Respiratory compensation during spontaneous ventilation with the Bain circuit

The volume of carbon dioxide rebreathed by spontaneously breathing patients under halothane anaesthesia at various fresh gas flow rates (FGF) with the Bain modification of the Mapleson "D" breathing circuit is measured. The effect of rebreathing on a heterogeneous patient population is shown to be unpredictable hypercapnia in those patients who cannot respond adequately to this carbon dioxide challenge. All adults rebreathe significant volumes of carbon dioxide at a FGF rate of 100 ml . kg-1 . min-1. This carbon dioxide load is a potential risk to every patient and this hypercapnia is preventable by using high FGF rates. Rebreathing occurs because the inspired carbon dioxide load is unpredictable in a given patient and the patient's response is uncontrolled. Patients respond to this carbon dioxide challenge by increasing inspiratory flow rate (Vt/Ti), which results in increased rebreathing of carbon dioxide from the expiratory limb of the circuit. To prevent potentially dangerous rebreathing of carbon dioxide in all patients the fresh gas flow rate must be much higher than presently recommended.     

Combining low inspired oxygen and carbon dioxide during mechanical ventilation for the Norwood procedure

BACKGROUND: Staged reconstruction has become the preferred approach for treating neonates with hypoplastic left heart syndrome (HLHS). The haemodynamic instability of a single ventricle providing blood flow in parallel to the systemic and the pulmonary circulation, combined with the effects of cardiopulmonary bypass (CPB), results in precarious perioperative conditions. The two ventilatory manoeuvres commonly used for increasing pulmonary vascular resistance are (i) decreasing the fraction of inspired oxygen to < 0.21 and (ii) adding carbon dioxide (CO2) to the ventilatory circuit. Whether molecular nitrogen (N2) or CO2 is used in these situations is a matter of physician and institutional preference. The effect of the two modalities in combination has not been studied in depth. METHODS: This prospective observational study was designed to look at the effects of adding inhaled CO2 to children with HLHS who were already on a hypoxic mixture during the immediate perioperative period. RESULTS: Twelve suitable neonates were enrolled in the study. Combining the two ventilatory modalities had an additive effect. The effect was more significant in the prebypass (83% of patients) compared with the postbypass period (25% of patients). CONCLUSIONS: Low inspired oxygen and CO2 have an additive vasoconstrictive effect on the pulmonary vessels. The use of both of these ventilatory manoeuveres is less effective postoperatively compared with the prebypass period.     

Predicted normocapnea in infants and children using the Bain circuit with controlled ventilation

We have constructed a nomogram for fresh gas flow (VFG) and minute ventilation (VE) for paediatric anaesthesia during controlled ventilation using the Bain coaxial Mapleson D circuit. VFG was based upon the assumption of a high fresh gas utilization because of a low VFG/VE ratio (0.67) and known figures of carbon dioxide elimination. The formulas VFG = 27.8 x VCO2 and VE = 1.5 x VFG were used to calculate the necessary flows to generate normocapnea. The nomogram was evaluated in 59 children (6-62 kg, age 5 months-14 years). PaCO2 (mean +/- s.d.) was 5.0 +/- 0.5 kPa (38 +/- 4 mmHg) with a total range of 3.9-6.3 kPa (29-47 mmHg). Ninety percent of the children had a PaCO2 of 5.7 kPa (43 mmHg) or lower. There was no correlation between body weight and PaCO2. Hence, there was no difference in mean values between children below or above a body weight of 20 kg.     

A new concept in controlled ventilation of children with the Bain anesthetic circuit

The Bain anesthesia circuit was studied as a semi-open or partial rebreathing system during controlled ventilation in 16 children weighing from 7.5 to 48 kg. During anesthesia the lungs were ventilated with a volume ventilator set at three times the calculated alveolar ventilation to provide optimum mixing in the exhalation tube of the Bain circuit. Fresh gas inflow rates initially were set equal to the calculated alveolar ventilation, and after 30 to 45 min, PCO2, PO2 and pH values were measured. At the same time, the fractional concentration of mixed expired carbon dioxide (FECO2) was recorded from a capnograph inserted between the ventilator and the Bain circuit. After initial readings, the fresh gas inflow was varied over a range of 1,400-3,000 ml/m2/min at 20-min intervals, with the arterial blood-gas values and FECO2 recorded at each setting. The results indicate that a lower fresh gas inflow than previously recommended can be used safely in children. When the minute ventilation is three times the fresh gas inflow, values for FECO2 correlate closely with PaCO2 values; with a fresh gas inflow of 2,500 ml/m2/min,PaCO2 values can be maintained near 40 torr.     

呼气末二氧化碳分压反映患儿置入喉罩机械通气时动脉血二氧化碳分压的准确性

目的 评价PETCO2反映患儿置入喉罩机械通气时PaCO2的准确性.方法 拟在全身麻醉下行骨科手术患儿52例,ASA分级Ⅰ级,年龄2～9岁,体重10～30 kg.采用分层随机法,将患儿随机分为2组(n=26):喉罩组(LMA组)和气管导管组(ETT组).常规麻醉诱导后行机械通气,待血液动力学稳定后,采集桡动脉血样测定PaCO2,同时记录PETCO2.结果 两组间PETCO2和PaCO2比较差异无统计学意义(P＞0.05);LMA组PETCO2与PaCO2比较差异无统计学意义(P＞0.05).结论 患儿置入喉罩机械通气时,PETCO2可反映PaCO2,用于指导调整机械通气参数.

Abstract：

Objective To investigate the accuracy of end-tidal carbon dioxide (PETCO2) in reflecting arterial carbon dioxide (PaCO2) during mechanical ventilation via laryngeal mask airway (LMA) in children. Methods Fifty-two ASA Ⅰ patients, aged 2-9 yr, weighing 10-30 kg, undergoing orthopaedic surgery under general anesthesia, were randomized into 2 groups (n = 26 each) : LMA group and endotracheal tube (ETT) group. After anesthesia was induced with fentanyl, propofol and succinycholine, LMA or ETT was inserted and the children were mechanically ventilated. After the hemodynamics was stable, arterial blood samples were obtained to detect PaCO2, and PETCO2 was recorded simultaneously. Results There was no significant difference in PaCO2 and PCT CO, between groups LMA and ETT ( P ＞ 0.05) . There was no significant difference between PaCO2 and PETCO2 in LMA group (P ＞ 0.05). Conclusion When mechanical ventilation is performed via LMA in children, PETCO2 can reliably reflect PaCO2 and guide the regulation of ventilatory parameters.     

Failure of a heat and moisture exchanger as a cause of disconnection during anaesthesia

Two cases of ventilator tubing mishaps, resulting from defective heat and moisture exchangers, are described. The report emphasises the need for preoperative inspection of the anaesthetic machine and associated equipment as well as the importance of a disconnect alarm device.     

人工鼻与热加湿器对机械通气患者影响的Meta分析

目的比较人工鼻与热加湿器对机械通气患者影响的差异。方法计算机检索PubMed、CENTRAL、EMbase、CNKI、VIP和万方数据库,查找人工鼻与热加湿器对机械通气的随机对照试验,检索时间为建库至2015年10月。采用RevMan5.2软件进行Meta分析。结果共纳入21篇文献,共3 147例受试者。Meta分析结果显示:人工鼻与热加湿器对机械通气患者影响比较,两者气道阻塞率RR=1.59,95%CI(1.00,2.51),P=0.05;在病死率及呼吸机相关肺炎发生率方面差异无统计学意义(均P0.05)。结论人工鼻与热加湿器治疗机械通气患者均取得较好的效果。人工鼻可减少每日吸痰次数,但是否会增加气道阻塞发生概率需进一步探讨,且本研究纳入文献质量偏低,需要更多高质量大样本的研究来进一步论证。     

Measurement of heat and moisture exchanger efficiency

Deciding between a passive heat and moisture exchanger or active humidification depends upon the level of humidification that either will deliver. Published international standards dictate that active humidifiers should deliver a minimum humidity of 33 mg.l^?1; however, no such requirement exists, for heat and moisture exchangers. Anaesthetists instead have to rely on information provided by manufacturers, which may not allow comparison of different devices and their clinical effectiveness. I suggest that measurement of humidification efficiency, being the percentage moisture returned and determined by measuring the temperature of the respired gases, should be mandated, and report a modification of the standard method that will allow this to be easily measured. In this study, different types of heat and moisture exchangers for adults, children and patients with a tracheostomy were tested. Adult and paediatric models lost between 6.5 mg.l^?1 and 8.5 mg.l^?1 moisture (corresponding to an efficiency of around 80%); however, the models designed for patients with a tracheostomy lost between 16 mg.l^?1 and 18 mg.l^?1 (60% efficiency). I propose that all heat and moisture exchangers should be tested in this manner and percentage efficiency reported to allow an informed choice between different types and models.     

Changes in delivered tidal volume with the addition of carbon dioxide to mechanical ventilation

Objective:  To determine discrepancies in delivered tidal volumes induced by the addition of carbon dioxide (CO₂) during mechanical ventilation.
Design:  Prospective, experimental, lung model study.
Setting:  Research laboratory at a University hospital.
Subjects:  Ventilator set-up using a mechanical lung model.
Interventions:  Tidal volumes were measured during pressure and volume limited ventilation at various pressures and volumes with the addition of inspired CO₂ concentrations of 0, 1, 2, and 4%. Three ventilator set-ups were used including the addition of CO₂ to the inspiratory limb (M1), the use of two external blenders to mix air, oxygen and CO₂ prior to entry to the ventilator (M2), and the use of one external blender to mix air and CO₂ prior to addition to the ventilator.
Measurements and main results:  Statistically significant increases in delivered tidal volume were noted with the addition of CO₂ with all three of the ventilator set-ups (M1, M2 and M3). However, the maximum increase was 2.4% above that of baseline (no CO₂ added). With the M2 set-up, there was also a significant discrepancy noted between the set and the delivered tidal volumes (16–17%) when using the volume limited mode even without the addition of CO₂.
Conclusions:  Either the M1 or the M3 set-up functioned efficiently without clinically significant alterations in ventilator performance. We prefer the M1 set-up as it is the one that is used most commonly in clinical practice and does not require significant alterations in the working configuration of the ventilator.     

Rebreathing and end-tidal CO2 during spontaneous breathing with the Bain circuit

In order to examine the relationship between end-tidal CO₂ (Fet_{co 2}) and inspired CO₂ (Fi_{co 2}) in anaesthetized patients breathing spontaneously with a Bain breathing circuit and afresh gasvolume ( \(\dot VF\) ) of 100 ml · kg^-1 · min^-1, the respiratory rate (f) and minute ventilation ( \(\dot VE\) ) was changed in two groups of six patients each by the induction or reversal of narcotic respiratory depression. During light nitrous oxide-halothane anaesthesia (Group I), the intravenous injection of 0.1 mg · kg^-1 of alphaprodine caused arapid fall in F_{co 2} from 2.3 ± 0.5 per cent to 0.7 ± 0.1 per cent concomitant with the reduction inf(37 ± 5 to 16 ± 4) breath -min^-1 and \(\dot VE\) (137 ± 29 to 55 ± 13 ml · kg^-1 · min^-1), while the Fet_{co 2} rose gradually from 5.2 ± 0.9 percent to 6.4 ± 0.9 per cent over a ten-minute period. During light nitrous oxide-halothane anaesthesia supplemented by alphaprodine (Group II), 0.2 mg of naloxone intravenously caused a rise in Fi_{co 2} from 0.5 ± 0.3 per cent to 2.9 ± 0.6 per cent simultaneous with arise in f (11 ± 2 to 25 ± 7 breath · min^-1) and \(\dot VE\) (70 ± 25 to 133 ± 34 ml · kg^-1 · min^-1), while the FET_{co 2} declined gradually over a ten-minute period from 7.6 ± 0.7 per cent to 6.4 ± 0.4 per cent. The change in Fl_{co 2} always occurred exactly at the same time as the drug-induced change in respiration. It was associated with acorresponding change in the degree of mixing of fresh gas and expired gas within the breathing system and appeared to correlate with the change in the ratio \(\dot VE/\dot VF\) . There was no indication that the Fl_{co 2} or the distribution of CO₂ within the system had any effect onFet _{co 2} or CO₂ elimination. Under these conditions theFl _{co 2} and the volume of rebreatked CO₂ can not be the cause but must be regarded as apassive change consequent to the altered pattern of breathing.     

The benefit of using a heat and moisture exchanger during short operations in young children

We studied the efficiency of a heat and moisture exchanging filter (HMEF; Pall BB25) as a means of compensating for the heat and moisture loss during anaesthesia in young children using cold and dry gas supplied from open circuits. Forty ASA I children (mean age: 48 months±20; mean weight: 16±3.5 kg) were randomized into two groups: Group I without HMEF/Group II with HMEF. The two groups did not show any significant differences for morphometric data or ventilation parameters. Relative humidity and temperature measurements in anaesthetic gases were taken using a combined temperature/humidity probe introduced into the circuit. Absolute humidity in the circuit was calculated from these measurements. In Group II, a significant increase (P<0.001) in absolute humidity was demonstrated (Group I: 12 mg H₂O·l^?1vs Group II: 22 mg H₂O·l^?1). This increase appeared immediately after introduction of the HMEF in the circuit and remained constant throughout the duration of the operation. Thus, the use of the device is recommended for young children, even for operations of short duration.     

Noninvasive monitoring of carbon dioxide during mechanical ventilation in older children: end-tidal versus transcutaneous techniques

We prospectively compared the accuracy of end-tidal CO(2) (ETCO(2)) and transcutaneous CO(2) (TCCO(2)) monitoring in older pediatric patients (4 yr or older) receiving mechanical ventilation for respiratory failure. ETCO(2) and TCCO(2) were simultaneously monitored and compared with arterial CO(2) (PaCO(2)) values when arterial blood gas analysis was performed. Eighty-two sample sets were compared. The ETCO(2) to PaCO(2) difference was 6.4 +/- 6.3 mm Hg, whereas the TCCO(2) to PaCO(2) difference was 2.6 +/- 2.0 mm Hg (P < 0.0001). The absolute difference of ETCO(2) and PaCO(2) was 5 or less in 47 of 82 measurements, whereas the absolute TCCO(2) to PaCO(2) difference was 5 or less in 76 of 82 measurements (P < 0.00001). Regression analysis of ETCO(2) and PaCO(2) values revealed a correlation coefficient of 0.5418 and an r value of 0.8745. Regression analysis of TCCO(2) and PaCO(2) values revealed a correlation coefficient of 1.0160 and an r value of 0.9693. Bland-Altman analysis revealed a bias of -5.68 with a precision of +/-6.93 when comparing ETCO(2) with PaCO(2) and a bias of 0.02 with a precision of +/-3.27 when comparing TCCO(2) and PaCO(2) (P < 0.00001). TCCO(2) monitoring provided an accurate estimation of PaCO(2) over a wide range of CO(2) values and was superior to ETCO(2) monitoring in older pediatric patients with respiratory failure. TCCO(2) monitoring may be considered as a useful adjunct to monitoring of ventilation in this patient population. IMPLICATIONS: The authors report on the accuracy of noninvasive, transcutaneous CO(2) monitoring during mechanical ventilation in children 4 yr or older. Application of this technique should be useful by decreasing the need for repeated, costly, and sometimes painful arterial blood gas analysis, and the continuity of assessment should facilitate proactive, rather than reactive, ventilator manipulations.     

Assessment of a hygroscopic heat and moisture exchanger for paediatric use

A laboratory study of a widely available heat and moisture exchanger marketed for paediatric use was undertaken. The deadspace, measured by volume displacement, was 12 ml, similar to that of a standard catheter mount for paediatric use. Pressure drop across the device was measured at several different flows in five samples of the device in both the dry and wet state. Calculated resistance proved to be markedly lower when compared with that of other anaesthetic equipment such as tracheal tubes, and with similar humidification devices for paediatric use.