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however these have yet to be elucidated fully. 5.5 The coefficient of gas transport: DCO2 Whilst not currently a controlled ventilator parameter, the gas transport/diffusion coefficient (DCO2 ) is a vital, and often under-recognised variable in HFOV as a critical determinant of the efficiency of gas mixing and hence success of HFO ventilation. Removal of carbon dioxide in conventional ventilation is determined by the product of alveolar tidal volume and respiratory rate, known as alveolar minute volume, or alveolar minute ventilation. As alveolar tidal volume is difficult to measure, the minute volume (determined by the product of tidal volume and respiratory rate) is used instead. 18 I I 19 6.1 Pressure damping A key purported advantage of HFOV is the achievement of adequate gas exchange with small tidal volumes and low intrapulmonary pressure amplitudes. During HFOV, the amplitude of the pressure waveform is damped progressively from the airway opening to the alveoli (Figure 6-1). In contrast, the full inspiratory pressure is transmitted from the airway opening to the alveoli during conventional ventilation (provided a sufficiently long inspiratory time is allowed). For many years, clinicians assumed that the intrathoracic damping of pressure amplitudes in HFOV occurred regardless of the patient’s condition, and that the babies were therefore protected from barotrauma. However, systematic reviews of HFOV versus conventional rate ventilation show increased pulmonary air leak for infants (13). Although at first this result seems surprising, an understanding of how mechanical factors influence the extent to which the pressure waveform is damped will help the clinician to use HFOV without causing barotrauma. Whereas tidal volumes are inevitably lower during HFOV than conventional ventilation, research has shown that these low tidal volumes are not always associated with similarly low oscillatory pressure amplitudes. 6.2 Pressure amplitude transmission and respiratory mechanics Damping of the oscillatory pressure amplitude as it traverses the airways to the alveoli is determined by the impedance to flow of the intubated respiratory system. 6 Understanding transmission of pressure amplitude Several studies show that the relationship between carbon dioxide removal and the factors tidal volume and respiratory rate changes as respiratory frequency increases. Above 5 Hz in particular, there is a transition such that CO2 elimination becomes approximately correlated to ƒ* VT 2 (9, 10). The ƒ* VT 2 product is commonly referred to as the CO2 diffusion (gas transport) coefficient, DCO2 . DCO2 is generally measured and displayed by oscillatory ventilators using flow and tidal volume monitoring. An increase in DCO2 decreases PaCO2 . As DCO2 is only linearly related to frequency, but increases exponentially with increases in tidal volume, improved CO2 removal is achieved most efficiently by increasing tidal volume. For example, a 10 % and 20 % increase in tidal volume increase DCO2 by 21 % and 44 % respectively. In contrast, changes in DCO2 with changes in frequency are dependent on whether HFOV is used with or without Volume Guarantee: if HFOV is used without Volume Guarantee, increase in frequency may be offset by a decrease in tidal volume, resulting in a decrease in DCO2 unless ΔP is increased simultaneously; however, in the setting of Volume Guarantee, an increase in frequency will increase DCO2 , as the set tidal volume will be maintained as long as the oscillatory pressure amplitude required to achieve that tidal volume is achieved. It is important to recognise that DCO2 is an absolute value dependent on the tidal volume. As tidal volume requirements increase with increasing size of the patient, DCO2 will similarly increase, but in an exponential fashion. However, DCO2 calculated relative to the square of an infant’s body weight (i.e. f * VT 2 /kg2 ) is a more reliable variable to predict the absolute DCO2 required for any given patient (see section 9). Impedance Impedance is a general mechanical term that describes the mechanical barrier to flow. Impedance incorporates the volume dependent (elastic), flow-dependent (resistive) and acceleration dependent (inertive) properties