Conway (1985) described the geometry of the Mapleson classification and the behaviour of the breathing systems under conditions of spontaneous and controlled respiration. The Mapleson A (Magill) system (Figure 48) has the fresh gas inlet remote from the subject while the expiratory valve is near the subject. Dead space is shown as the shaded area. This makes the system particularly economic to use during spontaneous respiration.
Figure 49 shows the hypothetical pressure and flow inside the Mapleson A system during expiration. Flow during expiration, which is usually a passive process, is maximal at the beginning of expiration, and it falls off exponen- tially. Pressure inside the system rises exponentially but at a faster rate than the expiratory flow falls because of accumulation of fresh gas in the system; this rise in pressure is cut off when the expiratory valve opens at 5–6 kPa and this pressure is then maintained until end-expiration. Areas under the flow curve give expired volumes (see the chapter on flow and volume measurement); dead space gas (VD), which is identical in composition to fresh gas is expired
first until point A on the graph. When the expiratory valve opens at point B, alveolar gas (VA) beyond that point is vented out. Furthermore, alveolar
gas that was deposited inside the system between points A and B (start of alveolar gas expiration and opening of expiratory valve) is after the opening of expiratory valve being pushed out by the fresh gas inflow (VF). If suffi-
cient time is allowed after the opening of the expiratory valve to allow venting of the alveolar gas deposited during the valve closure, at end-expiration the system will only contain fresh gas and dead space gas (identical in composi- tion). Thus, during spontaneous respiration, provided that fresh gas flow is
at least equal to alveolar ventilation, i.e. about 70% of the minute vol-
ume, there will be no rebreathing of CO2-containing gas. During controlled
ventilation the economy of gas flow is lost: positive pressure is applied
during inspiration and the pressure relief valve therefore opens, venting fresh gas; furthermore, the valve needs to be set to a higher pressure (tightened) and therefore during expiration it opens later, allowing retention of CO2-
containing gas inside the system. Higher flows are, therefore, needed during controlled ventilation. This disadvantage of the Magill system, and the large weight of the expiratory valve and scavenging close to the subject (i.e. to the mask or airway connection), caused a steady decline in the popularity of the Magill system. It is being increasingly replaced by the T-piece or circle absorption system.
1
The Mapleson A (Magill) breating systemFigure 48. The Mapleson A breathing system.
p P Ti Ti VD VA VF VE
1
Part
1
T-pieces
T-pieces (Figure 50) are geometrically opposite of the Magill system: the fresh gas flow inlet is near the subject while the expiratory valve or port is the furthest. E type is a valveless system suitable for spontaneously breathing subjects only. F type is a paediatric valveless system.
Figure 51 shows theoretical inspiratory gas flow during spontaneous
breathing in a T-piece system. The inspiratory waveform is a sine wave.
Integration of flow (area under the curve) yields volume inspired. This is again divided into gas that enters alveoli (VA) and dead space gas (VD). Super-
imposed on the graph is the fresh gas flow, which is constant. The area under the straight line of the fresh gas flow is the fresh gas volume delivered into the system. It can be seen that at point A the inspiratory flow exceeds the fresh gas flow. Until that point, excess fresh gas was being deposited in the tubing. After that point, inspiratory flow is supplied in part (the part exceeding fresh gas flow) by gas previously deposited in the tubing. Provided that the roughly triangular area between the fresh gas flow line, the inspiratory curve up to point A, and the y-axis, which is the volume deposited in the large bore
tubing, equals the area under the peak inspiratory flow above the fresh gas
flow line up to point B (which is the volume of gas drawn from the system
in excess of fresh gas flow during that time), no CO2-containing gas will
be rebreathed. These conditions will apply if fresh gas flow is approximately
twice the minute volume during spontaneous respiration.
During controlled ventilation, fresh gas flow mixes with expired gas as the peak ventilatory flows are higher; the inspirate will then contain CO2.
Under these conditions the magnitude of rebreathing will depend on fresh gas flow and minute ventilation: an increase in either of these will lower arterial CO2tension. In fact, arterial partial pressure of CO2becomes the product of
fresh gas flow and minute ventilation. To maintain the CO2level, an inverse
relationship must be maintained between fresh gas flow and minute volume. If minute ventilation is increased, fresh gas flow can be reduced; this provides economy of flow during controlled ventilation.
1
T-pieces D E F Figure 50. T-pieces. VD VF VE VA TimeFigure 51. Inspiratory flow and fresh gas flow in the Bain system. Reproduced with
1
Part
1