As an aide to understand this topic better, please see the Mechanical Ventilation Simulator by this author at the following web-site:

Mechanical Ventilation is a fundamental tool used in the PICU. The support of the patient in respiratory failure is an ever evolving field. The basic premise of mechanical ventilation, to assure adequate ventilation and oxygenation has not changed. But how we achieve ‘adequacy’ and just what do we accept as adequate has changed.

The purpose of this outline is not to make you experts in the field of mechanical ventilation, we like our jobs and frankly don’t need the competition. But there may well arise the occasion when you are called upon to stabilize a pediatric patient on a mechanical ventilator prior to the arrival of the transport team or the Pediatric Intensivist. These guidelines and outline should help steer you down the right path.

Blood Gases

The simplest way to look at mechanical ventilation is as a way to keep the blood gases normal. So what makes up a Blood Gas?

pH hydrogen Ion concentration

pCO2 partial pressure of Carbon dioxide

pO2 partial pressure of oxygen

You get several other values, but many of these are calculated and/or not reflective of pulmonary function which is what you are controlling with MV.

pH and pCO2 are closely related and are affected by minute ventilation.

pO2 is governed by oxygen delivery and ventilation and perfusion (V and Q) match.

Minute Ventilation - very simply is RRxTV, respiratory rate times tidal volume.

Because CO2 rapidly diffuses across the alveolar space the more air you can move into and out of the lungs the more rapidly the CO2 can be removed.

Oxygen Delivery and VQ match - is controlled by your FiO2 (fraction of inspired oxygen) and is related to your airway recruitment. Airway recruitment is indirectly reflected in your mean airway pressure (MAP). By increasing your mean airway pressure you can increase your airway recruitment (although this is not a linear relationship) MAP is a function of the PEEP (positive end expiratory pressure) and a fraction of the PIP (peak inspiratory pressure or Pmax).

Quick review

So, to control pH and pCO2, you manipulate the minute ventilation, ergo the respiratory rate and tidal volume.

To control pO2, you manipulate the oxygen delivery and the VQ match, ergo you adjust the FiO2 and the mean airway pressure (PEEP and PIP)

Another Quick Review

Volume Control Ventilation

• Controlling pH and pCO2 is done by controlling minute ventilation. You can set both the respiratory rate and the tidal volume.
• Controlling pO2 you can adjust the FiO2, the PEEP and, indirectly, the PIP by adjusting the tidal volume (bigger TV yields bigger Pmax) although we don’t do this so much in practice

Pressure Control Ventilation

• Controlling the pH and pCO2 is done by controlling the minute ventilation. You can set the RR, but the TV is managed indirectly. TV is directly proportional to your DP (PIP-PEEP). Over a pressure range this will vary, the higher the pressures the less TV for a given DP.
• Controlling the pO2, again you can adjust the FiO2 and the PEEP, in addition you control the PIP (again we rarely use this clinically)

Now you know the basics of mechanical ventilation. So we can move on.

Modes

In general we are trying to accomplish one of two things for a patient using mechanical ventilation: either to control their ventilation and oxygenation, which they are unable to do, or to support them as they wean from ventilatory support. Hence, although rather simplistically, we can look at ventilator modes as either Control Modes, or Support Modes.

Control modes, also commonly known as Assist Control Modes:

These include: CMV and IMV.(which are rarely , if ever, used today), VC, PC and PRVC (the details of the abbreviations will come later)

Control modes deliver a set breath, the size and duration determined by the physician, each part of the respiratory cycle. If the patient is breathing spontaneously above the set rate, he or she will generally receive a full set breath, regardless of how much effort they are generating

Support modes include: VS, PS, CPAP, BiPAP and SIMV with PS (which is partly Control and partly support

The ventilator mode determines both when a patient gets a breath and what kind of breath they receive. The goal is to select a mode that is both comfortable for the patient and allows adequate ventilation and oxygenation with minimal trauma. Here is a partial list of available modes, with a brief discussion. Unless otherwise mentioned these modes are all in volume control, meaning that you set the tidal volume, rather than the peak inspiratory pressure. Some newer ventilators, particularly the Servo 300 can do these modes in either pressure or volume control.

Control Modes

AC (assist control) or VC (Volume Control)

• Characteristics: preset rate and tidal volume (sometimes PIP), either on the patient's initiative or at the set interval a full mechanical breath is delivered.
• Uses: for patients who have a very weak respiratory effort, allows synchrony with the patient but maximal support. Not a weaning mode, as at any rate they are getting complete mechanical support.
• Contraindications: none in particular
• Advantages: a fairly comfortable mode, providing a lot of support
• Disadvantages: can lead to hyperventilation if not closely monitored, not able to wean in this mode.
• Ventilators: LP-10 (a portable ventilator), Servo 900, Infant Star

PC (Pressure Control)

• Characteristics: basically IMV, where the breath is controlled by the Pmax or Swing pressure and not the set tidal volume
• Uses: in neonates, or in patients with high airway pressures(such as ARDS) to avoid barotrauma
• Contraindications: none in particular, not a friendly mode in an awake patient
• Advantages: Pressure limited, decreases the risk of barotrauma
• Disadvantages: no guaranteed tidal volume
• Ventilators: all.

PRVC (Pressure Regulated Volume Control)

• Characteristics: a volume control assist control mode, that adjusts the flow rate of the delivered air to deliver the set tidal volume at or below the set maximum pressure.
• Uses: in patients with high airway pressures, although it can be used in any patient. Some authors feel that it is a more friendly mode of ventilation in awake patients than SIMV.
• Contraindications: none in particular
• Advantages: gives you a guaranteed tidal volume but minimizes barotrauma.
• Disadvantages: new, no particular disadvantages.
• Ventilators: only available on the Servo 300

IMV (Intermittent Mandatory Ventilation)

• Characteristics: set breath delivered at a fixed interval. No patient interaction, pressure or volume modes
• Uses: commonly in neonates on the Sechrist, can be a weaning mode
• Contraindications: none really, unfriendly to older patients
• Advantages: regular guaranteed breath
• Disadvantages: does not allow patient to breath with the ventilator except by chance..Does not work with the patient
• Ventilators: Sechrist, most others can do this as well.

The following modes fall into both Control and Support categories in that they have set rates, but the spontaneous breaths are not controlled, so they can be used in weaning.

SIMV (Synchronous IMV)

• Characteristics: set breath delivered within an interval based on the set respiratory rate. Ventilator spends part of the interval waiting for spontaneous breath from the patient, which it will use as a trigger to deliver a full breath. If not sensed it will automatically give a breath at the end of the period. Any other breaths during the cycle are not supplemented.
• Uses: commonly used in many settings. Can be a weaning mode (see also with PS)
• Contraindications: none in particular
• Advantages: allows work with the patient, somewhat more friendly.
• Disadvantages: Any other breaths during the cycle are not supplemented
• Ventilators: all but the Sechrist

SIMV/PS

• Characteristics: combination of the previous two modes. Extra breaths in the cycle are supplemented with pressure support.
• Uses: useful in most circumstances, including weaning.
• Contraindications: none in particular.
• Advantages: allows both synchrony with the patient and help in overcoming the resistance in the endotracheal tube, to allow easier spontaneous breathing
• Disadvantages: none in particular. PS does not add anything in the patient who is not spontaneously breathing. Sometimes patients will have difficulty with the pressure support on some ventilators.
• Ventilators: all but the Sechris

Support Modes

PS (Pressure Support)

• Characteristics: supports each spontaneous breath with supplemental flow to achieve a preset pressure. Gives a little push to get the air in, so to speak.
• Uses: In the spontaneously breathing patient this helps overcome the airway resistance of the endotracheal tube. Usually use 5 for older patients and 10 for smaller (smaller ETT has higher resistance, more impediment to flow). Can be very helpful for weaning.
• Contraindications: patient who is not spontaneously breathing, i.e. on muscle relaxants
• Advantages: helps overcome resistance of tube, making spontaneous breathing easier
• Disadvantages: the flow rate is very high on the Servo 900C, which can make pressure support uncomfortable for some small patients. This is hard to predict.
• Ventilators: All but Sechrist

Volume Support

• Characteristics: variable level of pressure support is delivered on each breath in order to maintain a minimum set goal minute ventilation. Note: because the goal the ventilator works from is a minute ventilation goal the patient's respiratory rate can fall below the 'set' rate as long as their breaths are large enough to maintain the goal minute ventilation
• Uses: a weaning mode. The concept is that as the patient becomes stronger, or more awake they will make more respiratory effort on their own. The more effort they make the less support they will need from the ventilator and hence the level of pressure delivered will get smaller, often into the single digits
• Contraindications: patient who is not spontaneously breathing, as there is no back-up rate.
• Advantages: greatly decreases the number of interventions needed to wean patient from a ventilator versus traditional weaning
• Disadvantages: can be tricky on chronically ventilated patients. Takes some experience to understand when a patient is ready to be extubated when in this mode
• Ventilator: Servo 300 only

CPAP (Continuous Positive Airway Pressure)

• Characteristics: just as it says. This is very similar to PEEP, except that the inspiratory pressure is also maintained at the CPAP level, leading to support on inspiration and resistance on exhalation.
• Uses: for patients with upper airway soft tissue obstruction or tendency for airway collapse. As a final mode prior to extubation in some patients.
• Contraindications: any patient without spontaneous respiratory effort. Not a good idea in a patient with obstructive pulmonary disease (like asthma, COPD)
• Advantages: simple, easy to use
• Disadvantages: provides no supportive ventilation.
• Ventilators: all, can also be done without a ventilator.

Where to start?

Every patient is different and it is hard to know exactly what a patient will need in terms of ventilatory support until they are actually on the ventilator. So many of us a have preset ideas as to where to start any patient and then adjust the ventilator afterwards to achieve the desired ventilation effect. This has a lot to do with the individual style of the attending physician.

Pressure vs. Volume: I generally choose Volume to start.

Why? generally a more straightforward in terms of meeting goals of ventilation.

Mode: PRVC, if available, otherwise SIMV with or without Pressure Support.

Why? PRVC has the advantages of guaranteed tidal volume AND limiting the peak pressure. The decelerating wave pattern on the flow is also generally more friendly.

Rate: 20

Why? A good place to start. You can always adjust later. For small children this is lower that their usual spontaneous rate but with the larger tidal volumes that are delivered this increases the minute ventilation. For large children I will decrease this to 15 or even less. For small infants or neonates I may increase this to 30 or higher.

PEEP: 5mm Hg

Why? a little above physiologic. Not so high as to cause problems.

FiO2: 100%

Why? You can start to wean once you are certain everything is stable. Allows maximal preoxygenation in case anything happens. The only patients who will suffer deleterious effects from 100% for brief periods are those with arterial to pulmonary shunts, such as a modified Blalock-Taussig shunt or a central shunt where pulmonary vasodilation can lead to systemic hypotension.

Tidal Volume: 8-10ml/kg

Why? Above physiologic, gives good distention without significant barotrauma. 10-12ml/kg used to be the standard range, but people are generally using PEEP to maintain lung volume and smaller tidal volumes to avoid baro or volutrauma.

Inspiratory Time: somewhere from 0.5 to 1 second

Why? physiologic. Longer for bigger kids. But this will vary on the situation. Asthmatics for example merit very short I-times to allow maximal time for exhalation.

Early things to worry about

Peak Pressures: You would like to keep these under 35 if at all possible. If they start climbing into the 40's to 50's you should consider changing to Pressure control ventilation. While there are several other manipulations that could also be tried, the implication is that the patient either has very restrictive lung disease and non-compliant lungs or a very severe obstructive lung pattern, in which case the pause pressure should be evaluated and attempts to improve bronchodilation should be increased.

Oxygenation: Inability to wean the FiO2 should be a concern. Once on the ventilator the goal should be to get the FiO2 under 60%. If you are unable to do this it implies shunting either from lack of airway recruitment (PEEP too low) or alveolar inflammation or disease (like ARDS). This is where increasing the Mean Airway Pressure will be of benefit.

Ventilation: Am I over or under ventilating this patient based on his needs. Remember a patient who is being intubated because of an upper airway problem may have an excellent respiratory drive and not need much support. While a patient in shock with profound metabolic acidosis may need a higher rate to help compensate. Keep in mind the reason you are putting the patient on the ventilator. Obtaining a blood gas early after intubation (15-20 minutes after being on the ventilator) will help you decide if you are moving in the right direction.

Bibliography:

1. Hammer GB, Frankel LR. Mechanical ventilation for pediatric patients. Int Anesthesiol Clin. 1997;35(1):139-67.

And to the many Respiratory Therapists who taught me in the wee hours of the night...

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