MECHANICAL VENTILATION

KENNEY WEINMEISTER M.D.

INDICATIONS FOR MV

• Hypoxemia

• Acute respiratory acidosis

• Reverse ventilatory muscle fatigue

• Permit sedation and/or neuromuscular blockade

• Decrease systemic or myocardial oxygen consumption

INDICATIONS CONTINUED

• Reduce intracranial pressure through controlled hyperventilation

• Stabilize the chest wall

• Protect airway– Neurologic impairment– airway obstruction

TYPES OF CONVENTIONAL MV

• Timed cycled– Home ventilators

• Pressure cycled– Pressure controlled

• Volume cycled

• Flow cycled– Pressure support

VOLUME VENTILATION

• Controlled mechanical ventilation CMV

• Assist-control AC

• Synchronized intermittent mandatory ventilation SIMV

• Which mode?

VENTILATOR SETTINGS

• Tidal volume– 10 to 15 mL/kg

• Respiratory rate– 10 to 20 breaths/minute– normal minute ventilation 4 to 6 L/min

• Fraction of inspired oxygen

• Flow rate and I:E ratio

PRESSURE SUPPORT VENTILATION

• Flow cycled– preset pressure sustained until inspiratory flow

tapers to 25% of maximal value

• Comfortable

• Used mainly as a weaning mode

• Wean pressure until equivalent to air way resistance– peak - plateau pressure

PRESSURE CONTROLED VENTILATION

• Pressure cycled

• Volume varies with lung mechanics

• Minute ventilation is not assured

• Improves oxygenation– recruitment of alveoli

• Lessens volutrauma?

SETTINGS FOR PRESSURE CONTROL VENTILATION

• Inspiratory pressure

• I:E ratio– 1:2, 1:1, 2:1, 3:1

• Rate

• FIO2

• Peep

PRESSURE REGULATED VOLUME CONTROLLED

• Ventilate with pressure control

• Preset volume

• Inspiratory pressure is adjusted breath to breath

• Minute ventilation is maintained

INDICATIONS FOR PEEP

• ARDS

• Stabilize chest wall

• Physiologic peep

• Decrease Auto-peep?

CONTRAINDICATIONS FOR PEEP

• Increased intracranial pressure

• Unilateral pneumonia

• Bronchoplueral fistulae

PEEP

• Increases FRC

• Recruits alveoli

• Improves oxygenation

• Best Peep– based on lower inflection of pressure volume

curve

TROUBLE SHOOTING VOLUME VENTILATION

• High pressure alarm– Breath sounds– CXR

• Low tidal volume– disconnected

• Desaturation

TROUBLE SHOOTING PRESSURE VENTILATION

• Low tidal volumes or minute ventilation

• Desaturation– Breath sounds– Patient agitation– CXR

Sedation in Mechanically Ventilated Patients

• Benzodiazepines

• Opioids

• Neuroleptics

• Propofol

• Ketamine

• Dexmedetomidine

Benzodiazepines

• Lorazepam– Half-life 12 to 15 hours– Major metabolite inactive

• Midazolam– Half-life 1-4 hours, increased in cirrhosis, CHF,

obesity, elderly– Active metabolite

Opioid

• Morphine

• Fentanyl

• Hydromorphone

Neuroleptics

• Haloperidol– Mild agitation .5mg to 2mg– Moderate agitation 2 to 5 mg– Severe 10 to 20 mg

• Side Effects– Acute dystonic reactions– Polymorphic VT– Neuroleptic malignant syndrome

Propofol

• Side Effect– Hypotension– Bradycardia

• Anticonvulsant

• Expensive

• Use short term

Ketamine

• Dissociative anesthetic state

• Direct cardiovascular stimulant

• Brochodilator

• Side Effects– Dysphoric reactions– increased ICP

Dexmedetomidine

• Centrally acting alpha 2 agonist

• Approved for 24 hours or less

• Side Effects– Hypotension– Bradycardia– Atrial fibrillation

Maintenance of Sedation

• Titrate dose to ordered scale– Motor Activity Assessment Scale MAAS– Sedation-Agitation Scale SAS– Ramsay

• Rebolus prior to all increases in the maintenance infusion

• Daily interruption of sedation

NEUROMUSCULAR BLOCKING AGENTS

• Difficult to asses adequacy of sedation

• Polyneuropathy of the critically ill

• Use if unable to ventilate patient after patient adequately sedated

• Have no sedative or analgesic properties

Neuromuscular Blocking Agents

• Depolarizing– Bind to cholinergic receptors on the motor

endplate

• Nondepolarizing– Competitively inhibit Ach receptor on the

motor endplate

Depolarizing NMBASuccinylcholine

• Rapid onset less than 1 minute

• Duration of action is 7-8 minutes

• Pseudocholinesterase deficiency– 1 in 3200

• Side Effects– Hyperthermia, Hyperkalemia, arrhythmias– Increased ICP

Nondepolarizing Agents

• Pancuronium– Drug of choice for normal hepatic and renal

function

• Atracurium or Cisatracurium– Use in patients with hepatic and/or renal

insufficiency

• Vecuronium– Drug of choice for cardiovascular instability

No bubble is so iridescent or floats longer than that blown by

the successful teacher.

Sir William Osler

MV IN OBTRUCTIVE AIRWAY DISEASE

• Decrease barotrauma– related to mean airway pressure

• Increase I:E– decrease TV and/or increase flow

• Minimize auto-peep– auto-peep shown to cause most barotrauma

• Permissive hypercapnea

ARDS

• Set peep to pressure shown at lower inflection point of pressure volume curve

• Tidal volumes set below upper inflection point of pressure volume curve

• Use pressure control ventilation early

• Minimize volutrauma

Ventilation With Lower Tidal Volumes

• Tidal volume: 6 ml/kg – Male 50 + 0.91(centimeters of height-152.4)– Female 45.5+0.91(centimeters of ht - 152.4)

• Decrease or Increase TV by 1ml/kg to maintain plateau pressure 25 to 30.

• Minimum TV 4ml/kg

• PaO2 55 - 88 mm Hg. Sats 88 to 95%

• pH 7.3 to 7.45

CASE EXAMPLE

• 34 y/o female admitted with status asthmaticus and respiratory failure

• You are called to see patient for inability to ventilate

• Tidal volume 800 cc, FIO2 100%, AC 12 Peep 5 cm

• PAP 70, returned TV 200 cc

Case example continued

• Examine patient

• CXR

• Sedate

• Assess auto-peep

• Increase I:E

• Lower PAP and MAP

• Reverse bronchospasm & elect. Hypovent.

CONCLUSION

• Three options for ventilation– volume, pressure, flow

• Peep, know when to say no

• Always assess to prevent barotrauma– ventilate below upper inflection point– assess static compliance daily– monitor for auto-peep