Ventilator Management

James Eakins, MD FACSDirector, Trauma and Surgical Critical Care

Hahnemann University Hospital

Overview

• Ventilation vs. Oxygenation

• Arterial blood gas analysis

• Modes of ventilation

• Methods of weaning

• Airway pressures

• ARDS

• Exotic ventilator modes

Ventilation / Oxygenation

• Ventilation is the process by which air enters the lungs

• Oxygenation is the process by which oxygen diffuses from inspired air into the blood

• Ventilation and Oxygenation occur, and should be looked at, independently

Ventilation

• Minute ventilation, abbreviated Ve, is the volume of air that is moved in and out of the lungs in one minute

• Ve = Respiratory Rate x Tidal Volume (Vt)

• Assessed by looking at the pCO2 on an arterial blood gas

• Normal pCO2 is 40

Oxygenation

• Assessed by looking at the pO2 and the oxygen saturation

• Most oxygen is carried bound to hemoglobin, so the saturation is more important (though they are related via the oxygen-hemoglobin dissociation curve)

• FiO2 and PEEP contribute to oxygenation

Blood gas analysis

• Blood gas components– pH – Normal 7.40– pCO2 – Normal 40– pO2– Bicarbonate (calculated) – Normal 24

• More accurate from Chem-7– Base excess – Normal +3 to -3– Oxygen saturation (usually calculated)

• pH and pCO2 important for assessing ventilation• pO2 and saturation (measured or pulse-ox)

important for evaluation of oxygenation

Blood Gas analysis

• pH of blood is a balance of acids and bases– Hydrogen ion– Bicarbonate– CO2– Other acids– Assorted buffers

• Acidosis vs. Alkalosis is determined by the pH

Metabolic acidosis

• Excess accumulation of any acid• pH less than 7.4• Patient will try to compensate by blowing

off extra CO2, so pCO2 on ABG will be decreased

• Example: 7.31 / 27 / 105• Compensation is never complete• Need to find and correct the cause of the

acidosis

Metabolic alkalosis

• pH greater than 7.4

• Accumulation of some base in the blood

• Body will attempt to compensate by allowing the pCO2 to rise above 40

• Example: 7.50 / 48 / 109

Respiratory Acidosis

• pH less than 7.4

• Accumulation of CO2 due to inadequate ventilation

• For some reason patient is not breathing enough

• Need more minute ventilation– Respiratory rate– Tidal Volume

Respiratory alkalosis

• pH greater than 7.4• Patient with too much minute ventilation,

therefore pCO2 is below normal• Why?

– Pain– Anxiety– Hypoxia– Head injured– Iatrogenic

• Example: 7.52 / 25 / 99

Practice

• 7.25 / 60 / 105• Respiratory acidosis• 7.49 / 48 / 99• Metabolic alkalosis• 7.22 / 27 / 88• Metabolic acidosis• 7.52 / 51 / 101• Metabolic alkalosis• 7.55 / 25 / 99• Respiratory alkalosis

Ventilator Terms

• Tidal Volume

• Respiratory Rate– Set– Spontaneous

• FiO2

• PEEP: Positive end expiratory pressure

• Minute Ventilation (Ve)

• Pressure Support

Ventilator Modes

• Volume Control (aka assist control)

• SIMV

• Pressure Control

• Pressure Regulated Volume Control (aka VC+, aka autoflow)

Volume Control

• Set rate, tidal volume, FiO2, and PEEP

• Machine will deliver the tidal volume you set at the rate you set

• If the patient attempts to take additional breaths, machine will sense it and deliver a full tidal volume

SIMV

• Synchronized intermittent mandatory ventilation• Set the same parameters as VC• Difference is that if patient initiates a breath the

machine does not help them• The machine will give pressure support on

spontaneous breaths to offset resistance to flow through ventilator circuit

• Synchronized because vent will not deliver a full breath while patient is taking a spontaneous breath

Pressure Control

• Developed to limit airway pressures in patients with stiff lungs

• Set FiO2, PEEP, rate, and peak inspiratory pressure

• With each breath the ventilator will deliver as much tidal volume as it can without exceeding peak pressure

Pressure regulated volume control

• Similar to volume control

• Only difference is that ventilator automatically adjusts flow rate to keep peak airway pressure as low as possible

Plateau Pressure

• The airway pressure after the entire tidal volume is in the lungs– No flow– Before exhalation

• A function of the tidal volume and the stiffness of the lung, aka static compliance

Peak inspiratory pressure

• Occurs during inspiration

• Dependent on the factors that determine plateau pressure

• Also dependent of flow rate and resistance to flow in the airway (dynamic compliance)

• 42 year old male, multiple rib fractures• Settings:

– VC– Rate 18– Vt 400 cc– FiO2 60%– Peep 10

• ABG: pH 7.25, pCO2 60, pO2 122, Sat 99%• Vent Changes?

• 75 year old female, stuck on vent after total abdominal colectomy for LGI bleed

• Settings– VC rate 14– Vt 350 cc– FiO2 80%– Peep 12

• ABG: pH 7.50, pCO2 28, pO2 55, Sat 87%• Vent Changes?

ARDS

• Originally named shock lung, or “DaNang” lung

• Multiple definitions• Multiple causitive factors• Heterogeneous disease process• Alveolar wall thickens and becomes

fibrotic– Decreased gas exchange– Stiff lung (poor compliance)

ARDS

• Generally need PEEP to keep alveoli open

• ARDSnet study showed better survival using higher peeps and lower tidal volumes or 4 – 6 cc/kg

Morbidity of mechanical ventilation

• Barotrauma

• Volutrauma

• Oxygen toxicity

• Opening and closing of alveoli

Methods of weaning

• First, make sure oxygenation is adequate

• Three main approaches– Daily spontaneous breathing trial– SIMV plus pressure support– Pressure support wean

• Generally equivalent if applied aggressively

Fancy modes

• Airway pressure release ventilation (APRV), also known as bi-level

• High frequency oscillatory ventilation

• Both are forms of “open lung ventilation,”