The delivery of a precise and safe mixture of oxygen and anesthetic gases to a pediatric patient, followed by the removal of carbon dioxide (CO2), presents unique challenges. Important issues to consider are the size of the child (from a 1000-g newborn to a 100-kg adolescent), the means of providing humidity and preventing heat loss, apparatus dead space, resistance, and work of breathing, and preventing rebreathing. Since 1998, the biggest advance in pediatric anesthesia has been the replacement of Mapleson and traditional circle breathing systems with the new anesthesia workstations. The modern anesthesia machine is able to precisely deliver small tidal volumes (TVs) accurately by compensating for breathing circuit compliance and changes in fresh gas flow with ventilators that can precisely deliver small volumes at high rates.
BREATHING CIRCUITS: PERFORMANCE CHARACTERISTICS
FGF becomes very important to pediatric patients where inhalation induction is used most commonly. Concentrations of oxygen and inhalation anesthetics can vary markedly between fresh gas at the inlet and inspired gas in the breathing hose. The lower the FGF, the greater the discrepancy. Higher flows speed induction and recovery, compensate for leaks in the circuit, and decrease the risks of unanticipated gas mixtures. Inspired oxygen and agent concentration monitors are used to assist in proper delivery of oxygen during low-flow anesthesia technique and assuring proper depth of anesthesia during rapid changes in FGF.
In the circle systems, one has to consider loss of TV from compliance of the distensible breathing circuit and gas compression in the circuit (mechanical dead space). The elbow, heat and moisture exchanger (HME), and the D-lite sensor contribute to real apparatus dead space where part of TV does not participate in gas exchange. Increasing dead space increases rebreathing of CO2. To avoid hypercarbia in the face of an acute increase in dead space, minute ventilation needs to be increased. Pediatric breathing circuits have smaller hoses, Y-connections, HMEs, and D-lite sensors. However, despite of smaller connections, dead space can still play a significant role in a small neonate with lung disease and with decreased lung compliance. Adjustments need to be made to remove all nonessential sensors, HMEs, and elbow connectors.
Laminar flow through tubes (blood vessels, endotracheal tube [ETT]) is directly proportional to the pressure gradient (P) and fourth power of the radius and inversely proportional to viscosity and length (Poiseuille’s law). Resistance is always high with turbulent flow, which can be caused by narrow diameter tubings and orifices, sharp bends, increasing circuit length, and valves in the apparatus. Circle system resistance is increased by unidirectional valves, the carbon dioxide absorber, and high minute ventilation. However, even the tiniest of premature infants can be successfully ventilated using the new anesthesia workstation circle systems.