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Ventilation - overview of ventilation in PICU

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A significant number of patients in ICU undergo ventilation for other than respiratory failure. Optimal ventilation in these patients can be achieved using a variety of ventilatory strategies and the main aim is to use a ventilatory mode that the patient tolerates, while at the same time avoiding ventilation induced lung injury.

The standard mode of ventilation in children has historically been some form of pressure control ventilation which compensates for a variable leak and protects against excess airway pressures. A small amount of PEEP is added to replace physiologic PEEP and to prevent atelectasis. Tidal volumes were not routinely measured and rates were set to match what the patient wanted to breathe at. Adults were ventilated in a volume control mode.

Modern ventilation practices for children are more volume control based and with ventilators having much better sensing and triggering there is the ability for more forms of triggered assisted ventilation. This has had the advantages of allowing less sedation/paralysis to be used and maintaining patient respiratory muscle use. There is however scant evidence that any of these new modes have a great impact on outcome.

This same technology has been applied to noninvasive ventilation allowing the use of this in even very small children which prevents the adverse effects of tracheal intubation.

Other noninvasive techniques based on the application of CPAP and/or high flows are increasingly used as they are well tolerated and highly effective when used appropriately.

Equipment

Ventilators

The advent of new ventilation modes has made classification difficult. Traditionally ventilators were classified according to type of flow generation and how they were cycled and this is perhaps the easiest way of understanding the machines although each ventilator can operate in a number of modes.

Below is a brief description of the ventilators that we use.

Drager V500
Repacement for the Drager XL.
Capable of ventilating all weights/ages.
Usual modes that we use are SIMV Volume or Pressure Control + Pressure Support.
Need the neonatal flow sensor for patients less than 5kg
Drager Babylog VN500
Ventilates from neonates and infants.
Continuous flow.
Offers true pressure control.
Additional modes are Volume Guarantee
Drager Evita XL
Advantages are active exhalation valve, ATC, variable flow triggering, MMV, autoflow, non-invasive ventilation, APRV
Biggest mistakes are not checking default settings and not confirming changes when selecting settings.
Sensormedics 3100A and 3100B
Oscillators for < 35kg (3100A) and 35-100kg (3100B)
Transport Ventilators
Incubator setups -Crossvent 2+ (neonates and small infants) < 5-6kg (limited by maximum flow rate and incubator size. Can be used to supply CPAP with constant flow up to 29lpm).
Stretcher setups - Elisse 300. Can be used for mask ventilation as well as invasive ventilation but NOT for high flow.
Bubble CPAP circuit
Allows the application of nasal CPAP using tight fitting nasal prongs, a high flow gas supply, wide bore tubing, and a column of water (usually 5cm high).
The important features of the system are obtaining a snug fit with the nasal prongs, ensuring adequate flow through the circuit, and preventing excessive mouth breathing.
This system rarely works well in infants older than 3-5 months
High Flow Circuits
These are used for spontaneously breathing larger children and consist of high flow flowmeters plus an oxygen blender plus a humidifier.
They allow for the provision of high inspired Fi02 and if used with a tight fitting mask with a PEEP valve can supply variable amounts of CPAP.
BIPAP and CPAP Machines
These can be used to supply BIPAP or CPAP via mask or ETT to patients. In the BIPAP mode they can be used for non-invasive ventilation but are limited by mask intolerance, lack of airway protection, and low FiO2 achievable.
The Respironics Vision is a major advance in Non-invasive ventilation due to sensive triggering and cycling.
Endotracheal Tubes
Traditionally in infants and children noncuffed ETTs have been used to avoid damage to the subglottis. This is not necessary as long as care is taken to avoid excess cuff pressures when using cuffed tubes (cuff pressure should be less than 20mmHg) or allowing a small leak at inspiratory pressures of 20cm H20.The advantage of the uncuffed tube is that a larger internal lumen can be achieved.
Nasal tubes are easier to secure in small children and are preferable for those under 5 years unless contraindicated.
Older children use oral ETTs.
Some longer term patients will have tracheostomies.

Modes

As well as various modes ventilation can be divided up into 2 broad types:

  1. IMV - Intermittent Mandatory Ventilation
    The machine will give the set number of breaths of a set type regardless of patient effort. These may be synchronised to patient effort (SIMV). If the patient takes additional breaths these may be supported (PS) or be solely patient effort related.
  2. Assist/Control
    The machine will give at a minimum the set number of breaths. Patient effort results in a machine breath being delivered. If the patients breath rate is > machine set rate then the set rate is a back-up rate only

Pressure Control

  • Constant pressure, decelerating flow, variable inspiratory time.
  • Distributes ventilation more evenly and avoids excessive inspiratory pressures.
  • Routine paediatric mode on Servo and mode to use for those difficult to ventilate.
  • Patient can trigger a machine breath.

Pressure Limited, Time Cycled

  • Increasing pressure, constant then decelerating flow, variable inspiratory time.
  • Not as good as pressure control.
  • Routine mode on Bird.
  • Can have either SIMV or Assist/Control settings. If using Termination Sensitivity the ventilator may cycle prior to the set inspiratory time.

Volume Control

  • Constant flow, increasing pressure, variable inspiratory time.
  • Some ventilators will give a decelerating flow wave which may improve oxygenation.
  • Delivers set volume regardless of compliance but doesn't account for leaks.
  • Good when CO2 control essential e.g. head injury.
  • If patient triggers ventilator they receive a machine breath.

Pressure Support

  • Spontaneous breathing mode that can be combined with a control mode.
  • Patient triggers ventilator and receives a set level of inspiratory pressure.
  • Breath is terminated when either flow has decreased to 5% of peak inspiratory flow or when 80% of the set breath cycle time is reached (whichever comes first).

Volume Support

  • Ventilator is able to adjust the inspiratory pressure support within set limits to deliver desired tidal volumes.
  • Spontaneously breathing mode. Back up mode for apnoea is PRVC.
  • Terminates as for pressure support.
  • Good weaning mode.

Pressure Regulated Volume Control

  • Assist/Control mode.
  • Adjusts pressure to achieve desired volume and delivers a set number of breaths if the patient is not breathing.
  • Limits peak pressure to 5cmH2O below the upper pressure alarm limit.
  • VT may be variable if patient intermittently breathing.
  • Provides decelerating flow.

High Frequency Modes

  • Rapid rates can be achieved with standard neonatal ventilators.
  • Oscillation differs by having an active expiratory phase and it requires a special ventilator. The machine uses small tidal volumes at very rapid rates (up to 900bpm).
  • There is evidence that this type of ventilation may be superior in difficult to ventilate patients - both in achieving oxygenation and in preventing VILI.

CPAP

  • Provides a continuous positive expiratory pressure through out the respiratory cycle.
  • If used with a control mode it is called PEEP.
  • When used with spontaneous ventilation adequate flow is required to meet peak inspiratory flow and maintain level of CPAP.

Inverse Ratio Ventilation

  • Not a specific mode as such but a ventilation strategy that can be used with any control mode by making the Inspiratory Time greater than the Expiratory Time.
  • Works by Auto-PEEP and evenly distributing ventilation in lungs with variable time constants.

MMV

  • Mandatory Minute Ventilation.
  • Allows for a preset minute ventilation to be set and the machine will give mandatory breaths depending on how much spontaneous ventilation is occuring. With the Drager this mode uses autoflow which is a combination of PC and VC and provides decelerating flow.

PCV + (= PCV +Pressure Support)

Pressure Control ventilation with Assist/Control for spontaneous breaths. The rate is a back up rate only.

Triggering

To enable patient synchrony with the ventilator the trigger MUST ALWAYS be set to the maximum sensitivity. Both over and undersensing are to be avoided.

Newer ventilator features

AUTOFLOW Uses a combination of VC and PC to provide a set VT with a decelerating flow.
ATC (AUTOMATIC TUBE COMPENSATION) Measures resistance continuously in the ventilator circuit and adjusts pressure to maintain flow.
Theoretically superior to PS.
VOLUME SUPPORT Used for a spontaneously breathing patient.
Adjusts level of pressure support to achieve a set tidal volume.
Essentially should be self-weaning.
VAPS (VOLUME ASSURED PRESSURE SUPPORT) Similar to VS

Weaning

The decision to wean the patient is based on an assessment of lung function (FiO2, CXR, Ventilatory pressures, PEEP, Respiratory rate), CVS function (Stability of BP, HR, Inotrope requirement), CNS function (Airway protection/maintenance,Cough, Muscle strength, Co-operation) and disease progress.

Complicated predictors of successful extubation have been formulated but most are no better than clinician judgement.

The 3 things that prolong patient ventilation are excessive sedation, lack of attempt to extubate, and use of some weaning mode rather than extubating the patient.

For the routine patient weaning is not involved. They merely need their prolonged "anaesthesia" terminated. Ie muscle relaxation is ceased, sedation is decreased to allow for a patient who can protect their airway and have adequate respiratory drive.
Once the patient begins making respiratory effort they are switched to a spontaneously breathing mode and when they are breathing adequately on minimal support (e.g. PS 10cmH2O) they can be extubated.

THIS IS NOT WEANING AND SHOULD NOT BE TREATED NOR DESCRIBED AS SUCH.

Remember that going from ventilated to spontaneous breathing changes intrathoracic pressures and may result in different loading conditions for the heart and alterations in lung water.

Weaning of the more complex patient is not so straight forward. Patients who have been ventilated for some time or who have muscle weakness may need a period of respiratory muscle retraining over a prolonged period. This may include a gradual weaning of support levels and/or increasing lengths of time off the ventilator. It is important to not let patients perform excess work during the weaning process as this will prolong weaning. It is also important to optimise their nutrition - provide adequate calories but do not overfeed with CHO nor water.

The management of ventilation for a serious respiratory illness has 3 components to it.

These are rest/repair, recovery/retraining, and extubation.

Prolonged intubation in children tends to be well tolerated and the need for tracheostomy is very uncommon unless for airway protection or very long term ventilation.

Once weaned from the ventilator extubation is usually not a problem but can be in cases of airway, CNS, or neuromuscular abnormalities.

Specific Diseases

Diseases with air trapping as the main feature

These include asthma, bronchiolitis, tracheobronchomalacia etc.

The aims of ventilation are to oxygenate the patient, stent the airway open, and allow time for exhalation.

Patients are often best breathing at slower rates with short inspiratory times and with variable amounts of PEEP/CPAP (NB avoid auto-PEEP).

In the PICU setting this may entail sedating spontaneously breathing patients.

PaCO2 is seldom used as an indicator of the need for intubation. Clinical status is much more important. If the patient requires ventilation do not try to ventilate to a normal PaCO2.

The patient with airway collapse often requires a prolonged period of airway support, either via an ETT or with nasal or mask CPAP. Airway collapse will be exacerbated by anything that causes increased respiratory effort.

Diseases associated with neuromuscular problems

These include any of the congenital myopathies. Certain very sick PICU patients may develop a critical illness polyneuropathy which presents as diffuse weakness.

There are often 3 components to respiratory failure in these patients: respiratory pump weakness, inability to cough, and inability to protect the airway.

Although many of these diseases are progressive and fatal short term respiratory support is indicated for some of these patients.

If the problem has been respiratory pump failure these patients will need a rest on the ventilator before considering weaning.

Noninvasive ventilation with mask BiPAP can be useful if tolerated.

Bronchopulmonary Dysplasia

Patients are often on home oxygen with a degree of respiratory distress/failure when "well" and can therefore deteriorate rapidly with minor respiratory insults.

There is often a considerable obstructive component to their disease which may or may not be reversible.

It is important to minimise ventilatory pressures, avoid fluid overload, use antibiotics as indicated, and think of increasing diuretics and adding steroids.

Pulmonary Oedema

Oxygenation can be markedly improved with CPAP.

If the pulmonary oedema is on the basis of myocardial dysfunction this needs treating (inotropes to improve contractility and IPPV to unload the LV). The usual cause of this is severe sepsis with large volume infusion and a stiff heart.

Do not disconnect the patient from PEEP to repeatedly suction as this will worsen the pulmonary oedema.

If severe pulmonary oedema can result in significant hypovolemia.

Intraparenchymal lung disease

The commonest cause of this is infective pneumonia but this can be associated with other lung disease or systemic diseases.
These patients may be more hypoxic with ventilation.

Often there are large areas of collapsed/consolidated alveoli that can be recruited with PEEP and this is the initial treatment to use.

If there is progressive disease diagnosis is important and in severe intraparenchymal disease from an unknown cause BAL and lung biopsy may be needed to guide treatment.

Neonatal Ventilation

There are major differences in ventilation strategies between neonatologists and intensivists.

Neonatologists seldom paralyse or heavily sedate babies and aim to achieve patient-ventilator synchrony by ventilating at rapid rates with short inspiratory times.

Intensivists tend to ventilate at slower rates with longer inspiratory times, use more sedation/paralysis, and accept higher PaCO2s.
Each believes that their ventilation strategy is the best and for the majority of patients there is little difference. It is becoming more common to minimise ventilation in the hope of decreasing the incidence of BPD/VILI (see below) but how best to do this is unclear.

Some neonatal conditions do require a high pH to maximise pulmonary vasodilatation and these in these patients higher rates of ventilation may be required. It would be rare to ventilate at a rate above 40bpm using conventional ventilation.

Neonates who are failing conventional ventilation should be switched early to HFO and if appropriate discussed re ECMO.

Surfactant is used for RDS and should be given early then repeated if effect wears off.

Severe Lung Disease

Severe lung disease requires a precise approach.

Important factors are:

  1. Diagnostic accuracy.
  2. Avoiding VILI.
  3. Optimising oxygen delivery.
  4. Minimising lung insult.
  5. Additional treatments.
      
  1. Diagnostic Accuracy
    Microbiology Blood cultures
    NPA for respiratory viruses
    Tracheal Aspirate
    Early BAL to diagnose infection
    Pathology Early open lung biopsy in the immunosuppressed.
    ? Late lung biopsy in ARDS. 
    Serology ? Connective tissue disease. 
    Radiology  CXR.
    ? CT scan (High resolution). 
    Look for secondary infection
    Drain fluid (for diagnostic and treatment reasons).
    Drain air outside the lung
  2. Avoiding VILI
    Limit FiO2 to < 0.5.
    Accept SpO2 in 80's.
    Optimise PEEP.
    Optimise patient-ventilator interaction (Sensitivity, Mode, Flows, Circuit, Triggering, Sedation, Paralysis).
    Avoid excessive volume/pressure (Small Vt, Pressure control, Long Tinsp, HFV).
    Avoid atelectasis (Alter patient position, Prone ventilation, Good suctioning technique, ???Chest physio).
  3. Optimising O2 delivery
    Optimise PEEP.
    Target Hb.
    Maximise CO.
    Minimise patient O2 demand.
  4. Minimising lung insult
    Eliminate infection elsewhere (antibiotics, drainage).
    Maintain hemodynamics.
    Optimise fluid balance.
    Optimise nutrition.
  5. Additional treatments
    Inverse ratio ventilation.
    Surfactant.
    Nitric oxide.
    HFO.

Stepwise Approach to Ventilation in Severe Lung Injury

All these patients will be ventilated and most will require heavy sedation. Most will require paralysis. Some patients will deteriorate with positive pressure ventilation due to increased V/Q mismatch, increased intrathoracic pressure, and/or decreased RV function. These patients may oxygenate better if they are breathing spontaneously.

Current vogue in ARDS is the open lung strategy. This uses a prolonged high pressure inflation (eg 40cmH2O for 40 sec) to open recruitable lung units followed by high PEEP and small tidal volumes at low rates to maximise oxygenation while limiting injury. A consequence of this approach is that there is permissive hypercarbia.

  1. Supplemental oxygen
    Toxic oxygen concentration in the diseased lung is unknown. Aim to get FiO2 under 50% if possible.
  2. PEEP
    Aim for 'best PEEP' ie PEEP that maximises oxygen delivery. Start at 5cmH2O and increase in 2.5cmH2O increments watching SpO2, PIP, P - V curve, and hemodynamics. An alternative in lungs that are known to be very diseased is to start with a prolonged high pressure inflation then use high PEEP e.g. 10 - 20cmH2O and decrease it until saturations or PaO2 drop, then return to level just above this.
    Changes in PEEP may take up to 30 mins to affect PaO2.
    May need very high PEEP (20 - 25cmH2O).
    Will need IV fluids +/- inotropes to maintain hemodynamics with high PEEP.
    If needing >10cmH2O PEEP do not disconnect to suction but use a Bodai connector and suction through this. If you do need to disconnect the patient reinflate manually before reconnecting.
  3. Tidal Volume
    Limit to no more than 6ml/kg ideal body weight.
  4. Pressure control
    A decelerating flow pattern will more evenly distribute ventilation.
    This can also be achieved using volume control with a decelerating flow.
    Limit plateau pressure to < 30cmH2O.
  5. Prevent/Treat excess lung water
    Patients with severe respiratory disease on a ventilator often have inappropriate ADH secretion with retention of water. This excess fluid tends to manifest itself as lung water.
    Aim is for euvolemia as hypovolemia will cause hemodynamic instability and decreased oxygen delivery.
  6. Prolong inspiratory time
    Start at an inspiratory time:expiratory time (I:E ) ratio of 1:2 and increase to 1:1.
    If oxygenation is still inadequate increase the I:E beyond 1:1 to create inverse ratio ventilation. Few patients will tolerate this unless paralysed.
    This improves oxygenation primarily by causing increased PEEP due to inadequate time for complete expiration (this is called autoPEEP).
    AutoPEEP needs to be monitored to ensure that it doesn't become excessive and interfere with hemodynamics and/or ventilation. This is done by using expiratiory hold on the ventilator and seeing what the expiratory pressure is, or by disconnecting the patient from the ventilator and watching how long expiration continues. Either of these manoevurs may result in much improved hemodynamics.
  7. Recruitment Manoeuvers
    Safest to perform using the ventilator with insp hold or very long I time and slow rate.
    30-40cmH2O for 30-40sec.
    May get CVS depression.
    Repeat whenever disconnected from the ventilator.
  8. Permissive hypercapnia
    Allow PaCO2 to be as high as it can as long as pH > 7.1.
    High CO2 may actually protect the lung.
    Not in head injury or reactive pulmonary hypertension (may still allow a high PaCO2 in pulmonary hypertension and control pH with bicarbonate).
  9. Prone Ventilation
    Often improves oxygenation by minimising dependent atelectasis.
    Unstable patients will often deteriorate initially and take up to 20-30 mins to show improvement.
    Leave prone for >12hrs.
    Need manpower to turn larger patients.
  10. Decrease oxygen demand
    Sedate and paralyse.
    Consider cooling.
  11. Goal PaO2/SpO2
    Accept a lower PaO2 (7-8kPa) and a SpO2 in the mid-high 80s.
  12. Nitric Oxide
    Variable responses.
    Start at 20ppm and decrease as tolerated. Most responders need only 1-2ppm for effect.
  13. High frequency oscillation
    Suitable for all sizes with the right equipment.
    Uses high mean airway pressures and small tidal volumes at very rapid rates.
    Used early in infant lung disease this mode of ventilation does reduce long term lung injury.
    3100A for < 30kg and 3100B for large children and adults.
    MAP 2-4cmH2O > conventional
    Hz neonate 15 child 10 adult 5
    Bias flow 20L (40L on the B), power to wiggle, I time 33%, FiO2=1
    Do not make rapid nor large changes in settings.
    Check CXR to ensure adequate inflation - should show 9-10 ribs posteriorly
  14. Surfactant
    Effective in a number of conditions but prohibitively expensive for all but neonates and infants.
    Natural better than artificial.
    Term baby dose = 1 ampule via feeding tube through ETT.
    Doubts about improving long term outcome.
  15. Tracheal gas insufflation
    Augments tidal volume and PEEP, increases FiO2, and washes out CO2.
    Small catheter placed beyond ETT tip and low flow oxygen is given.
  16. ECMO
    VV preferred over VA for respiratory disease.
    Limited availability.
    Patient selection is vital.
    Single organ disease with reversible lung disorder and no contraindications
  17. Steroids
    No place in acute management but can be beneficial in preventing excessive fibrosis during the resolution of ARDS.
    The trick is getting the timing right and avoiding the complications.

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Document Control

  • Date last published: 06 June 2016
  • Document type: Clinical Guideline
  • Services responsible: Paediatric Intensive Care Unit
  • Editor: John Beca
  • Review frequency: 2 years