Anatomy of a ventilator (Part 3)

Anatomy of a ventilator (Part 3)

In our medical practice is very important the next sentence: “Once a patient needs a ventilator to support the ventilation, our main goal is to remove this effort as soon as possible.”

Why ? Because it is a temporary support and we should not take a long time, because they can have grave and serious problems related to time (infections, injuries from using the same fan that would lead to problems like “pneumothorax” by breaking a bronchus and air out the pleura and loss of ventilation capacity to be a complication dreaded by us, plus others …).

Perform artificial ventilation is to go against the natural laws of respiratory physiology. It is therefore necessary support when patients need it, but alters the very nature of ventilation and breathing. So if a doctor tells you that this is necessary to note that the patient is critical and life-threatening.

Currents mechanical ventilators used in our ICU
We have several models such as the Evita XL Dragüer (see photo) SERVO Series 300 and Series “SERVO i” (Siemens Elema, SWEDEN)

For you to know the difference between them is relatively small, the only change is certain denomination of ventilatory work mode but in the end are very similar.

The ICU physicians, in addition to our medical training must be trained in Bio-engineering, because our work is fundamental to understanding and management of multiple mechanical devices that support life (such as mechanical ventilators, which will be discussed and the renal support (CRRT) or continuous renal replacement techniques (or dialyzer), as well as knowledge of following devices “pacemaker”, “defibrillator”, “contrapulsations” or mechanical cardiac support systems, “infusion Pumps” drug, “monitors” invasive pressure, “echocardiography” systems “PICCO monitoring” or hemodynamic cardiopulmonary systems “Vigilance” by the continuous cardiac output pulmonary artery catheter, etc ….

The essence of a mechanical ventilator are the following:

“A ventilator gets a lot of air volume, exerting a positive pressure (through the tracheal tube) into the air during a given time”.  Three parameters are very important in an ventilator : Volume, Pressure and Time

Volume: amount of air. Tidal Volume: The volume of air depends on a single breath.

Pressure: airway pressure (after pulse). At a level of pressure-driven ventilator is set to “x” of tidal volume. The amount depends on how stable is the airway (trachea, bronchi, bronchioles) so if there is already an altered pressure, more pressure is needed to put more volume. This depends on how elastic is the lung. The more rigid (chronic bronchitis, asthma, etc) for the same pressure, the volume depends on the elasticity and the changes that are in the air.

Imagine a balloon as a lung. You know that to inflate the balloon needs to put air in the beginning with much effort (that would make the fan) and once defeated the resistance to airflow (trachea, bronchi) hich the balloon is easily (meaning that we have overcome initial resistance) and open the lung depends on whether you are sick or not.

Then we know that the air cycle also depends on 3 factors: Inspiration (volume of air inspired or tidal volume) and expiration (a passive exercise, the air outlet of the lung) and the duration of the cycle (usually 1:2 means that the expiration lasts twice as long on inspiration).

What do you ask a ventilator?: Be robust, easy to learn, reliable, and that support at least the following ventilation modes:

* Modes of Ventilation: Each mode responds to a need we have to apply on the patient’s lung connected to the ventilator. Often ventilate several ways to find the ideal and sometimes are mixed ventilatory) … see below! I will name the most common ways to ventilate and liar not to those who read this humble blog, I will avoid classifications sometimes complicated to understand.


The standard size for patients without previous chronic disease is easily calculated by the weight of it: for example, for every breath a person brings in 10 ml of air per kg of weight (10 per 75 kg adult is 750 ml, Bone 3 / 4 liter of air). To calculate the number of times the machine (ventilator) enter that amount is called respiration, we must bring ourselves naturally (about 12 minutes). There are patients who need many more breaths to compensate for their disease (even 20 to 25 breaths per minute). The total volume (the volume of a breath or tidal volume called multiplied by respiratory rate) if we apply for a minute volume minute call. But all that air does not reach the lungs with each breath, because our trachea and bronchi to reach the area of ​​gas exchange (alveoli) are air handling systems, so after breathing of 750 milliliters (or three quarters of liter) only 600 milliliters (a little over a pint) is involved in oxygenation. This is called the alveolar ventilation, so if we make a few small accounts is as follows:

Tidal volume: 750 ml normal times 12 breaths a minute is equal to 9,000 milliliters (9 liters in a final minute or minute volume). Of these only 600 ml are reaching the alveoli (600 to 12 breaths / minute is equal to 7,200 ml or 7.2 liters of alveolar ventilation. So if you look there is a loss of 1.8 liters (which is the amount of air into the lungs in one minute and not swapped to give us oxygen, it is called anatomic dead space or plumbing or bronchi which circulates the air)

Well we know some quantities that enter the lungs if they were healthy, but these if they intubation means that there were healthy, the disease (pneumonia, pulmonary edema, gas embolism, pulmonary embolism, acute respiratory distress OR OTHERWISE) affected to them. It is also possible that aired a healthy lung INCLUDING BUT THE PATIENT CAN NOT BREATHE, as patients who are comatose or who have suffered cardiac arrest and respiratory any other cause lung. In an operation that required general anesthesia, patients with healthy lungs need to be intubated and artificially ventilated until the withdrawal of anesthesia (usually several hours, depending on the time of the operation). After the anesthesia, the patient recovers and leaves the ventilator (most often does not know) and then breathe normally. Now a patient in need of a long operation, may not be immediately withdrawn and ventilation must be done more gradually over time.

It is therefore important that any move in the lung (healthy or sick) sometimes involves some serious risks.

C / A MV: (Assistant/Control Mechanical Ventilation) controlled ventilation is the device that controls everything and assisted the patient who sets the pace. Virtually intestate except for patients who are very severe lung damage, 80% of the vents are assisted to go accommodating the patient at the time to disconnect (or English weaning Weaning) a day as soon as possible.  Modes:

 IPPV (intermittent positive pressure ventilation, if read backwards acronym that comes from the English language) means to introduce a volume of air at positive pressure (effect of introducing air through a tube by applying an external pressure) intermittent. The patient should breathe normally without being intubated, breathe (breathe all) negative pressure (mean atmospheric pressure that surrounds us normally to your body).
 PRVC: Ventilation pressure regulated volume control (volume ventilation within the lung that we previously adjusted to the needs of the patient, but usually a lot of pressure that is sufficient in order not to damage the airway).

BIPAP, BIPAP / ASB (Biphasic Positive Airway Pressure, Spontaneous Assisted Breathing) in Christian … it would be a positive pressure, such as IPPV or under positive pressure PRVC but both inspiration and expiration (biphasic). Administered with a special mask and a ventilator that puts pressure on a system that recovers the losses generated by the mask. This system is applied with or without intubate intubated patients (here called non-invasive mechanical ventilation). The ASB is a system that allows spontaneous breathing or assisted called pressure support when the patient breathes on its own.
 PS: pressure support. Positive pressure is used when the patient recovers and can evaluate a switch or weaning!

    PCV: pressure-controlled ventilation in some models called BIPAP. Here we limit the pressure in the airway and administer the volume that we let the patient. It is a method performed in difficult cases such as ventilation in ARDS (Acute Respiratory Distress Syndrome). By limiting the pressure that we do is to limit the volume, but in these cases we do not care too much volume, because if not break the lung (tension pneumothorax) and the air would accumulate between the pleura and rib of the chest, lung would collapse and the patient would have many opportunities to die, we will put a drain in the chest to let the air and the lung to recover. This type of ventilation is more complicated than we are.

CPAP: continuous positive pressure airway. It can be done several ways: 1 CPAP support (like the PS) but here the patient is breathing spontaneously through a flow (volume per minute) continuous gas that exceed the air becomes positively . 2 º CPAP in patients without intubation: Exactly the same but are exercised through a special mask that overcomes the atmospheric pressure.
PEEP: Positive End expiratory Pressure … (note to write an “x” in Castilian for “expire” is to die). This is one of the most important elements for artificial ventilation. The concept of PEEP is to always maintain a slight positive pressure when the air leaves the lung to prevent collapse of alveoli ventilated and do not participate in gas exchange. As I said, artificially ventilate is “against nature”. When we breathe normally without tubes in the windpipe, our lungs hold a certain volume of air that goes to make the breath (called residual volume or VR) keeping the alveoli open during this phase. When artificially ventilated, the only way the socket is not damaged, is exerting a little pressure (typically a pressure of 5 cm of water).

The lung is an elastic organ, whose natural tendency is to be deflated, to reach the residual volume (RV), while the chest is a set musculoskeletal, whose natural tendency is towards its maximum expansion, seeking the position of total lung capacity (TLC) which is the maximum volume of air. Both should be good neighbors, and this balance is achieved precisely in the middle, the end of a natural unforced expiration, which is called the position of functional residual capacity (FRC). In this position, the forces of swollen chest and lung deflation are balanced.

Here come new concepts PEEP exerts pressure in the alveoli and alveolar pressure at all times the sum of the pleural pressure plus the elastic recoil pressure of the lung itself, which leads to the definition of back pressure elastic (Pel, Pst), and is the difference between alveolar pressure and pressure in the pleural space, with the coefficient of elastic recoil (Pel / TLC) a similar term, based on the value of total lung capacity, value normal, 2.5 to 8 cm water / L (<2.5 in patients with pulmonary emphysema, and> 8 in patients with pulmonary fibrosis). Later we will study these processes, we now sounds like a “Chinese”. For all this we can measure it indirectly, because the alveolar pressure is impossible (for now) to measure directly.

Our teams of ventilators (our machines) can indirectly evaluate the pressure on the socket. Then we will see some examples!

   SIMV, SIMV / ASB: Mandatory Ventilation Synchronized intermitent.El objective of this mode allows the patient undergoing mechanical ventilation make spontaneous breathing cycles interspersed fan. The patient makes spontaneous breathing on opening the inspiratory valve of the fan while it synchronizes the mandatory cycles with spontaneous or forced. The trigger (sensitivity of the respiratory stimulation that triggers a self-ventilation) must be set to low values ​​for the effort made by the patient to open the inspiratory valve is mínimo.El mandatory minute volume is fixed, fitted with a tidal volume and frequency. Among the mandatory ventilation strokes the patient can breathe spontaneously and contribute to the total minute volume. Spontaneous breathing can be assisted by ASB (pressure support). This mode is useful for the withdrawal of mechanical ventilation in patients who have been exposed to weaning by a gradual reduction in the mandatory throughout the cardiac output and insufficient spontaneous breathing patients. Curiously, in Spain, do not consider this method to a reduction of ventilation and weaning or walk down the fan off. This method is used against almost 90% of ICUs in the United States, especially postoperative (postoperative). Does your explanation, but as I said is not shared (almost nothing) by the Spanish intensivists (personally I agree with American colleagues): The true explanation is that the SIMV at least for me, is a good way to vent abdominal postoperative patients, since the “hiccup” is common and is best suited SIMV than any other method (gulp! ! Hip!)
All ventilatory modes, described and undescribed

CLASSIFICATION ventilatory modes

Conventional ventilatory modes
· Ventilation assist-control.
. Synchronized Intermittent Mandatory Ventilation. SIMV.  Only the United States of América  and I personally in patients with postoperative abdominal
· Ventilation pressure support. PSV. For disconnection of ventilation

Alternative ventilatory modes
· Ventilation Controlled Pressure. PCV. Cases in Acute Respiratory Distress Syndrome (ARDS)
· Ventilation with I: E ratio inversed. Cases in ARDS
· Permissive hypercapnia.  Cases in ARDS
· Ventilation mandatory minute. VMM. Not used by us
· Ventilation with release of pressure. APRV. Not used by us
· Pressure biphasic positive airway. BIPAP. In Non-invasive ventilation (Biphasic)
· Pressure Continuous Positive Airway. NIV in CPAP (Flow-by)
· Ventilation high frequency. HFV. Not used by us

New ventilatory modes
Dual control modes

1. In the same cycle. No used by us!
Increased pressure
Pressure support with volume insured. VAPS.

  2. Cycle to cycle
a) Limited to pressure and flow cycled
Volume Assisted. VA Not used by us, except for a colleague!
Variable pressure support. VPS used by us

b) Limited pressure and cycle time
• Volume controlled ventilation and pressure regulated. PRVC. The most common for us
• Adaptive Support Ventilation. APV not used by us
• Autoflow if when used as IPPV + Autoflow commonly used by us
• Pressure control variable. Not used by us

    3.Patient-ventilator synchrony
· Automode. Commonly used by us
· Compensation Automatic Tube. ATC. Commonly used by us
· Flow-by or continuous flow. Commonly used by us as CPAP
. Spontaneous Amplified Pattern ·. SAP. Not commonly used by us
· Proportional Assisted ventilation. PAV. Not commonly used by us
· Liquid Ventilation. Not used by us (only in research in some centre of USA)
. Continuous gas flow (extratracheal). Research and used as a last resort in severely ill patients for us!

Next Update Part IV


Herrero S. “Anatomy of a ventilator (Part 3)”  Journal of Pearls in Intensive Care Medicine 2011. Vol. 1. Nº 17

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Journal of Pearls in Intensive Care Medicine - Perlas en Medicina Intensiva

Herrero-Varon's MD Editors. Asturias (Gijón) and Houston (TX, USA). Languaje EN/ES 2011-2016