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Intensive Care 69

The two bags commonly used in bag-mask valve ventilation When making adjustments to the mode and frequency of mechanical
include the self-inflating bag and the standard anaesthetic circuit. ventilation, effective oxygenation is determined by manipulations of
The self-inflating bag consists of a bag, oxygen inlet, connector for inspired oxygen (FiO ) and mean airway pressure (MAP). The factors
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the face mask or tracheal tube, pressure relief valve, and a reservoir. that most reliably influence MAP are the amount of set PEEP and
The self-inflating bag is relatively easy to use and more available, the inspiratory time. To optimise ventilation and thus carbon dioxide
and when used with the reservoir, it can provide near 100% oxygen. clearance, minute volume should be increased by increasing the peak
It can provide emergency ventilation without a fresh gas source inspiratory pressure as well as the ventilator rate. Note, however,
because the gas movement generated by bag inflation will inflate the that changes in mechanical ventilation to achieve improvements in
chest with room air, even without an external gas source. Because the oxygenation and ventilation have an impact on each other as well
valve mechanism opens only in response to manual bag inflation, the as on other organ systems, most notably the cardiovascular system.
self-inflating bag is not appropriate to deliver oxygen or continuous Increasing mean airway pressures, for example, may potentially impede
positive pressure in the spontaneously breathing child. The bag in a venous return and thus negatively affect cardiac output. With any
standard anaesthetic circuit, however, requires a constant supply of adjustment, the clinician should establish whether a positive change has
fresh gas in order for it to fill. The bag must therefore be connected been effected with the fewest possible negative clinical consequences.
to a fresh gas supply to inflate the lungs via either a face mask or The inability to achieve effective oxygenation and ventilation
tracheal tube. The advantage of the standard bag over the self-inflating by using conventional positive pressure ventilation may suggest
bag is the ability to deliver fresh gas and continuous positive pressure the need for nonconventional modalities of ventilator support,
to the spontaneously breathing child and to control the pressures such as high-frequency oscillatory ventilation, high-frequency jet
administered with each breath. The system can be difficult to use, ventilation, or extracorporeal membrane oxygenation (ECMO).
however, in all but experienced anaesthetic hands. Generally, these therapies are available mainly as rescue therapy in
Mechanical ventilation intensive care centres.
Mechanical ventilation provides a way of supporting the respiratory Circulation
system while waiting for the natural history of the pathological pro- Many children with severe disease require cardiovascular monitoring
cess to improve or for specific treatment to be effective. The goals and support. Circulatory compromise frequently accompanies critical
of mechanical ventilation are to ensure adequate oxygen delivery, illness and may be either a primary cause or be secondary to the pres-
decrease the work of breathing, and ensure adequate elimination of ence of untreated respiratory failure and hypoxia. Early recognition and
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carbon dioxide. Mechanical ventilation is generally provided by using intervention is therefore essential to prevent further progression to cir-
positive pressure-ventilated breaths superimposed on a background of culatory collapse and death. When the cardiovascular system is unable
positive end expiratory pressure (PEEP) to maintain alveolar patency to provide adequate perfusion of end organs to supply adequate oxygen
during expiration. PEEP can stabilise alveoli, decrease ventilation- and nutrients to cells, the situation is referred to as shock. Table 12.2
to-perfusion (V:Q) mismatch, and reduce the alveolar shear injury shows a common scheme for the classification of shock.
incurred through repetitive inflation with positive pressure—the so- Table 12.2: Classification of shock.
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called “ventilator-induced lung injury”. Excessive PEEP, however,
may result in overdistention of recruited alveoli and have a negative Shock classification Aetiology
impact on cardiac output. Hypovolaemic Haemorrhage
Diarrhoea and vomiting
The goals of mechanical ventilation are to optimise alveolar
Burns
ventilation, maximise V:Q matching, decrease the work of breathing, Peritonitis
and minimise the risk of ventilator-associated injury.
Distributive Sepsis
The usual starting point when calculating the initial inflation Anaphylaxis
pressure requirement is a simple visual assessment of the amount of Vasodilating drugs
Spinal cord injuries
pressure required to move the chest. Adjustments can then be made
according to the clinical situation and oxygen requirement, and, if Cardiogenic Arrhythmias
available, as dictated by blood gas analysis. If the bedside ventilator Cardiomyopathy
Myocardial infarction or contusion
system enables tidal volume calculation, the ventilator pressure should Congenital structural heart disease
be adjusted with the aim of delivering a tidal volume of 5–10 ml/ Cardiac tamponade
kg. This will vary with chest compliance, but it often requires peak Obstructive Tension haemo/pneumothorax
inspiratory pressures in the range of 20–25 cm H O, with a PEEP of Flail chest
2
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3–5 cm H O. Higher PEEP may be required to achieve adequate Pulmonary embolism
2
oxygenation when extensive airspace disease or pulmonary oedema Dissociative Anaemia
Carbon monoxide poisoning
is present. Wherever possible, avoid excessive inflation pressures to
Methaemoglobinaemia
avoid ventilator-associated lung disease, which has been associated
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with high inspiratory pressures. Tidal volumes of 6 ml/kg and
limiting the peak inspiratory pressure to less than 32 cm H O has In evaluating a child with signs of shock, the earliest and most
2
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demonstrated a significant reduction in mortality. In setting the rate, sensitive—but not exclusively reliable—sign is tachycardia. This may
both the inspiratory and expiratory times—that is, the proportion of the also be caused by pain, anxiety, fever, or medications, but these causes
respiratory cycle occupied by inspiration and expiration, respectively— are often easily excluded. The presence of additional significant signs
can be adjusted. The inspiratory time is usually decided based on the consistent with inadequate blood supply to end organs, such as altered
age, size, and disease process of the patient. As a guide, inspiratory mental state, poor peripheral skin perfusion due to vasoconstriction,
times increase from approximately 0.5 seconds in a neonate to 1 thready rapid pulses that may be difficult to palpate, and decreased
second in children older than 5 years of age. A useful starting point is urine output as a result of poor organ perfusion, help to establish a
to simply start with a ventilator rate of 20 breaths per minute and adjust diagnosis of shock. Vasodilatation as a sign of shock is less common
as necessary. Small infants and neonates may require a higher starting in children as compared to adults. Metabolic acidosis commonly
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rate, in the range of 40–60 breaths per minute. accompanies suboptimal perfusion due to tissue anaerobic metabolism,