Mechanical Vent Chapter 4- Establishing the need for mechanical ventilation

Establishing the Need for Mechanical Ventilation

Assessing the Patient

• Physical Examination:

  • Level of consciousness: Awake/asleep? Can they be roused and stay awake?

  • Skin Color and appearance: Warm and pink vs. cyanotic? Cool, clammy? Diaphoretic?

  • Vital Signs: HR, BP, respirations, breath sounds, and SpO2 - are they normal?

  • Mental Status: AAO x 3 (Alert and Oriented to person, place, and time)?

  • Anxiety/Agitation: Determine if anxiety alone is the problem.

• Patient History and Diagnosis:

  • PMH (Past Medical History) and diagnosis.

  • Pre-existing pulmonary or neuromuscular diseases.

  • Traumatic injuries to head or spine.

Respiratory Failure

• ARF (Acute Respiratory Failure): Respiratory activity is absent or insufficient for CO2 removal and O2 uptake.

  • Inability to maintain acceptable PaO2, PaCO2, and pH levels.

  • Criteria:

    • PaO2 < 70 on > 0.6 FiO2

    • PaCO2 > 55 and rising

    • pH < 7.25

• Types of Respiratory Failure:

  • Hypoxic Respiratory Failure

  • Hypercapnic Respiratory Failure

Hypoxic Respiratory Failure

• May not require mechanical ventilation; O2 therapy and/or CPAP may suffice.

• Causes: V/Q mismatch, diffusion defects

Hypercapnic Respiratory Failure

• Inability to maintain normal PaCO2.

• Causes: CNS, neuromuscular, and WOB (Work of Breathing) disorders.

Conditions of Hypercapnia and/or Hypoxia

Hypoxia:

• Mild to moderate:

  • Respiratory findings: Tachypnea, Dyspnea

  • Cardiovascular findings: Paleness, Tachycardia, Mild hypertension, Peripheral vasoconstriction

  • Neurological findings: Restlessness, Disorientation, Headaches, Lassitude

• Severe:

  • Respiratory findings: Tachypnea, Dyspnea

  • Cardiovascular findings: Cyanosis, Tachycardia and eventual bradycardia, arrhythmias, Hypertension and eventual hypotension

  • Neurological findings: Somnolence, Confusion, Blurred vision, Tunnel vision, Loss of coordination, Impaired judgment, Slow reaction time, Manic-depressive activity, Coma

Hypercapnia:

• Mild to moderate:

  • Respiratory findings: Tachypnea, Dyspnea

  • Cardiovascular findings: Tachycardia, Hypertension, Vasodilation

  • Neurological findings: Headaches, Drowsiness

• Severe:

  • Respiratory findings: Tachypnea and eventual bradypnea

  • Cardiovascular findings: Tachycardia, Hypertension and eventual hypotension

  • Neurological findings: Hallucinations, Hypomania, Convulsions, Coma

  • Signs: Sweating, Redness of the skin.

CNS Disorders

• Generally cause hypoventilation.

• Causes:

  • Drugs: sedatives, narcotics, pain medications

  • Brain trauma or pathology: stroke, CHI, ICB, CVA – “Cheyne-Stokes or Biot’s” respirations

  • Pathology induced CO2 narcosis: PaCO2 > 70 mmHg acts as CNS depressant.

Neuromuscular Disorders

• Paralytic disorders: M.G. (Myasthenia Gravis), G.B. (Guillain-Barré syndrome), Tetanus, M.S. (Multiple Sclerosis)

• Paralytic Drugs: Succinylcholine, Norcuron, Pavulon, Zemuron, Curare, nerve gas

• Other drugs affecting muscle function: certain antibiotics, long-term steroids

• Impaired muscle function: electrolyte imbalances, malnutrition, air-trapping.

Increased WOB (Work of Breathing)

• Pleural lesions: pneumo/hemothorax, effusions, empyema

• Chest wall deformities: Trauma, obesity, kyphoscoliosis

• Increased airway resistance: bronchospasm, mucosal edema, airway inflammation

• Lung tissue pathology: ARDS, pulmonary edema

Physiologic Measurements

• MIP (Maximum Inspiratory Pressure) or NIF (Negative Inspiratory Force):

  • Maximum negative pressure a patient can generate during an inspiratory effort.

  • Requires significant patient effort.

  • Indicates ability to take a deep breath.

• VC (Vital Capacity):

  • Maximum inspiration followed by maximum expiration.

  • Indicates ability to generate a significant cough.

• NIF and VC can be performed every 2-4 hours.

• PEFR (Peak Expiratory Flow Rate):

  • Indicates ability to maintain adequate airway patency.

  • Routinely measured pre and post bronchodilator therapy during Asthma exacerbation.

• RR (Respiratory Rate): Normal range is 12 – 20 breaths/min.

• MV (Minute Ventilation): Normal range is 5 – 6 L/min.

Indications of Acute Respiratory Failure and Need for Mechanical Ventilation

• Indicates need for mechanical ventilatory support.

• Indicates need for oxygen therapy or PEEP/CPAP.

Ventilation Failure

• PaCO2 is the best indicator of adequate ventilation.

• Increased Dead Space: VD/VT ratio 0.3 – 0.4.

  • Dead space is an area that is ventilated but not perfused.

  • Pathological example: pulmonary embolus.

Oxygenation Failure

• PaO2 is the best indicator of adequate oxygenation.

• SpO2 is an acceptable non-invasive alternative (except with abnormal hemoglobin types).

• CaO2 is a good indicator of a patient’s total oxygen carrying capacity.

  • CaO2 = (Hb \times 1.34 \times SaO2) + (PaO2 \times 0.003), normal = 19.5 vol.%

Alternatives to Invasive Ventilation

• Oxygen Therapy, Hi-Flo O2 therapy.

• Hi-Flow nebulizer, Heated or unheated.

• NPPV (Non-invasive Positive Pressure Ventilation)

Objectives of Mechanical Ventilation

• Physiological objectives

  1. To support or manipulate pulmonary gas exchange:

     • Alveolar ventilation-normal or deliberate hyperventilation (sustained hypocapnia)

     • Alveolar oxygenation-maintain oxygen delivery (CaO2 x cardiac output) at or near normal

  2. Increase lung volume:

     • Prevent or treat atelectasis with adequate end-inspiratory lung inflation

     • Achieve and maintain an adequate functional residual capacity (FRC)

  3. Reduce the work of breathing

• Clinical objectives

  1. Reverse acute respiratory failure

  2. Reverse respiratory distress

  3. Reverse hypoxemia

  4. Prevent or reverse atelectasis

  5. Reverse ventilatory muscle fatigue

  6. Permit sedation and/or paralysis

  7. Reduce systemic or myocardial oxygen consumption

  8. Reduce intracranial pressure

  9. Stabilize the chest wall

Criteria for Mechanical Ventilation

• Apnea or absence of breathing/airway protection

• Acute Respiratory Failure

• Impending Respiratory Failure

• Refractory hypoxic Respiratory Failure with increased WOB.

• Acute exacerbation of COPD with complicating factors.

Ethical Considerations

• Does the patient have a “DNR” (Do Not Resuscitate) or “DNI” (Do Not Intubate) status?

• Does the patient have an “Advanced Directive?”