RET 2284 Mod 1.0 Review of Phys and Pathophys

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RET 2284 Mod 1.0 Review of Phys and Pathophys Powered By Docstoc
					Principles of Mechanical Ventilation
RET 2284
Module 1.0
Respiratory Physiology and
Pathophysiology Review
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
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 <60 mm Hg and /or a PaCO2 >50 mm Hg in
        otherwise healthy individuals breathing room air at sea
        level
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”
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
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
Physiology and Pathophysiology

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

Shunt               Pneumonia




       Shunt shows little to no improvement with O2
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
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)
Physiology and Pathophysiology

  Acute Hypoxemic Respiratory Failure (Type I)
      Alveolar Hypoventilation
         Will be discussed with Acute Hypercapnic Respiratory
         Failure (Type II)
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
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
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)
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
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)
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
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
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
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
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
Physiology and Pathophysiology

  Acute Hypercapnic Respiratory Failure (Type II)
      Ventilatory Failure (pump failure)
         Decreased Ventilatory Drive
              Clinical Presentation
                  Bradypnea – (respiratory rate <12, ultimately apnea)



                    Drug overdose and brain disorder (altered level of
                     consciousness, lethargic, obtunded, comatose)
                    Hypothyroidism (fatigue)
                    Obesity-hypoventilation (rapid, shallow breathing)
                    Sleep Apnea (hypersomnolence)
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
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)
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)
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)
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
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)
Physiology and Pathophysiology

  Differentiating the Causes of Hypoxemia
      Three main causes
           V/Q Mismatch
           Shunt
           Hypoventilation
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)
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
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
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
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

				
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