Chapter 4 - Mechanical Ventilation Pt. 1

What is mechanical ventilation?

• use of an external device/machine that either breathes for the patient OR helps the patient breathe on his/her own

• a _____ is, “a device that delivers air into the lungs through a tube placed within the windpipe”

  • Delivers O2, removes CO2

What’s a ventilator?

• device for moving air

  • can be complex computerized; or, a simple bag mask

  • Ventilation → air exchange

  • Respiration → gas exchange

  • Perfusion → oxygen actually reaching the blood

What’s a respirator?

• device for filtering air

What are indications for mechanical ventilation?

• failure of the respiratory system

  • failed/weak respiratory muscles → alveolar hypoventilation

  • absent brainstem respiratory drive → central alveolar hypoventilation

  • hypoventilation → inadequate ventilation

    • over time → affects gas exchange at the level of the alveoli

      • hypoxemia

      • hypoxia

What is hypoventilation?

• theoretical: deviation from the usual state of ventilatory control resulting in decreased minute ventilation relative to metabolic requirements

• operational: PaCO2 EXCEEDS normal upper limit (45mmHg)

(Terms - Arterial Blood Gasses [ABGs]) SaO2/SpO2 - Oxygen Saturation:

• extent to which the hemoglobin is saturated w/ oxygen

(Terms - Arterial Blood Gasses [ABGs]) What is FIO2?

• Fraction of Inspired Oxygen

  • Room air FIO2 = ~21%

(Terms - Arterial Blood Gasses [ABGs]) What is the difference between PAO2 & PaO2?

• _____ the force necessary to move oxygen from the air into the blood

• _____ amount of O2 in the blood

  • 80-100 mm Hg

(Terms - Arterial Blood Gasses [ABGs]) What is the difference between PaCO2 & PACO2?

• _____ amount of CO2 in the blood

  • 35-45 mm Hg (millimeters of mercury)

• _____ the force necessary to move CO2 from the blood & into the air

(ABGs - Acidosis vs Alkalosis) What is normal blood pH?

• 7.34-7.45

(Homeostasis) Which systems work together to keep the body in homeostasis?

• respiratory & renal system

(Homeostasis) What does acute (uncompensated) impaired breathing impact?

• → Renal system (healthy) will compensate

• → Need ventilation/respiration support

(Homeostasis) What does acute (uncompensated) impaired kidney function impact?

• Respiratory system (healthy) will compensate

  • May need ventilation/respiration support

  • May need dialysis

(Homeostasis) What does chronic (compensated) breathing and/or kidney function impact?

• the body does its best to keep in homeostasis w/ either or both ventilation/respiratory support & renal support (dialysis)

• If unable →

  • Hypoxemia

  • Hypoxia

What is hypoxemia?

• decreased oxygen in the blood

What is hypoxia?

• decreased oxygen in the tissue

How do you treat hypoxia?

• Deliver more oxygen by increasing FIO2 & using nasal cannula (40% is common)

• Apply (Extrinsic) PEEP “Positive End-Expiratory Pressure”

  • Needs mechanical ventilation to get more PEEP

What is hypercapnia?

• Too much CO2 in the blood

• it’s bc ur hypoventilating from kidney → ur not urinating

What is PEEP?

• the amount of pressure needed to maintain air in the lungs after exhalation (at the “end” of exhalation).

• healthy systems have a little bit of natural or “physiologic” PEEP

• impaired systems may need to have external PEEP added

PEEP - Extrinsic maintains airway pressure above atmospheric pressure at the end of _____

• EXHALATION

What are the advantages of PEEP - Extrinsic?

• Mitigates alveolar collapse

• Increases gas exchange/oxygenation

• May reduce Ventilator Assisted Pneumonia (VAO)

• Helps decrease leakage of pharyngeal secretions into the lower airway

• Helps mitigate effects of Auto-PEEP

What is the typical small amount of PEEP-Extrinsic?

• 3-5 cm H2O is pretty routine

  • “Physiologic PEEP”

What is the typical large amount of PEEP-Extrinsic?

• > 5 is needed in acute lung injury, ARDS, or some other hypoxemia conditions

  • “Supraphysiologic PEEP”

What is Dead Space (DS)?

• volume of inspired air that is not used in gas exchange

  • ventilation w/o perfusion

• volume ≈ 1/3 of resting tidal volume

  • ≈ mL of lean mass lbs (ex: 100 lbs = 100 mL DS)

Total or Physiologic DS = Anatomic DS + Alveolar DS

• Anatomic (air that never made it to capillaries)

  • mouth, pharynx, trachea, bronchioles

• Alveolar (dead space in between alveoli)

  • sum of the volume of the individual alveoli that have little or no blood flow through their capillaries

  • Negligible in healthy people; can increase dramatically in lung diseases

(DS) Is breathing faster & shallower more effective?

• NO! Breathing slower & deeper is more effective than breathing faster & shallower, even if total volume remains the same

  • 10 bpm x 500 ml = 5000 ml IS BETTER THAN

  • 20 bpm x 250 ml = 5000 ml exchanged per minute

Why is breathing slower and deeper more effective?

• More DS w/ faster shallower breathing

• “wasted” respiratory effort, leading to fatigue & possible acidosis

• More CO2 retention = Higher PaCO2, & possible respiratory acidosis

• Less Oxygen Perfusion

(Mechanical Ventilation Options) What are the Types of Ventilation?

• Positive Pressure Ventilation

• Negative Pressure Ventilation

• High Frequency Ventilation

What are the 2 air delivery methods?

• Non-Invasive

• Invasive

Non-invasive =

• External (air gets blown into mouth externally)

Invasive =

• Internal

  • Endotracheal tubes (air being blown through tube in you)

  • Tracheostomy tubes

What is Positive Pressure Ventilation?

• Machine provides pre-set positive pressure (above atmospheric pressure), pushes a pre-set volume of gas into the airway, inflates the lungs, increases intra-alveolar pressure

• Pressure stops, exhalation occurs

• Cycles at pre-set interval or Respiratory Rate

• Passive inspiration

• Passive exhalation

• Invasive or Non-Invasive

• Invasive is Common in ICU’s

What is the sequence of Positive Pressure Ventilation?

• Machine provides positive pressure → Air is pushed into lungs → Lungs & Alveoli Expand

Can positive pressure ventilation be only be invasive?

• No, can be invasive or non-invasive!

  • Invasive: Endotracheal tube, Tracheostomy

  • Non-Invasive (external): CPAP, BiPAP, VAPS

What is Negative Pressure Ventilation?

• Ventilators go “around the body”

• Machine creates a vacuum around the chest wall & acts as inspiratory musculature

• Chest wall is sucked outward, diaphragm is lowered, & pleural pressure falls (goes below atmospheric pressure), lungs & alveoli expand as air rushes in

• Vacuum stopped, exhalation occurs

• Cycle repeats at pre-set interval

• “Active-ish” inspiration

• Passive exhalation

• Not so common these days

• Non-invasive only

What is the sequence of Negative Pressure Ventilation?

• Vacuum sucks out chest; creates negative pressure → Lungs & Alveoli expand → Air rushes in

What are the Negative Pressure Types?

• Iron Lung

• Cuirass

• Rocking Bed

• Pneumobelt

Iron Lung:

• full body surrounded by machine (Polio)

Cuirass:

• chest piece attached to a vacuum

  • replaced iron lungs

Rocking Bed:

• uses gravity to pull on abdomen & displace diaphragm

  • Head of bed UP → Diaphragm pulled down → Inhale

  • Head of bed DOWN → Diaphragm pulled up → Exhale

Pneumobelt:

• forces diaphragm upward to assist w/ exhalation

What is High Frequency Ventilation (HFV)?

• respiration is set much higher than normal rates

• Low tidal volumes; i.e., very shallow, fast breathing

• Limit the amount of lung inflation

• Used in neonates & in cases of adults w/ severe ARDS

• Lowers risk of Ventilator-Associated lung injury

• Disadvantage - higher risk of atelectasis

What are the 3 types of High Frequency Ventilation (HFV)?

• HF Positive Pressure Ventilation (HFPPV)

• HF Jet Ventilation (HFJV)

• High Frequency Oscillating Ventilation (HFOV)