Mechanical Ventilation

Mechanical Ventilation

Mechanical Ventilation • Definition: • Positive pressure ventilation provides pressure and flow to the airway in order to effect Oxygen and CO2 transport between the environment & pulmonary capillary bed.

Indications for Mechanical Ventilation • Ventilation abnormalities • Respiratory muscle dysfunction • Respiratory muscle fatigue • Chest wall abnormalities • Neuromuscular disease • Decreased ventilatory drive • Increased airway resistance and/or obstruction

Indications for Mechanical Ventilation • Oxygenation abnormalities • Refractory hypoxemia • Need for positive end-expiratory pressure (PEEP) • Excessive work of breathing

Goals of Mechanical Ventilation • Provide appropriate oxygen supplementation • Assure adequate alveolar minute ventilation • Reduce work of breathing • Increase patient comfort during respiration

Mechanical Ventilation • Types: • Total: The mechanical device completely unloads the ventilatory muscles & provides all the work of breathing. • Partial: The mechanical device partially unloads the ventilatory muscles requiring the patient to provide the remainder of work of breathing.

Mechanical Ventilation • Use: • Total: Respiratory failure with patient ms. Are clearly overloaded or fatigued. Gas exchange is very unstable or unreliable. • Partial: Respiratory failure less severe & / or during recovery or weaning phase.

Device Design Features for total ventilatory support: • What initiates breath? Trigger variable Patient effort Machine timer

Device Design Features for total ventilatory support: • What controls gas delivery during breath? Target variable Flow target Pressure target

Device Design Features for total ventilatory support: • What terminates breath? Cycle variable Volume Pressure limits Inspiratory time

Modes Of Mechanical Ventilation • Controlled Mechanical Ventilation • Assist-Control Ventilation • Synchronized Intermittent Mandatory Ventilation (SIMV) • Pressure-Support Ventilation (PSV) • Pressure-Controlled Ventilation • Continuous Positive Airway Pressure (CPAP)

Point of Reference: Spontaneous Ventilation

Controlled Mechanical Ventilation • Preset rate and tidal volume • No patient interaction with ventilator • Advantages: rests muscles of respiration • Disadvantages: requires sedation/neuro-muscular blockade, potential adverse hemodynamic effects

Assist-Control Ventilation • Preset tidal volume and minimal ventilator rate • Additional patient-initiated breaths receive preset tidal volume • Advantages: reduced work of breathing; allows patient to modify minute ventilation • Disadvantages: potential adverse hemodynamic effects or inappropriate hyperventilation

Synchronized Intermittent Mandatory Ventilation (SIMV) • Preset tidal volume at a preset rate • Additional spontaneous breaths at tidal volume and rate determined by patient • Often used with pressure support

Synchronized Intermittent Mandatory Ventilation (SIMV) • Potential advantages • Better patient-ventilator interaction • Less hemodynamic effects • Potential disadvantages • Higher work of breathing than CMV, AC

Pressure-Support Ventilation (PSV) • Pressure assist during spontaneous inspiration • Pressure assist continues until inspiratory effort decreases • Delivered tidal volume dependent on inspiratory effort and resistance/compliance of lung/thorax

Pressure-Support Ventilation • Potential advantages • Patient comfort • Less WOB than spontaneous breathing • May enhance patient-ventilator synchrony • Used with SIMV to support spontaneous breaths

Pressure-Support Ventilation • Potential disadvantages • Variable tidal volume if pulmonary resistance/compliance changes rapidly • If sole mode of ventilation, apnea alarm is only backup • Gas leak from circuit may interfere with cycling

Pressure-Controlled Ventilation • Used to limit inflationary pressures • Allows setting of inspiratory time • Complexity of interacting ventilatory variables necessitates critical care consultation

Continuous Positive Airway Pressure (CPAP) • No machine breaths delivered • Allows spontaneous breathing at elevated baseline pressure • Patient controls rate and tidal volume

Inspiratory Time: Expiratory Time Relationship (I:E ratio) • Spontaneous breathing I:E = 1:2 • Inspiratory time determinants with preset volume breaths • Tidal volume • Gas flow rate • Respiratory rate • Inspiratory pause • Expiratory time passively determined

I:E Ratio during Mechanical Ventilation • Expiratory time too short for exhalation • Breath stacking • Auto-PEEP • Reduce auto-PEEP by shortening inspiratory time • Increase gas flow rate • Decrease tidal volume • Decrease respiratory rate

Auto-PEEP • Can be measured on some ventilators • Increases peak, plateau, and mean airway pressures • Potential harmful physiologic effects

Permissive Hypercapnia • Acceptance of an elevated PaCO2, e.g., lower tidal volume to reduce peak airway pressure • Contraindicated with increased intracranial pressure • Consider in severe asthma and ARDS • Critical care consultation advised

Mechanical Ventilation