Tracheobronchial consequences of the use of heat and moisture exchangers in dogs

0 From the Departments of Anesthesiology 1 Otorhynolaringology, Ophthalmology and Head and Neck Surgery 2 " and Morphology, ~ College of Medicine , UNESP, Botucatu, S~o Paulo, Brazil . MG - Brazil, CEP 38010-350 Purpose: To determine the effect of heat and moisture exchange(HME) on the tracheobronchial tree (TBT) using a unidirectional anesthesiccircuit with or without CO 2 absorber and high or low fresh gas flow (FGF), in dogs. Methods: Thirty-two dogs were randomly allocated to four groups: G I (n=8) valvular circuit without CO 2 absorber and high FGF (5 L'min-~); G2 (n=8) as G I with HME; G3 (n=8) circuit with CO 2 absorber with a low FGF (I L.min-~); G4 (n=8) as G3 with HME. Anesthesia was induced and maintained with pentobarbital. Tympanic temperature CI-I-),inhaled gas temperature (IGT), relative (RH) and absolute humidity (AH) of inhaled gas were measured at 15 (control), 60, 120 and 180 rain of controlled ventilation. Dogs were euthanized and biopsies in the areas of TBT were performed by scanningelectron microscopy. Results: The G2 and G4 groups showed the highestAH (> 20 mgH20' L-I) and G I the lowest (< 10 mgH20' L-j) and G3 was intermediate (< 20 mgH20-L-~) (P < 0.01). There was no difference of-l-i- and IGT among groups. Alterations of the mucociliary system were greatest in G I, leastin G2 and G4, and intermediate in G3. Conclusion: In dogs, introduction of HME to a unidirectional anesthetic circuit with/without CO 2 absorber and high or low FGF preserved humidity of inspired gases. HME attenuated but did not prevent alterations of the mucociliary system of the TBT. Objecdf: D&erminer, chez des chiens, I'effet de I'&hange de chaleur et d'humidit~ (ECH) sur I'arbre trach~obronchique (ATB) en utilisant un circuit anesth&ique unidirectionnel avec ou sansabsorption de CO 2 et un haut ou bas d~bit de gaz frais (DGF). M6thode : Trente-deux chiens ont &6 r~partis au hasard en quatre groupes : G I (n=8), un circuit & valve sans absorption de CO z et avec un haut DGF (5 L'min-I); G2 (n=8), comme G I avec ECH; G3 (n=8), un circuit avec absorption de COzet un bas DGF (I L.min-I); G4 (n=8), comme G3 avec ECH. L'anesth&ie a ~t~ induite et maintenue avecdu pentobarbital. La temp&ature tympanique Cl-l-), latemp&ature des gaz inhal& (TGI), rhumidit~ relative (HR) et absolue (HA) des gaz inhal& ont ~t~ mesur~es apr& 15 (t~moin), 60, 120 et 180 min de ventilation contr61~e. Les chiens ont ~t~ sacrifi& et des biopsies de I'ATB ont ~t~ r~alis&s par microscopie ~lectronique ~ balayage. REsultats : Les groupes G2 et G4 ont affich~ les plus hauts taux d'HA (>20 mgH20L-~ ), G I avait le plus bas taux (< 10 mgH20.L-I ) et G3 ~tait interm~diaire (< 20 mgH20-L-I ) (P < 0,01). II n'y a pas eu de diff&ence intergroupe concernant la -IF et la TGI. Les modifications du syst~me mucociliaire ont &~ plus importantes dans le G I, moindres dans les G2 et G4 et interm~diaires darts le G3. Conclusion : IJintroduction, chez des chiens, d'un ECH ~ un circuit anesth&ique avec ou sans absorption de CO 2 et avec un haut ou bas DGF a pr~serv6 I'humidit~ des gaz inhal&. I'ECH a diminu~, mais n'a pas emp&h~, les modifications du syst~me mucociliaire de I'ATB. Tracheobronchial consequences o f the use o f heat and moisture exchangers in d o g s U R I N G respiration, inhaled air is heated and humidified when passing through the nose and upper airway, reaching the alveoli at body temperature (37~ with 100% relative humidity and approximately 44 mgH20.L-I absolute humidity. During expiration, heat and moisture are conserved by the upper airway and nose to minimize these losses from the lower airways. ~; When tracheal intubation or tracheostomy is performed, this counter-current mechanism is partly bypassed and ventilation with dry (i.e. < 5% relative humidity) and cold medical gases produces loss o f water from the respiratory tract, unless appropriate means of humidification and heating are used. 23 Failure to achieve and maintain adequate humidification may predispose patients to airway damage, such as destruction o f cilia and mucus glands, decrease in surfactant and ciliary transport, s-~ During anesthesia, ventilator systems are frequently equipped with circle systems with carbon dioxide absorbers, that have inherent humidifying properties as a result o f rebreathing o f expired humidity and o f the production o f water in the CO 2 absorber, but data concerning the level o f absolute humidity from studies with rebreathing system are contradictory. 9,x~ Heat and moisture exchangers ( H M E ) placed between the endotracheal tube and the ventilation circuit conserve heat and moisture during expiration, mixing expired air with the cool, dry gases inspired by the patient, l~ The aim of the present investigation, in dogs, was to determine the effect o f H M E on the tracheobronchial tree assessed by scanning electron microscopy, using a unidirectional anesthetic circuit with or without CO z absorber and high or low fresh gas flow. Methods The study was approved by the University Ethics Commission for Research in Animals. Thirty-two mongrel dogs, o f both sexes, weighing 15 - 20 kg were used. They were considered healthy after clinical examination and normal red blood cell count. Before induction o f anesthesia, the dogs were randomly allocated, by opening a sealed envelope, to one of four groups, as follows: G1 (n=8) a unidirectional anesthesia system without CO 2 absorber with a high fresh gas flow of 5 L-min-l (2 L.min-l O 2 and 3 L.min-1 air); G2 (n=8) as G1 with an H M E ; G3 (n=8) a circuit with CO 2 absorber was used with a low fresh gas flow of I L.min -l (0.4 L.min-1 O 2 and 0.6 L.min -1 air); G4 (n=8) as G3 with an HME. In G2, the hydrophobic H M E (Hygrobac S, from DAR, Italy) was placed between the tracheal tube and the unidirectional valve, and in G4, between the tracheal tube and the Y piece o f the breathing system. After starvation of 12 hr, a 20 G catheter was placed in the cephalic vein. The dogs received an initial dose o f 25 mg.kg-1 pentobarbital, followed by an intravenous infusion maintenance at 5 mg.kg-l.hr -1, using an infusion pump (Anne, Abbott, Chicago , USA). The dogs were placed in dorsal recumbency and the tracheas intubated with a tracheal tube 8.5 - 9.0 mm internal diameter. A 20 G catheter was placed in the right femoral artery for blood pressure measurement (AS 3, Datex Engstrom, Helsinki, Finland). An 18 G catheter was placed in the right jugular vein for infusion of S ml.kg-l-hr-I lactated Ringer's solution. The dogs were maintained under controlled ventilation according to the group, using a pneumatic ventilator, generating a continuous input in the inspiratory phase with pressure cycling from inspiratory to expiratory, and volume limited (K.Takaoka, model Nikkei, S~o Paulo, Brazil), converted to lowflow anesthesia. In groups G3 and G4, the capacity o f the circuit, including the CO 2 absorber, was about 5.12 L, the volume o f the soda lime in the absorbe (...truncated)