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MOCA Time: Mechanical Ventilation Basics

Mindy Ju MD and Solomon Behar, MD
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24:50

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UCSF pediatric intensivist Mindy Ju breaks down the basic principles of mechanical ventilation in children with Sol.

 
  • The essential difference between mechanical ventilation and spontaneous ventilation is that spontaneous validation relies on negative pressure - the muscles in the diaphragm and in between the ribs will contract and expand to then create a negative pressure in the chest and pull air in to do gas exchange. In mechanical ventilation, air is pushed in through a variety of methods and modes to allow for  gas exchange to occur.

  • Tidal volume is the amount of air delivered with each breath. 

    • It is now known that large tidal volumes increase morbidity and so  it is now recommended that providers start with tidal volumes of 5 to 8 mL/kg.

  • Ventilator rate determines how frequently a supported breath is delivered. For most patients, an initial set ventilatory rate should match their native rate (for example, in adolescents, a rate of 10 or 12 breath per minutes). 

    • Although a higher respiratory rate  is often needed to maintain CO2 levels within safe range, it can alter the inspiratory-to-expiratory ratio, leading to breath stacking  due to short expiratory time.

  • Positive end expiratory pressure (PEEP) is generally added to mitigate end-expiratory alveolar collapse. A typical initial applied PEEP is 5 cm H2O.

  • Inspiratory time is the time over which the tidal volume is delivered  or the pressure is maintained (depending on the mode). Typically, the inspiratory time is set to target an inspiratory to expiratory ratio of 1:2 to 1:3.

    • The expiratory time is determined by the  respiratory rate and the inspiratory time. 

  • Peak inspiratory pressure (PIP) is the maximum pressure measured at end inspiration. Higher PIP, especially above 30 cmH2O, is associated with increased mortality rates. 

  • In pressure support, the ventilator provides the driving pressure for each spontaneous breath. The amount of pressure support selected is generally determined by the size of the patient's ET tube. For example, neonates generally require a higher level of pressure support because the diameter of their ET tubes are more narrow. 

  • In volume-controlled assist control ventilation the clinician determines the minimal minute ventilation by setting the respiratory rate and tidal volume. Each patient-initiated breath receives the set tidal volume from the ventilator.

  • In volume-controlled synchronized intermittent mandatory  (SIMV) ventilation the clinician determines the minimal minute ventilation by setting the respiratory rate and tidal volume but the patient-initiated breaths  that are over the set rate will not be supported. 

  • In pressure-controlled ventilation, the clinician sets the inspiratory pressure level.

    • During pressure-controlled assist control, the set respiratory rate and inspiratory pressure level determine the minimum minute ventilation. All breaths are fully supported.

    • During pressure-controlled SIMV, the set respiratory rate and inspiratory pressure level determine the minimum minute ventilation. The patient is able to increase the minute ventilation by initiating spontaneous breaths.

  • The primary difference between pressure-controlled and volume-controlled ventilation is how the breath is delivered through flow. Flow (represented in liters per minute) is how much gas is moving and how quickly it is moving. 

    • In pressure-controlled ventilation,  breaths are delivered via  decelerating flow.  In  volume-controlled ventilation, breaths are delivered via constant flow (which is not how individuals normally breathe).  

  • Adaptive support ventilation is a ventilatory mode in which  the respiratory mechanics dictate adjustments to the inspiratory pressure that are necessary to achieve a desired minute ventilation. How this mode of ventilation is referred to will depend on the brand of the ventilator (e.g. pressure regulated volume control, AutoFlow or volume guarantee). 

    • In this mode of ventilation, the clinician sets the desired tidal volume and the ventilator delivers it in a decelerating flow manner. The machine will give a certain pressure and then after that breath is given,  it measures how much volume that pressure generated.  If the pressure given results in too low of a tidal volume, the ventilator will increase the pressure to achieve the desired tidal volume. 

  • Think of the mnemonic DOPE for the most common causes of post-intubation hypoxemia.

    • Dislodged or displaced tube 

    • Obstruction (check for kinked tube or mucous plug)

    • Pneumothorax

    • Equipment malfunction

  • In the setting of a rising CO2, you want to increase the patient’s minute ventilation. This can be achieved by increasing the respiratory rate and/or tidal volume.

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Peds RAP May 2020 Written Summary 509 KB - PDF

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