Fig 1.
Risk of rebreathing with traditional bubble CPAP, the original Pumani and the revised Pumani CPAP.
The conventional bubble CPAP (top) has the submersion bottle located on the expiratory limb. The fresh gas flow is equal to the set flow and dead-space (brown) is small (with sufficient fresh gas flow). The original Pumani (middle) had a risk for total rebreathing when there was no leakage at the patient interface. The revised Pumani (bottom) has a bleed port to ensure leakage and fresh gas flow. The illustration of the revised Pumani has been simplified by removing the narrow parts of the tubing and the bleed valve between the Y-piece and the bottle (a detailed illustration provided as Fig 2).
Fig 2.
Design and dimensions of the revised Pumani.
Fresh gas flow is supplied to the breathing circuit at the bifurcation. The patient is connected to the prongs and gas can leave the system through leakage at the patient, the two bleed ports and at the bubble bottle (right side). Internal diameters (ID) were estimated by the authors. Blue circle 1–6 indicate cutpoints used to determine tube resistance. A simplified illustration of the revised Pumani components is provided in Fig 1 (bottom).
Fig 3.
Fresh gas flow and generated CPAP.
An increase in fresh gas flow resulted in higher CPAP. The conventional bubble CPAP systems were less sensitive to adjustments in fresh gas flow than the revised Pumani.
Fig 4.
Pumani bleed port flow at different levels of fresh gas flow (left) and CPAP (right). The bleed port flow exceeds 1 L/min at fresh gas flows of approximately 2 L/min or CPAP of 1 cm H2O. The revised Pumani is equivalent to a ‘Mapleson A’ system and a bleed port flow higher than the minute ventilation will prevent re-breathing even if there is no leakage at the interface or through the mouth.
Fig 5.
Resistance of tubing and connectors.
The resistance for the revised Pumani system was comparable to an uncut endotracheal tube size 3. The resistance was reduced as parts of the Pumani tubing were disconnected. Removing the bottle and attached tubing have small effects on resistance (curves cutpoint 1 to 3 close to identical, Fig 2). A main component in the resistance was from the driver (driver removed in cutpoint 4 to 6, Fig 2) but all connectors (to the prongs) had to be removed before resistance was lower than for the two conventional bubble CPAP systems. The resistance was measured without CPAP (no submersion or fresh gas flow). Negative flows correspond to patient inspiration.
Fig 6.
Resistance for each system with increased submersion depths and a fresh gas flow of 10 L/min.
There is an increase in pressure during expiration and a decrease during inspiration. The revised Pumani has higher resistance (steeper slope) than conventional bubble CPAP systems. Simulations without submersion were included to illustrate resistance from the connectors, tubing and prongs. Negative flows correspond to patient inspiration.
Fig 7.
Resistance for each system with increasing fresh gas flows and a submersion depth of 5.0 cm.
Due to the system resistance, with increasing fresh gas flows, CPAP (pressure at 0 L/min of simulated airway flow) increased slightly in the Diamedica and the Fisher & Paykel and more pronounced in the revised Pumani. For all systems the resistance (slope) was similar at different levels of fresh gas flows. An uncut endotracheal tube size 3 (no fresh gas flow) was included for comparison. Negative flows correspond to patient inspiration.
Fig 8.
Imposed work of breathing at increasing CPAP levels.
The Pumani system was tested with a constant fresh gas flow (6, 8 or 10 L/min) and increasing submersion depths from 0 to 8.5 cm. This increase resulted in CPAP rising with approximately 4 cm H2O. The imposed work of breathing was higher than for the other CPAP systems. The Neopuff T-piece resuscitation system was included for comparison (tested without interface). Simulations were performed with a 32 mL symmetrical and sinusoidal flow pattern (I:E 1:1, flow maximum 6 LPM, respiratory rate 60).