory Failure (Type II)
• Ventilatory Failure (pump failure)
• Respiratory Muscle Fatigue or Failure
• Neuromuscular Dysfunction
• Amyotrophic Lateral Sclerosis (ALS)
• Guillain-Barré
• Myasthenia Gravis
• Muscular Dystrophy

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Physiology and Pathophysiology

• Acute Hypercapnic Respiratory Failure (Type II)
• Ventilatory Failure (pump failure)
• Respiratory Muscle Fatigue or Failure
• Clinical Presentation
• Respiratory muscle fatigue and elevated PaCO2
• ALS (drooling, dysarthria, weak cough, supine
  paradoxical breathing in advanced cases)
• Guillain-Barré (lower extremity weakness, weak
  cough and gag)
• Myasthenia Gravis (ocular muscle weakness)

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Physiology and Pathophysiology

• Acute Hypercapnic Respiratory Failure (Type II)
• Ventilatory Failure (pump failure)
• Increased Work of Breathing
• An imposed respiratory workload that cannot be
  overcome
• COPD (increased deadspace)
• Asthma (elevated airway resistance)
• Thoracic abnormalities (pneumothorax, rib
  fractures, pleural effusions, etc.)
• Increased CO2 production requiring increased
  minute ventilation (hypermetabolic states as in
  extensive burns)

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Physiology and Pathophysiology

• Acute Hypercapnic Respiratory Failure (Type II)
• Ventilatory Failure (pump failure)
• Increased Work of Breathing
• Clinical Presentation
• Hyperventilation (COPD/Asthma exacerbation)
• Rapid but shallow respirations (indicative of
  impending respiratory failure)
• Irritability and confusion (signs of worsening
  hypercapnia)

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Physiology and Pathophysiology

• Chronic Respiratory Failure (Type I and II)
• Respiratory failure that has developed over weeks
  or months and has become chronic
• Compensatory mechanisms have developed to adapt
  to the chronic hypercapnia
• Elevated bicarbonate levels - renal response to
  elevate the blood ph
• Polycythemia may result from the prolonged
  hypoxemic respiratory failure
• Hemoglobin releases O2 more easily O2
  dissociation curve shift to right in acidosis
• O2 delivery to the brain is enhanced
  hypercapnia results in increased cerebral blood
  flow

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Physiology and Pathophysiology

• Chronic Respiratory Failure (Type I and II)
• Acute-on-Chronic Respiratory Failure
• Exacerbation of chronic respiratory failure
• Most Common Precipitating Factors
• Bacterial or viral infections
• Congestive heart failure (CHF)
• Pulmonary Embolus
• Chest wall dysfunction (e.g., pneumothorax,
  fractured ribs, pleural effusion)
• Medical noncompliance (not following prescribed
  treatment plans)

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Physiology and Pathophysiology

• Differentiating the Causes of Hypoxemia
• Three main causes
• V/Q Mismatch
• Shunt
• Hypoventilation

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Physiology and Pathophysiology

• Differentiating the Causes of Hypoxemia
• V/Q Mismatch
• Elevated P(A-a)O2 gradient
• Shunt
• Elevated P(A-a)O2 gradient
• Hypoventilation
• Differs from the other causes in presenting with
  a normal P(A-a)O2 gradient
• Normal P(A-a) O2 gradient 10 (young) to 25
  (elderly)

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Physiology and Pathophysiology

• Differentiating the Causes of Hypoxemia
• Alveolar Air Equation
• PAO2 FiO2 (PB PH2O) PaCO2/R
• Example
• PAO2 0.21 (760-47) 40/0.8
• PAO2 100

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Physiology and Pathophysiology

• Differentiating the Causes of Hypoxemia
• P(A-a)O2 Gradient
• Example (2 patients)
• A B
• pH 7.45 7.21
• PCO2 33 72
• PaO2 40 53
• HCO3- 22 28
• SaO2 70 81
• FiO2 0.21 0.21

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Physiology and Pathophysiology

• Differentiating the Causes of Hypoxemia
• P(A-a)O2 Gradient
• Patient A PAO2 0.21 (760-47) 33/0.8 108
  mm Hg
• PaO2 40 mm Hg
• P(A-a)O2 108 40 68 mm Hg
• Patient B PAO2 0.21 (760-47) 72/0.8 60 mm
  Hg
• PaO2 53 mm Hg
• P(A-a)O2 60 53 7 mm Hg

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Physiology and Pathophysiology

• Differentiating the Causes of Hypoxemia
• P(A-a)O2 Gradient
• Patient A Has hypoxemic respiratory failure with
  a P(A-a)O2 of 68 mm Hg (elevated) indicating a
  oxygen defect. Significant response to 100
  oxygen would indicate V/Q mismatch, while shunt
  would be implicated in the PaO2 did not respond
  (treat with PEEP)
• Patient B Has hypercapnic respiratory failure
  (ventilatory failure) with hypoxemia with a
  P(A-a)O2 of 7 mm Hg indicating a pure ventilatory
  defect. Treatment should be focused on improving
  ventilation because the hypoxemia is purely a
  result of alveolar displacement of oxygen by
  elevated carbon dioxide