Anatomy of a ventilator (Part 4)

Anatomy of a ventilator. Part 4

Continuing with the “Anatomy of a ventilator” (11/07/2011 Today, PART IV), an important aspect is to understand what gives us the monitoring of mechanical ventilation.

Note the picture of a monitor from a mechanical ventilator, click here for show picture in another window and from there you can understand what you mean.

Monitoring of ventilation allows us to check several aspects:

Above all it puts the IPPV mode with AutoFlow®.

What is AutoFlow®? This is a new advance in volume controlled modes of mechanical ventilation where the ventilator
automatically regulates inspiratory flow. This auto regulation is in accordance with the set VT and current lung compliance. The set VT is always given at minimum possible pressure and spontaneous breathing is possible (open valves) through the whole Inspiratory and Expiratory phases of the mechanical ventilatory cycle. A decelerating flow pattern reduces Peak pressures
and as lung compliance changes further they are recognised and responded to. More about AutoFlow

This patient is a complicated and difficult ventilatory mode because it requires a lot of help with the ventilation.

1.- Look, the numbers are on the right of the photo:

Monitor de un ventilador mecánico tipo Dräguer* FiO2 fraction of inspired oxygen: The amount in percent of administered oxygen with each breath. If you watch the picture the amount shown on the ventilator comes as 56%, but for us and do calculations we use the fraction (FiO2: 0.56)

* EtCO2: The value displayed is 43 mmHg. It is called the amount of CO2 eliminated with each breath: It gives us an estimate or fraction of CO2 expelled to the expiration of the ventilator and is an important value for us because it reports the status of ventilation.

The tidal volume or “tidal” is the amount of air in each breath (inspiration and expiration). Normally the inspired and expired volumes are not identical and which is greater than that which comes out except as in this case, not yet updated the last value.

  •      Inspired tidal volume (Vti): 550 ml
  •      Exhaled tidal volume (Vte): 656 ml, is usually a slightly lower value to the inspired, because a certain volume is residual. Here there was no time to update.

* Respiratory rate we put on the ventilator (here 25) and then would come the patient does (here 0), which means that the vent is under control and not attended.

* Volume minute: Volume per unit time (minute or VM) can be inspiratory and expiratory when the patient is able to vent itself. If you look here the VM is 15.2 liters / minute (608 ml Vti multiply x 25 = 15,200 breaths mitu ml (15.2 liters) of the last ventilation.

* The air pressure is transmitted (trachea, bronchi) with each breath and the air resistance, which may occur as a result of any change in the passageway for air. The airway resistance is the opposition to flow caused friction forces post (optional on other side of the screen). Is defined as the ratio of boost pressure and air flow rate. The flow resistance in the airways depends on whether the flow is laminar or turbulent, the size of the airway and the viscosity of the gas. In principle the flow of the airway is laminar, but if you are straitened in your way, you can vary to a vortex or turbulent flow if other abnormalities coexist.

  • The total inspiratory pressure or ppeak: Here comes the picture as 34 cmH2O. It tells us that there is maximum pressure in the airway and is an expression of whether good or bad air enters the lungs. Informs us whether rapid or slow, we would say for obstruction (mucus or secretions usually the patient can not clear and should help you get through aspirations within the tracheal tube).
  •     Plateau pressure or the pressure just before the start of inspiration (depends on the mode of ventilation).
  •     PEEP, or pressure to positive end expiratory pressure ( default 5 cmH2O) If more is needed to improve oxygenation of the patient, keeping the alveoli to more tension (pressure), here comes with a value of 11 cm H2O is already a high value. The higher the value, the greater risk of potential patient complication such as pneumothorax (see previous lesson). The PEEP will always be extrinsic to apply, but a PEEP intrinsically makes the same patient and we can only find out when we stop the machine expiratory

Parameters that are not here (but available to modify the data of the screen):

* Airflow is the amount of lung volume per unit time (which is in minutes or seconds, depending on what we measure) and unit of pressure. This would appear in another option of the monitor screen.

* Compliance (C): Dynamics (Cdin) and Static (Cst) Compliance means static relationship between volume and pressure points without gas flow (static), while talk of Compliance dynamically when measurements of V and P are points where the flow is not interrupted (dynamic or moving), but it takes (flow reversal) at the end of inspiration, and end-expiration at the ends of VT, with the C static and dynamic nearly equal in normal subjects except at high frequencies (in this patient are different from having frequencies above 18 breaths per minute)

The calculation of the Cdin (dynamic compliance) is very simple, do not take any action, so does the fan breath by breath because the calculation is movement of each cycle.

The calculation of Cst (static compliance) is also simple: First we make the fan stops with a move to take the tidal volume at peak maximum (inspiratory stop) and then after a normal tidal expiratory we stop with another special lever . The calculation is done automatically the fan, but we can calculate when the ratio between the fan flow volume divided by the peak pressure minus the total PEEP (the pressure that we apply or extrinsic and that the patient is called intrinsic) . Measured in ml/cmH20 (normal 50 to 80). Here the patient has 25 ml/cmH20 Compliance which is a low expression of a rigid lung disease patient.

Cst: Vt: 550 / PPIC (34) – total PEEP 12 (e-PEEP- i-PEEP and 11 +-i 1) = 25 ml/cmH2O

Well now look at the numbers there are down on the monitor and are inside a circle round green. View from left to right as you look …

The first is the FIO2 delivered (55%), the second set volume with each breath or Vt (560), the third is the inspiratory time (0.90 almost 1), then programmed respiratory rate (24) and we finally PEEP we (10). These are the programming data that we made in the patient. Finally, before the first number is a flag that says I: E = 1.0: 1.8 (this is the time I: E or inspiratory time: expiratory) means that the expiration time is almost twice as inspiration and is also programmable. The increased or decreased depending on the ventilation process.

These data, together with others as we can now see a very strict control of what happens in the lungs! and patients’ lives depend on these adjustments as sensitive. To ensure the proper functioning of multiple alarm systems for all data and avoid a catastrophe!

The patient to be controlled ventilation as possible, does what we ask, but sometimes there are some minor variations. So when a ventilator  sounds an alarm, is that the address is incorrect or the patient is presenting some variation. This sometimes happens very frequently and is especially important the work of nursing in these patients in their care. The high specialization of nursing is very important, because patients are critically ill.


Herrero S. Monitoring Mechanical ventilation (Part IV)  Journal of Pearls in Intensive Care Medicine 2011. Vol. 1. Nº 18

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