Pediatric & Neonatal Mechanical Ventilation Praveen Khilnani
Abnormal waveforms 119
Acidosis 76
lung injury (ALI) 128, 168
myocardial infarction (AMI) 233
pulmonary edema 168
respiratory distress syndrome 244
respiratory failure 172
Adjustments after initiation 42
Advanced mechanical ventilation 57
Advantages of NIPPV 168
Aerosol therapy 91
leak syndrome 200
trapping 120
-entrainment mask/venturi mask 26
Airleak syndrome 48
injury from mechanical ventilation 163
pressure (PAW) 109, 110
pressure release ventilation (APRV) 58
resistance 9, 16
Alkalinization 80
Alkalosis 76
Altering inspired oxygen and carbon dioxide 64
Alternative modes of
neonatal ventilation 52
ventilation 196
capillary interface 23
overdistention 122
Anatomical dead space 10
Anterior horn cell disease 168
Apneic oxygenation 63
Applied respiratory physiology for mechanical ventilation 16
Approach to
child with acidosis 78
patient with alkalosis 81
ARDS controversies with INO therapy 232
Argyl nasal prongs 185
Art of ventilation 107
Artificial lung 198
Assessing outcome 200
Assist/control ventilation 52, 196
Assisted mode (volume-targeted ventilation) 124
Asthma 118
Asynchrony during SIMV-PS 66
Auto-peep or air trapping 119
Auto-triggering 70, 71
Barotrauma and oxygen toxicity (BPD) 200
Barotrauma/volutrama 162
concepts of HFV 203
fundamentals of ventilation 38
mechanical ventilation 34, 37
physiology 9, 35
principles of ventilation 194
respiratory physiology 9
Benefits of HFO 267
Benzodiazepines 138
Bi-level positive airway pressure (BIPAP) 168, 170
Blood gas
and acid-base interpretation 76
monitoring 200
parameters 193
cycling asynchrony 74
delivery asynchrony 71
Breathing circuit 2
Bronchiolitis 168
Bronchopulmonary dysplasia (BPD) 11
Bruises and erosions 176
Bubble nasal CPAP system 184
Buffering system 76
Bunnell jet ventilator 204
Cardiac case statistics 238
Cardiovascular factors 149
Care of ventilated patient 88
Cavopulmonary connection 168
Central nervous system (CNS) 147
Cerebral malaria 233
Cesar-trial 244
Characteristic flow-volume loops 119
Characteristics of aerosol generating device 91
mechanics 15
physiotherapy (CPT) 88
Child with severe tracheomalacia 119
Choose the mode 42
Chronic respiratory failure 170
characteristics 92
disconnect alarm 269
application 205
applications and significance 125
Collapse of upper airway 176
Common causes of extubation failure 159
available ventilators 247
used nomenclature 36
Complication of invasive monitoring 200
and sequelae 200
associated with bubble nasal CPAP 188
of mechanical ventilation 162
of NIPPV 176
related to adjunctive therapies 165
Components of inflation pressure 113
Compressor 1
Conditions when CPAP fails 187
Conductance is reciprocal of resistance 11
Constant flow ventilation 63
Content of
oxygen (CAO2) 22
in blood 22
positive airway pressure (CPAP) 27, 36, 50, 170, 194
rotational therapy 89
venovenous hemofilteration loop 244
Contraindications to CPAP 182
Control of respiration 15
hypercapnia 46
hypoventilation (permissive hypercapnia) 142
mode (volume-targeted ventilation) 124
neonatal ventilation 51
ventilation 195
CPAP delivery system 28, 182, 183
for intubation 137
to assess ventilator dependence 150
Cycling off 70
Cystic fibrosis 168
Dead space ventilation 10
Decreased lung compliance during volume ventilation 121
Delayed termination 74
Descending ramp flow waveform 113
Determinants of weaning outcome 148
Diagnosing acute lung injury 128
Diaphragmatic palsy 168
Differences in high frequency jet ventilation 199
Diffusing capacity 14
Disease specific ventilation 45
Distribution of inspired gas 10
Double triggering 70
Drager Babylog 8000
controls 252, 253
plus 254
Duration of treatment 230
Dynamic hyperinflation 139, 142
circuit 241
management 242
Effects of
CPAP in infant with respiratory disease 181
intubation 139
metabolic acidosis 79
Effects on
circulatory system 165
lung 163
Elevated pulmonary capillary wedge pressure 234
ELSO registry 2010 data 245
intubation and ventilation 130
suctioning 94
Esophageal pressure (PES) 109
Evidence for CPAP 189
Extracorporeal membrane oxygenation 198, 237
Extubation after trial of CPAP 190
Extubation 158
Eye care 94
Factors affecting
mean airway pressure and oxygen 43
oxygen delivery ACI 23
Fiberoptic bronchoscopy (FOB) 89
Fixed upper airway obstruction 118
patterns 112
trigger sensitivity level 262
volume loop on spirometry 118
volume loop 110, 117, 118
vs volume 111
Fontan procedure 168
expiratory flow 118
inspiratory flow (FIF) 118
Fraction of bias flow 262
FVL indicates positive bronchodilator response 120
Gas exchange 18, 35
factors 149
related problems 42
Goals of
mechanical ventilation during weaning 156
ventilation in ARDS 47
Guillain-Barré syndrome 168
Heart transplantation 233
Heat and moisture exchangers (HMEs) 90
water humidifiers (HWHs) 90
wire circuit 90
Helium-oxygen mixture (heliox) 64
High flow 121
High frequency
jet ventilation (HFJV) 203
oscillation (HFO) 267
oscillatory ventilation (HFOV) 199, 204, 248
positive pressure ventilation (HFPP) 203
ventilation 53, 62, 197, 202, 203, 206
High PACO2 43
High raw 120
Homeostasis 77
Humidifier 2
Hypercarbia 18, 35
Hypoxemia 17, 35, 141
and hypoxia 23
Improved RDS with CPAP 187
Improves non-invasive ventilation 68
Improving patient ventilator synchrony 74
IMV modes 38
Inadequate oxygenation 42
Inadvertent (auto) peep 109
Increased airway resistance (RAW) 120
Indications for
CPAP 182
reintroducing NCPAP 189
Indications of mechanical ventilation 36
Ineffective trigger 70, 71
flow driver CPAP system 184
star 500/950 ventilator system 259
ventilator CPAP system 183
β-agonists 144
nitric oxide (INO) 54, 197, 227
Initial ventilator settings 41
Initiating and maintaining optimal NCPAP 182
Initiation of
non-invasive mechanical ventilation 172
ventilation 45, 193
Injury and ARDS in children 128
in cardiology 233
in chronic lung disease 231
therapy in children with ARDS 231
Inspiratory time (TI) 2, 42
Inspired oxygen concentration (FIO2) 3
Intermittent mandatory ventilation (IMV) 155
Interpretation of respiratory alkalosis 84
Intubation technique 138
Inverse ratio ventilation (IRV) 57
Ketamine 138
Kyphoscoliosis 168
Life-threatening status asthmaticus 233
Limitations of NIPPV 174
Liquid ventilation (LV) 63, 199
Loops 117
Low functional residual capacity 16
compliance changes in P-V loop 121
infection 168
transplantation 233
Management of pediatric ALI and ARDS 129
Mandatory minute ventilation (MMV) 52, 196
Manual hyperinflation 89
Maquet 67
Mean airway pressure (Map) 2, 111
Measurement of
end-inspiratory plateau pressure 143
intrinsic positive end-expiratory 144
the end-inspired volume (VEI) 143
for high PACO2 43
to reduce barotrauma and volutrauma 44
misadventures 166
operational problems 162
ventilation in acute asthma 137
ventilation 9
Mechanism of improvement with non-invasive ventilation 167
Metabolic acidosis 77, 80
and respiratory alkalosis 86
factors and ventilatory muscle function 149
Meter dose inhaler (MDI) 92
Method linear regression analysis 253
Methods to monitor patients in ICU 108
Miscellaneous uses and ongoing trials 233
acid-base disorders 85
metabolic alkalosis and respiratory acidosis 85
metabolic and respiratory acidosis 85
metabolic and respiratory alkalosis 85
Modes of ventilation 36, 122
Modified from recommendations by HESS 92, 93
Monitoring during NIPPV 175
Mucolytics 93
Muscle relaxation 200
Myasthenia gravis 168
cannula 25
obstruction 188
Nasopharyngeal catheters 25
Naturally adjusted ventilatory assist 65
chronic lung disease 234
CPAP (continuous positive airway pressure) 181
intensive care unit (NICU) 247
respiratory case statistics 239
ventilation 50, 192
ventilator model bearcub 750
PSV–VIASYS H 250, 251
Neurally adjusted ventilatory assist (NAVA) 65
Neurologic issues 148
diseases 168
-blocking agents 139
Newer modes 57
wave E 120 258
breeze E 150 258
E 100 m ventilator 258
E 100 m 258
ventilators 258
Nitric oxide
delivery system 229
synthase (NOS) 227
double nasal tube 174
facial mask 174
via double nasal tube 174
via facial mask 173, 174
via tracheostomy 175
Nonconventional techniques 62
mechanical ventilation 145
negative pressure ventilation (NINPV) 168
positive pressure ventilation (NIPPV) 169
ventilation 167, 171
Non-rebreathing masks 25
curve 120
flow-volume loop 119
infections 162
pneumonias 165
Objectives of ventilation 195
pulmonary diseases 168
sleep apnea 168
Old and newer versions 256
Open heart surgery 49
Opioids 139
Other bronchodilators 145
Overdistention 122
carriage 14
concentrator 29
delivery devices 24
dissociation curve 23
hood 26
in arterial blood 21
therapy 20
toxicity 31
Oxygenation 17, 35
Oxyhemoglobin-dissociation curves 15
PAO2 and FIO2 is safe 32
Paradoxical worsening 234
Partial pressure of oxygen 20
in alveolus (PAO2) 35
Partial-rebreathing masks 25
Parts of a ventilator 1
Pathophysiology of ventilator dependence 147
comfort 175
on NIPPV 173
selection criteria 239
triggered ventilation (PTV) 52, 196
ventilator dyssynchrony 44, 70
with symptomatic severe asthma/cystic fiber 119
-ventilator synchrony 157
expiratory flow rate (PEFR) 118
inspiratory flow rate (PIFR) 118
inspiratory pressure (PIP) 13, 109, 111
intensive care unit (PICU) 247
respiratory case statistics 239
Percent survival without ECMO 229
Percussion and vibration 89
hypercapnia 44
hypoxemia 44
Persistent pulmonary hypertension (PPHN) 34
Phrenic palsy, injury or disease 168
Physiological effects of INO in ARDS 232
PIP vs Pplat 120, 121
Plateau pressure (Pplat) 109
Pneumotaxic and apneustic 15
Position of device 92
Positive end expiratory pressure (PEEP) 1, 111, 141
Postoperative ventilation 49
Potential benefits with nava 67
Practical tips to approach acid-base disorders 84
Premature termination 74
Prerequisites to weaning 157
Pressure control
mode 140
ventilation 39, 169
control 39
flowtrace 62
limited time cycled ventilation 51
limited 42
-regulated volume control (PRVC) 41, 61
regulated volume control (PRVC) 41, 62
support (PS) 38, 156
-support 7
support/CPAP 60
ventilation (PSV) 52, 60, 155, 170, 196, 267
ventilators 248
volume and flow against time 111
volume curve 110
-volume loop 117
Prevent gastric distention 188
Preventing injury to nasal septum 188
Prevention of barotrauma 164
Primary pulmonary hypertension 233
Principles of oxygenation 29
Procedure of weaning from mechanical ventilation 153
Procedures for removal of NCPAP 189
Product benefits 261
Prolonged bleeding time 234
Prophylactic CPAP in VLBW infants 190
Propofol 138
Proportional assist ventilation (PAV) 52, 61, 196
Protection from contaminants 268
Psychosocial factors 149
capillary flow is best at functional 18
circulation: 162
or cardiac shunt 24
Puritan Bennett® 840 ventilator 268
Raised ICP (intracranial pressure) 50
Ramp flow waveforms comparing fast and slow space 114
Ratio of inspiratory to expiratory time 6
Rebound effects 234
Recent evidence on use of ECMO 244
Recognition of hypoxia 23
Rectangular flow waveforms
square wave 113
comparing fast and slow 114
Rescue therapies for children with ALI/ARDS 132
acidosis 82, 137
alkalosis 83
care protocol 45
control 66
failure on CPAP 188
failure 36
rate (RR) 2, 5, 111
support in children with ALI and ARDS 129
system muscle/load interactions 148
Respironics BIPAP 264
and non-invasive ventilator 264
Restrictive lung disease 118, 119
Routine ventilator management protocol 44
Scalar waveforms
during common modes of ventilation 123
of pressure and volume controlled 122
Scalars and loops 111
Sechrist ventilator
new 257
old 256
and muscle relaxation during ventilation 44
during intubation and ventilation 138
Self-diagnostic testing 269
Sensor medics high frequency oscillatory ventilation 263
Sensormedics 3100A 263
oscillator 262
oscillator 205
Servo-I infant (MAQUET) 261
Severe RDS 187
Shortens weaning time 68
Shunt 13
Siemens servo 300/300A ventilator 255, 256
Siemens servo 900C ventilator 254, 255
Siemens servo 260
I (MAQUET) 260
humidifier 2
oxygen masks 25
SIMV + PS—volume-targeted ventilation 125
SIMV with pressure support (PS) 124
Sine flow waveform 113
2000 for infant ventilation 264
2000 265
5000 266
Smartalert™ alarm system 268
Specific ventilators 249
Structure and function of a conventional ventilation 1
Success with NCPAP 186
Suction support 262
Suggested method for delivery of drug by nebulization 92
Support mode 38
Supportive therapy with mechanical ventilation 200
Surgery on right heart 168
Synchronized intermittent mandatory ventilation (SIMV) 124
Systemic corticosteroids 144
Targeted tidal volume (TTV) 267
Technical specification 261, 265
Technique of respiratory mechanics monitoring 110
Terminology 2
Tetralogy of Fallot 168
Three types of CPAP delivery systems 182
Tidal volume (VT) 1, 6, 109
Time constant 36
resistance × compliance 11
Timely delivery of assistance 68
T-piece weaning 155
Tracheal insufflation of oxygen 63
Transairway pressure (PTA) 109
Transport ventilator 258
bird avian 258
Transpulmonary pressure 11, 109
Treatment of underlying cause 79
Trigger variable 71
Trigger/sensitivity 40
Triggering 70
Types of
high frequency ventilation 203
humidifiers 2
hypoxemia 24
ventilatory support 194
waveforms 111
Unresponsiveness to INO therapy in PPHN 230
Upper airway 16
Use of peep 46
extrathoracic obstruction 118
intrathoracic obstruction 118
VELA ventilator 249, 250
Ventilation 17, 35
control 140
for acute respiratory distress syndrom 47, 128
perfusion (V/Q) 228
mismatch 24
strategies 45, 49
causes of patient agitation 70
dependent 147
graphics and clinical applications 107
induced lung injury 202
model AVEA- VIASYS health care 251, 252
waveforms 111
Ventilatory parameters 92
Venturi principle for air entrainment 27
Viasys health care 249
VIP bird ventilator 257
Vital signs 175
assured pressure support (VAPS) 156
descending ramp flow waveform 113
mode 141
rectangle flow waveform 114
rectangle ramp flow waveform 115
rectangular flow waveform with flow 117
rectangular flow waveform 113, 116
sine flow waveform 114
ventilation 40
limited 42
targeted ventilation (SIMV) 125
ventilation 169
ventilators 247
vs time scalar 115
Weaning 131, 176, 230
from mechanical ventilation 147
methodology 157
modes 155
Work of breathing (WOB) 110
Zone of perfusion in lung 10
Chapter Notes

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Structure and Function of a Conventional VentilatorChapter 1

Praveen Khilnani,
S Ramesh
This chapter is intended to get the reader familiar with basic aspects of the ventilator as a machine and its functioning. We feel this has important bearing in the management issues of a critically-ill child requiring mechanical ventilation.
A ventilator is an automatic mechanical device designed to move gas into and out of the lungs. The act of moving the air into and out of the lungs is called breathing, or more formally, ventilation.
Simply, compressed air and oxygen from the wall is introduced into a ventilator with a blender, which can deliver a set FiO2. This air oxygen mixture is then humidified and warmed in a humidifier and delivered to the infant by the ventilator via the breathing circuit.
The peak inspiratory pressure (PIP) or tidal volume (Vt), positive end expiratory pressure (PEEP), inspiratory time and respiratory rate are set on the ventilator.
The closing of the exhalation valve initiates a positive pressure mechanical breath. At the end of the preset inspiratory time, the exhalation valve is opened, permitting the infant to exhale. If this end is partly occluded during expiration, a PEEP is generated in the circuit proximal to the occlusion (or CPAP if the infant is breathing spontaneously). Expiration is passive and gas continues to flow delivering the set PEEP.
Parts of a Ventilator
  1. Compressor: This is required to provide a source of compressed air. An in-built wall source of compressed air, if available, can be used instead. It draws air from the atmosphere and delivers it under pressure (50 PSI) so that the positive pressure breaths can be generated.
The compressor has a filter which should be washed with tap water daily or as directed. If this is not done, it greatly increases the load on the compressor. The indicator on the compressor should always be in the green zone. It should not be placed too close to the wall as it may get overheated. There should be enough space to permit air circulation around it.2
  1. Control panel: The controls that are found on most pressure-controlled ventilators include the following:
    • FiO2
    • Peak Inspiratory Pressure: PIP (in pressure controlled ventilators).
    • Tidal volume/Minute volume (in volume controlled ventilators).
    • Positive End Expiratory Pressure (PEEP).
    • Respiratory Rate (RR).
    • Inspiratory Time (Ti).
    • Flow rate.
The other parameters displayed on the ventilator include mean airway pressure (MAP), I:E ratio (ratio of the inspiratory time to expiratory time). The expired tidal volume will be displayed in all volume controlled ventilators and some pressure controlled ventilators.
Newer ventilator models have digital display controls. Some ventilators also display waveforms, which show the pulmonary function graphically.
  1. Humidifier: Since the endotracheal tube bypasses the normal humidifying, filtering and warming system of the upper airway, the inspired gases must be warmed and humidified to prevent hypothermia, inspissation of secretions and necrosis of the airway mucosa.
    Types of humidifiers available:
    1. Simple humidifier: It heats the humidity in inspired gas to a set temperature, without a servo control. The disadvantage is excessive condensation in the tubings with reduction in the humidity along with cooling of the gases by the time they reach the patient.
    2. Servo-controlled humidifier with heated wire in the tubings: These prevent accumulation of condensate while ensuring adequate humidification. Optimal temperature of the gases should be 36-37°C and a relative humidity of 70 percent at 37°C. If the baby is nursed in the incubator, temperature monitoring must take place before the gas enters the heated field. At least some condensation must exist in the inspiratory limb which shows that humidification is adequate. The humidifier chamber must be changed daily. It should be adequately sterilized or disposable chambers may be used.
  2. Breathing circuit: It is preferable to use disposable circuits for every patient. Special pediatric circuits are available in the market with water traps. If reusable circuits are used, they must be changed every 3 days. Reusable circuits are sterilized by gas sterilization or by immersion in 2 percent glutaraldehyde for 6-8 hours and then thoroughly rinsing with sterile water. Disposable circuits may be changed every week.
Ventilatory controls that can be altered in mechanical ventilation include the following:
  1. Inspired oxygen concentration (FiO2).
  2. Peak inspiratory pressure (PIP).3
  3. Flow rate.
  4. Positive end-expiratory pressure (PEEP).
  5. Respiratory rate (RR),or Frequency (f).
  6. Inspiratory/Expiratory Ratio (I:E Ratio).
  7. Tidal volume (in volume controlled ventilators).
  8. Pressure support.
Inspired Oxygen Concentration (FiO2)
An improvement in oxygenation may be accomplished either by increasing the inspired oxygen concentration (FiO2) or by different ventilator settings.
  1. Increasing peak inspiratory pressure (PIP)
  2. Increasing inspiratory/expiratory ratio
  3. Applying a positive pressure before the end of expiration (PEEP).
FiO2 is adjusted to maintain an adequate PaO2. High concentrations of oxygen can produce lung injury and should be avoided. The exact threshold of inspired oxygen that increases the risk of lung injury is not clear. A FiO2 of 0.5 is generally considered safe. In patients with parenchymal lung disease with significant intrapulmonary shunting, the major determinant of oxygenation is lung volume which is a function of the mean airway pressure. With a shunt fraction of > 20 percent oxygenation may not be substantially improved by higher concentrations of oxygen.
The administration of oxygen and its toxicity is a clinical problem in the treatment of neonates, especially low birth weight infants.
The developing retina of the eye is highly sensitive to any disturbance in its oxygen supply. Oxygen is certainly a critical factor (hyperoxia, hypoxia), but a number of other factors (immaturity, blood transfusions, PDA, vitamin E deficiency, infections) may interact to produce various degrees of Retinopathy of Prematurity (ROP).
Another complication of oxygen toxicity induced by artificial ventilation in the neonatal period is a chronic pulmonary disease, Bronchopulmonary Dysplasia (BPD), mostly seen in premature infants ventilated over long periods with a high inspiratory peak pressure and high oxygen concentration.
High oxygen concentration may play a role in the pathogenesis of BPD, but recent studies have shown, that the severity of the disease is correlated to the Peak inspiratory pressure (PIP) during artificial ventilation rather than to the doses of supplementary oxygen.
Peak Inspiratory Pressure (PIP)
Peak Inspiratory Pressure is the major factor in determining tidal volume in infants treated with time cycled or pressure cycled ventilators. Most ventilators indicate inspiratory pressure on the front and it can be selected directly.
The starting level of PIP must be considered carefully. Critical factors that must be evaluated are the infant's weight, gestational age (the degree of maturity), the type and severity of the disease and lung mechanicssuch as lung compliance and airway resistance.4
The lowest PIP necessary to ventilate the patient adequately is optimal. In most cases, associated with increased tidal volume, increased CO2 elimination and decreased PaCO2.
Mean airway pressure will rise and thus improve oxygenation.
If PIP is minimized, there is a decreased incidence of barotrauma with resultant air leak (pneumothorax and pneumomediastinum) and BPD.
Hacker et al demonstrated that more rapid ventilator rates and lower PIP are associated with a decreased incidence of air leaksa mode of ventilation which may be recommended in infants with congenital diaphragmatic hernia.
High PIP may also impede venous return and lower cardiac output.
Flow Rate
The flow rate is important determinant during the infant's mechanical ventilation of attaining desired levels of peak inspiratory pressure, wave form, I:E ratio and in some cases, respiratory rate.
In general, a minimum flow at least two times the minute volume ventilation is usually required. Most pressure ventilators operate at flows of 6-10 liters per minute.
If low flow rates are used, there will be a slower inspiratory time (Ti) resulting in a pressure curve of sine wave form and lowering the risk of barotrauma.
Too low flow relative to minute volume, may result in hypercapnia and accumulation of carbon dioxide in the system.
High inspiratory flow rates are needed if square wave forms are desired and also when the inspiratory time is shortened in order to maintain an adequate tidal volume. Carbon dioxide retention in the ventilator tubing will be prevented at high flow rates.
A serious side effect of high flow rate is an increased risk of alveolar rupture, because maldistribution of ventilation results in a rapid pressure increase in the non-obstructed or non-atelectatic alveoli.
Positive End Expiratory Pressure (PEEP)
Positive pressure applied at the end of expiration to prevent a fall in pressure to zero is called Positive End Expiratory Pressure (PEEP).
PEEP stabilizes alveoli, recruits lung volume and improves the lung compliance. The level of PEEP depends on the clinical circumstances. Application of PEEP results in a higher mean airway pressure, and mean lung volumes.
The goals of PEEP are:
  1. Increasing FRC (Functional Residual Capacity) above closing volume to prevent alveolar collapse
  2. Maintaining stability of alveolar segments
  3. Improvement in oxygenation, and
  4. Reduction in work of breathing.
The optimum PEEP is the level at which there is an acceptable balance between the desired goals and undesired adverse effects. The desired goals are: (1) reduction in inspired oxygen concentrationnontoxic levels (usually less than 50%); (2) maintenance of PaO2 or SaO2 of > 60 mm Hg or > 90 percent respectively, (3) improving lung compliance; and (4) maximizing oxygen delivery.
Arbitrary limits cannot be placed to determine the level of PEEP or mean airway pressure that will be required to maintain adequate gas exchange. When the level of PEEP is high, peak inspiratory pressure may be limited to prevent it from reaching dangerous levels that contribute to air leaks and barotrauma. In children with tracheomalacia or bronchomalacia, PEEP decreases the airway resistance by distending the airways and preventing dynamic compression during expiration.
The compliance may be improved. Improved ventilation may result (improvement in ventilation/perfusion ratio) by preventing alveolar collapse.
Low levels of PEEP (2-3 cm H2O) are often used during weaning from the ventilator in conjunction with low IMV rates only for a short amount of time.
Medium levels of PEEP (4-7 cm H2O) are commonly used in moderately ill patients.
High levels of PEEP (8-15 cm H2O) benefit oxygenation in ARDS (Acute Respiratory Distress Syndrome); tidal volume, and PaO2 increases. HigherPEEP level can also reduce blood pressure and cardiac output explained by a reduced preload. Very high levels of PEEP results in overdistention and alveolar rupture leading to increased incidence of pneumothorax and pneumomediastinum.
Respiratory Rate (RR) or Frequency (f)
Respiratory rate, together with tidal volume, determines the minute ventilation. Depending on the infant's gestational age and the underlying disease, the resulting pulmonary mechanics (resistance, compliance) require the use of slow or rapid ventilatory rates.
Moderately high ventilator rates (60-80 breaths per minute) employ a lower tidal volume and therefore, lower inspiratory pressures (PIP) are used to prevent barotrauma.
High rates may also be required to hyperventilate infants with pulmonary hypertension and right-to-left shunting to achieve an increased pH and reduced PaCO2, thereby reducing pulmonary arterial resistance and shunting associated with increased PaCO2. Respiratory rate is the primary determinant of minute ventilation and hence, CO2 removal from lungs.
Tidal volume RR, increasing the RR lowers the PaCO2 level. A respiratory rate of 40-60 is usually sufficient in most conditions. High rates are necessary in Meconium Aspiration Syndrome (MAS) where CO2 retention is a major problem. It must be recognized that increasing the RR while keeping the IT the same, shortens expiration and may lead to inadequate emptying of lungs and inadvertent PEEP.6
One of the major disadvantages in using the high ventilator rates is an insufficient emptying time during the expiratory phase, resulting in air trapping, increased FRC, and thus decreased lung compliance.
A slow ventilation rate combined with a long inspiratory time, both in animals and infants with RDS resulted in fewer bronchiolar histological lesions, better lung compliance and in infants, a reduction in the incidence of BPD.
Ratio of Inspiratory to Expiratory Time (I:E ratio)
One of the most important ventilator control is the ratio of inspiratory to expiratory time (I:E ratio). This ventilator control has to be adjusted depending on the pathophysiology and the course of the respiratory disease, always with respect to pulmonary mechanics, such as compliance, resistance and time constant.
In infants with, Respiratory Distress syndrome (RDS) with decreased compliance but normal resistance, resulting in shortened time constants inspiratory times I:E with ratios 1:1 are usually used.
Reversed I:E ratios, as high as 4:1 have been shown to result in improvement in oxygenation and in a retrospective study decreased the incidence of BPD. Other investigators also advocated the use of prolonged inspiratory time, since infants in the ‘2:1’ group required less inspired oxygen and a lower expiratory pressure to achieve satisfactory oxygenation. Extreme reversed I:E ratio with a short expiratory time will lead to air trapping and alveolar distention. In addition, prolonged inspiratory time may adversely affect venous return to the heart and decreased pulmonary and systemic blood flow. The concept becomes especially important when higher respiratory rates are used.
If inspiratory time is shorter than three to five time constants, inspiration will not be complete and tidal volume will be lower than expected. If expiratory time is too short, expiration will not be complete which will lead to air trapping.
An IT of 0.3-0.5 sec is sufficient for most disorders. In low compliance condition like RDS use closer to 0.5 sec. In disorders with increased airway resistance like MAS use shorter IT. Once set, IT is usually not changed unless there is persistent hypoxemia unresponsive to changes in PIP and FiO2.
Increasing the IT shortens the expiratory time increasing the I:E ratio. Normal ratio is 1:3. Avoid 1:1 ratio to prevent air trapping.
While ventilating a case of lower airways obstruction (asthma, bronchiolitis) use short IT and slow rate with a longer expiratory time as there is gas trapping and increased risk of air leaks.
Tidal Volume (Vt)
In most volume cycled ventilators, tidal volume of 6 to 8 ml/kg can be set, or a particular flow rate and minute ventilation can be set to get a particular tidal volume. Siemens Servo 300 ventilator measures expired tidal volume and gives a display. If set tidal volume is significantly (the difference 7between set inspired and expired tidal volume is more than 15%) higher than the expired tidal volume, then circuit leak or an endotracheal leak should be looked for and corrected.
Pressure-support ventilation is a form of assisted ventilation where the ventilator assists a patient's spontaneous effort with a mechanical breath with a preset pressure limit. The patient's spontaneous breath creates a negative pressure, which triggers the ventilator to deliver a breath. The breath delivered is pressure-limited; very high inspiratory flow results in a sharp rise in inspiratory pressure to the preset pressure limit. The inspiratory pressure is held constant by servo-control of the delivered flow and is terminated when a minimal flow is reached (usually < 25% of peak flow), just before spontaneous exhalation begins. Pressure-support ventilation depends entirely on the patient's effort, if the patient becomes apneic, the ventilator will not provide any mechanical breath. Pressure-support ventilation allows better synchrony between the patient and the ventilator than IMV, volume-assisted ventilation, or pressure control ventilation. Pressure-support allows ventilatory muscle loads to be returned gradually during the weaning process like IMV techniques. Since each breath is assisted, it alters the pressure volume relationship of the respiratory muscles in such a way so as to improve its efficiency. With ventilatory muscle fatigue, muscles can be slowly retrained and titrated more efficiently than IMV and thus, promote the weaning process. The emphasis with weaning with pressure support ventilation is endurance training of the respiratory muscles, especially, the diaphragm. The parameters that can be manipulated to titrate the muscle loading are the magnitude of the trigger threshold and the preset pressure limit. PEEP is provided to maintain FRC and prevent alveolar collapse. The amount of pressure-support to be provided depends on the clinical circumstance. A pressure-limit that delivers a VT of 10 to 12 ml/kg has been termed PSV max because at this level respiratory work can be reduced to zero. It is not necessary to provide PSV max at the beginning. The level of pressure support selected should allow for spontaneous respiration without undue exertion and still results in normal minute ventilation. No strict criteria can be established; it has to be applied and titrated on an individual basis. Weaning of pressure-support ventilation is accomplished by reducing the pressure-limit decrementally. Similar to weaning guidelines previously mentioned with each wean, the effect of weaning on muscle loading has to be evaluated clinically. Increase in respiratory rate is an early indication of increasing muscle load. Retraction and use of accessory muscles would indicate a more severe muscle load. If respiratory rate increases during the weaning process, the level of pressure-support should be increased until there is reduction in the respiratory rate. While this method of weaning is attractive theoretically, its benefit in the weaning process is yet to be established in infants and children. A relative contraindication to the use of pressure-support ventilation is a high baseline spontaneous respiratory rate. There is a finite lag time involved from the initiation of a breath to the 8sensing of this effort and from the sensing to the delivery of a mechanical breath. In infants breathing at a relatively fast rate (40 to 50 breaths/minute), this lag time may be too long and result in asynchrony between the patient and the ventilator. Pressure-support has been mainly used to wean adult patients off mechanical ventilation. Its use in pediatrics is gaining popularity. When used at our institution, we tend to keep a base line low SIMV rate (5 to 6 per min) along with pressure support before extubation.