Acid–base values change slightly from the moment of birth until adulthood (Table 9.1). The first hour after birth is characterized by a mixed (predominantly respiratory) acidosis which steadily normalizes in the normal neonate. Preterm neonates may have a lower HCO₃ and lower PaCO₂ for the first month to 6 weeks of life.

Table 9.1 Acid–base normal values from birth to adulthood

Acid–base diagram

Figure 9.1 can be used to elucidate the nature of an acid–base disturbance by plotting pH against PCO₂ or HCO₃.

Fig 9.1 An example of an acid–base chart.

Universal acid–base formula

There is also a ‘rule-of-thumb’ formula that may be used to rapidly assess acid–base status:

Where Δ = change, ≈ = approximately equal to, PaCO₂ = arterial partial pressure of carbon dioxide (mmHg), HCO₃ = serum bicarbonate (mmol/L) and K⁺ = serum potassium (mmol/L).

This predicts that a change in pH of 0.1 may be caused by a change in PaCO₂ of 12 mmHg or a change in HCO₃ of 6 mmol/L. Similarly, changes in PaCO₂ or HCO₃ of these amounts will be required to compensate for a change in pH. Potassium will also rise by 0.3 to 0.5 mmol/L for each 0.1 fall in pH and vice versa.

There are more specific equations to predict the expected values for compensation in acid–base derangement (Box 9.1).

Box 9.1

Equations to predict compensation in acid–base derangements

Respiratory compensation for metabolic acidosis:

Respiratory compensation for metabolic alkalosis:

Respiratory compensation – simplified formula:

Metabolic compensation for respiratory acidosis:

Metabolic compensation for respiratory alkalosis:

PaCO₂ = arterial partial pressure of carbon dioxide (mmHg); Actual HCO₃ = actual measured bicarbonate (mmol/L). *Or use age-specific normal values of PaCO₂ for a more precise calculation (see table 9.1); ^†or use age-specific normal values of HCO₃ for a more precise calculation (see table 9.1).

Electrolytes and fluids

Anion gap

Where AG = anion gap (mmol/L), Na⁺ = serum sodium (mmol/L), HCO₃⁻ = serum bicarbonate (mmol/L) and Cl⁻ = serum chloride (mmol/L).

The upper limit of normal for the anion gap is 12 mmol/L. Some formulas include K⁺ on the cationic side of the equation, in which case the upper limit of normal is slightly higher (16 mmol/L).

Sodium deficit in hyponatraemia

Where NS = volume (mL) of 0.9% normal saline to administer to correct Na⁺ deficit, wt = weight (kg), desired Na⁺ = the minimum level required (normally 130 mmol/L), and rate of administration = (mL/hr) based on a maximum infusion of 0.6 mmol/kg/hr.

Parkland formula

Where fluid requirement (mL) = the total fluid volume to infuse (over and above normal maintenance fluids): half in the first 8 hours from the time of the burn and the second half in the subsequent 16 hours; wt = weight in kg and %BSA = burn surface area as a percentage of total body surface area (see Fig. 1.7).

Respiratory system

Acute lung injury ratio

Where PaO₂ = partial pressure of oxygen (mmHg) and F_iO₂ = fraction of inspired oxygen (in decimal form, e.g. 0.4).

This is also known as the P/F ratio. A value of <300 indicates the presence of acute lung injury and a value of <200 the presence of acute respiratory distress syndrome (ARDS).

Oxygen index

Where F_iO₂ = fraction of inspired oxygen (in percentage form, e.g. 40%), MAP = mean airway pressure (mmHg) and PaO₂ = partial pressure of oxygen (mmHg).

An oxygen index >40 suggests the need for extracorporeal membrane oxygenation techniques in appropriately selected neonates.

Estimated minute ventilation requirements

This formula allows for the estimation of ventilation requirements of the critically ill child (based on metabolic factors) to maintain a normal PaCO₂:

Where MV = minute ventilation (L/min), wt = weight (kg) and RR = ventilator respiratory rate.

This formula allows for the determination of the estimated required ventilation rate for children with a tidal volume of 6 mL/kg, based on expected metabolic requirements.

Predicted peak expiratory flow rate

Where PEFR = peak expiratory flow rate (L/min) and ht = height (cm).

This formula approximates the 50th centile for peak expiratory flow rate in children taller than 100 cm. More precise values should be obtained from tables or electronic applications as soon as time permits (Table 9.2).

Table 9.2 Peak expiratory flow rate normal values for stature (or length) from 85 to 160 cm

Endotracheal tube size and length

Where ETT = endotracheal tube internal diameter (mm) and Age = age in years. For infants less than a year use a 3.5 mm ETT; for children between 1 and 2 years of age a 4 mm ETT; after 2 years use the formula above.

For infants less than a year use 3.0 mm ETT; for children between 1 and 2 years of age a 3.5 mm ETT; after 2 years use the formula above.

Nervous system

Pain scoring systems are an essential component of analgesic practice in the ED. These two systems have been used across the world and can be simply and quickly applied. Although they have been used mostly in the setting of chronic pain, they have been validated in the ED.

Wong Baker faces

Pain in children is often underestimated by parents and doctors alike. This system (Fig. 9.3) can be used by asking children to point out the face that best represents how they are feeling. A decrease in the indicated pain will provide proof of analgesic efficacy or the need for additional analgesia.

Fig 9.3 The standardized Wong Baker faces which can be used for self-assessment of acute pain by children in the ED.

Behavioural observational pain scale (BOPS)

Younger children and infants cannot self-report pain effectively. There are several observational pain systems, of which the BOPS is an example (Table 9.3). A pain scale should be used to obtain an indication of pain or discomfort in young children and the need for analgesia.

Table 9.3 The behavioural observational pain score (BOPS)

Sedation scores

Procedural sedation is a core skill for ED practice and patient safety is of paramount importance in its performance. Although dissociative agents are most frequently used in children (for which there is no sedation scale) other agents need to be used to target a particular level of sedation. The Ramsay (Table 9.4) and American Society of Anaesthesiologists (Table 9.5) sedation scales are useful to evaluate and document sedation levels.

Table 9.4 The Ramsay sedation scale

Score	Response
1	Anxious or restless or both
2	Cooperative, orientated and tranquil
3	Responding to commands
4	Brisk response to stimulus
5	Sluggish response to stimulus
6	No response to stimulus

Table 9.5 The ASA sedation scale

Level of sedation	Definition
Minimal sedation (anxiolysis)	• Drug-induced state • Patients respond normally to verbal commands • Cognitive function and co-ordination may be impaired • Ventilatory and cardiovascular functions are unaffected

Moderate sedation/analgesia

• Drug-induced depression of consciousness

• Patients respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation

• No interventions are required to maintain a patent airway

• Spontaneous ventilation is adequate

• Cardiovascular function is usually maintained

Deep sedation/analgesia

• Drug-induced depression of consciousness

• Patients cannot be easily aroused

• Patients respond purposefully following repeated or painful stimulation

• The ability to independently maintain ventilatory function may be impaired

• Patients may require assistance in maintaining a patent airway

• Spontaneous ventilation may be inadequate

• Cardiovascular function is usually maintained

General anaesthesia

• Drug-induced loss of consciousness

• Patients are not arousable even by mild painful stimulation

• Ability to independently maintain ventilatory function is often impaired

• Patients often require assistance to maintain a patent airway

• Positive pressure ventilation may be required due to depressed spontaneous ventilation and drug-induced depression of neuromuscular function

• Cardiovascular function may be impaired

Paediatric Glasgow Coma Scale

The adult Glasgow Coma Scale (GCS) has been modified for application to infants and children (Table 9.6).

Table 9.6 The Glasgow Coma Scale as modified for children and infants

Spinal nerve root assessment

Accurate evaluation of spinal cord function or injury is essential after trauma in children. Sensory and motor maps (Table 9.7) allow for determination of the level of spinal cord injury as well as the exclusion of spinal cord injury by examining the function of the spinal nerve roots.

Table 9.7 The sensory and motor pathways to map out the testing of spinal nerve roots

Dermatome map (sensory function)	Myotome map (motor function)
C5 – area over the deltoid muscle C6 – thumb C7 – middle finger C8 – little finger T1 – medial aspect of forearm T4 – nipple line (remember the cervical cape of C4) T8 – level of xiphisternum T10 – level of umbilicus T12 – level of pubic symphysis L4 – medial aspect of calf L5 – webspace between 1st and 2nd toes S1 – lateral border of foot S3 – skin overlying ischial tuberosity S4 and S5 – perianal area

Dermatome map (sensory function)

Myotome map (motor function)

Body composition

Body mass index

Where wt = weight (kg) and ht = height (m).

Body surface area

Both Mosteller’s formula and Haycock’s formula may be used for children.

Where BSA = body surface area (m²), wt = weight (kg) and ht = height (cm).

Boyd’s formula may be used to estimate body surface area by using weight alone, which is useful if height is not known or cannot be measured. This formula is accurate to within about 10% of BSA estimations by Mosteller’s formula. (Note that weight must be entered as grams (kg × 1000) and the resultant BSA is reflected in cm² (m² × 10 000).)

Ideal body weight

Where Age_mo = age (months), and Age_yr = age (years).

Adjusted body weight

Where ABW = adjusted body weight (kg), IBW – ideal body weight (kg) and TBW – total body weight (kg).

Catzel’s percentage method of drug dose calculation

There are many ways to calculate drug doses in children, the most accurate of which are length-based tape systems. Table 9.8 represents another method of estimating drug doses which can be used if a more accurate system is not available.

Table 9.8 Catzel’s percentage method of drug dose estimation

Conversions

Weight, length and temperature

Table 9.9 provides the conversion factors for commonly used metric and imperial units of measurement.

Table 9.9 Weight, length and temperature conversion factors

Weight	Length	Temperature
1 kg = 2.2 pounds 1 pound = 0.45 kg

C – degrees Centigrade

F – degrees Fahrenheit

Measurements of pressure

In Table 9.10, find the row of the unit that you wish to convert from and follow it across to the unit that you wish to convert to. Multiply by that factor to convert to the new unit.

Table 9.10 Conversion factors for commonly encountered units of pressure measurement

Laboratory test units

See Table 9.11 for conversion factors applicable to lab results.

Table 9.11 Conversion factors for SI and non-metric units of measurement for laboratory investigations commonly used in the ED

Laboratory units
Bilirubin	1 µmol/L = 0.058 mg/dL	1 mg/dL = 17.2 µmol/L
Calcium	1 mmol/L = 4 mg/dL	1 mg/dL = 0.25 mmol/L
Cholesterol	1 mmol/L = 38.61 mg/dL	1 mg/dL = 0.0259 mmol/L
Creatinine	1 µmol/L = 0.013 mg/dL	1 mg/dL = 76.9 µmol/L
Glucose	1 mmol/L = 18.02 mg/dL	1 mg/dL = 0.055 mmol/L
Lactate	1 mmol/L = 9.01 mg/dL	1 mg/dL = 0.111 mmol/L
Magnesium	1 mmol/L = 2.43 mg/dL	1 mg/dL = 0.412 mmol/L
Phosphate	1 mmol/L = 3.1 mg/dL	1 mg/dL = 0.323 mmol/L
Urea	1 mmol/L = BUN 2.8 mg/dL	1 mg/dL BUN = 0.357 mmol/L

Steroid equivalence

The potency of systemic steroids is represented by a comparison to intravenous hydrocortisone. In general, the choice of steroids is related to the specific clinical condition but alternatives may be used in equipotent doses.

• Hydrocortisone – short acting with a relative potency of 1.

• Prednisone or prednisolone – intermediate acting with a relative potency of 4.

• Dexamethasone – long acting with a relative potency of 25.

• Betamethasone – long acting with a relative potency of 30.

• Triamcinolone – intermediate acting with a relative potency of 5.

Opioid equivalence

The strength of intravenous opioid analgesics is generally related to the potency of a 10 mg dose of morphine. These figures are not precise but do offer an indication of relative dosing requirements.

• Morphine 10 mg – duration of action 3 to 4 hours.

• Diamorphine 5 mg – duration of action 3 to 4 hours.

• Fentanyl 0.1 mg – duration of action 1 hour.

• Alfentanil 1 mg – duration of action 20 minutes.

• Tramadol 100 mg – duration of action 4 to 6 hours.

• Pethidine (meperidine) 75 mg – duration of action 2 to 3 hours.

Opioid doses need to be individualized in all patients, but especially children. Infants less than 6 months metabolize opioids slowly and are at greater risk for side effects from repeated doses or infusions than older children. Doses need to be a bit higher in children from 2 to 6 years of age, when compared to adults, because of their higher metabolic rates.

Miscellaneous

Insulin infusion

An insulin infusion at 0.1 U/kg/hr is, in general, preferable to an insulin sliding scale. There are times when a sliding scale is required and Table 9.12 is an example of one such scale.

Table 9.12 A weight-based insulin sliding scale for children and infants

Paediatric trauma score

Early objective assessment of the severity of injury (see Table 9.13) is an essential component of managing the paediatric trauma victim in the ED. Severely injured children should ideally be treated at a facility that specializes in treating children as outcomes are better.

Table 9.13 The paediatric trauma score

APGAR score

The APGAR score (Table 9.14) is useful, and of particular relevance, for neonates that are born in the ED as unplanned or unexpected deliveries.

Table 9.14 The APGAR score

Lund and Browder Chart

The Lund and Browder chart (Fig. 9.4) is used to accurately determine burn surface area in infants and children of all ages. The relative contribution of head and lower limbs to total body surface area changes with increasing age, as is shown on the chart. The total burn surface area is obtained from the sum of each component area affected by the burn.

Fig 9.4 The Lund and Browder chart.

The PAWPER tape values

From Table 9.15 one can estimate the child’s weight, as long as the child’s length can be measured with a regular tape measure. See Chapter 4 for how to estimate the Habitus Score.

Table 9.15 The base measurements and estimated weight from which the PAWPER tape is derived

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