Total Fresh Gas Flow Research Articles

Optimizing Pediatric Mask Induction Fresh Gas Flow.

The environmental impact of inhaled anesthetics is a subject of increasing research. However, little attention has been paid to optimizing high-concentration volatile anesthetics during the inhalational (mask) inductions that begin most pediatric anesthetics. The performance of the GE Datex Ohmeda TEC 7 sevoflurane vaporizer was analyzed at different fresh gas flow (FGF) rates and two clinically relevant ambient temperatures. We found that an FGF rate of 5 liters per minute (LPM) is likely optimal for inhalational inductions, rapidly achieving dialed sevoflurane concentrations at the elbow of an unprimed pediatric breathing circuit while minimizing waste associated with higher FGF rates. We began educating our department regarding these findings, first with QR code labels on anesthetic workstations, then with targeted e-mails to pediatric anesthesia teams. We analyzed peak induction FGF in 100 consecutive mask inductions at our ambulatory surgery center at three different periods - baseline, post-labels, and post-emails - to assess the efficacy of these educational interventions. We also analyzed the time from induction to the start of myringotomy tube placement in a subset of these cases to determine if reducing mask induction FGF was associated with any change in the speed of induction. Our institution's median peak FGF during inhalational inductions decreased from 9.2 LPM at baseline to 8.0 LPM after labels were placed on anesthetic workstations to 4.9 LPM after targeted e-mails. There was no associated decrease in the speed of induction. Total fresh gas flow can be limited to 5 LPM during pediatric inhalational inductions, decreasing anesthetic waste and environmental impact without slowing the speed of induction. Educational labels on anesthetic workstations and direct e-mails to clinicians were effectively used in our department to enact change in this practice.

I-gel supraglottic airway use is efficient and safe during minimal flow anesthesia with volume controlled ventilation - a randomized controlled trial

Background: A fear of potential gas leak limits the wide spread use of minimal flow anesthesia with supraglottic airway devices. Second generation supraglottic airway devices have been claimed to be come with good airway seal. I-gel is one of these and it has been extensively evaluated for spontaneous as well as assisted ventilation. We conducted this study to evaluate its use with low flow anesthesia and volume controlled ventilation.
 Methodology: In a prospective randomized controlled trial, 50 patients undergoing laparoscopic surgeries for less than 2 h duration were administered general anesthesia, with controlled ventilation, using either endotracheal tube or an I-gel for airway control. The total fresh gas flow was gradually reduced until it was 400 mL/min. Patients were monitored for evidence of loss of respiratory gas volume in the breathing circuit and other parameters for ventilation failure.
 Results: Ventilation could be maintained at a fresh gas flow of 400 mL/min, without clinical or volumetric evidence of gas leak in all patients in the two groups. Two patients in the I-gel group had intraoperative gas leak leading to ventilator failure alarm but the same was corrected by repositioning of the head. The vital signs, peak airway pressure, end-tidal carbon dioxide, set tidal volume and expired tidal volumes were statistically similar in the two groups.
 Conclusion: I-gel supraglottic airway can be safely and efficiently used for laparoscopic surgery using minimal flow anesthesia and volume controlled ventilation.
 Citation: Khan A, Kapoor MC, Garg S, Puri A, Sinha A. I-gel supraglottic airway use is efficient and safe during minimal flow anesthesia with volume controlled ventilation - a randomized controlled trial. Anaesth pain intensive care 2020;24(1):__
 Abbreviations: Supraglottic airway device – SAD; Volume controlled ventilation – VCV; Pressure controlled ventilation – PCV; Endotracheal tube – ETT; Laryngeal Mask Airway - LMA
 
 DOI: https://doi.org/10.35975/apic.v24i1.
 Received – 11 June 2019;
 Reviewed – 20, 23 October 2019;
 Revised – 31 January 2020;
 Second Review – 2, 7 January 2020;
 Accepted – 02 February 2020;

Sevoflurane sparing effect of dexmedetomidine in patients undergoing laparoscopic cholecystectomy: A randomized controlled trial.

Background and Aims:Sevoflurane is an excellent but expensive anesthetic agent for laparoscopic cholecystectomy. To decrease sevoflurane consumption during surgery adjuvants like dexmedetomidine may be used. Dexmedetomidine is a recently introduced drug which alleviates the stress response of surgery, produces sedation and analgesia. We aimed to evaluate sevoflurane sparing effect of dexmedetomidine in patients undergoing laparoscopic cholecystectomy under entropy-guided general anesthesia (GA).Material and Methods:In this prospective randomized control study, 100 American Society of Anesthesiologists physical status I–II adult surgical patients scheduled to undergo laparoscopic cholecystectomy were enrolled. Patients were randomly divided into two groups (n = 50). In dexmedetomidine group, patients received intravenous (IV) dexmedetomidine 0.5 μg/kg over 10 min before induction followed by 0.5 μg/kg/h infusion while in control group, patients received the same volume of normal saline.Results:Sevoflurane consumption was 41% lower in dexmedetomidine group as compared to control group (7.1 [1.6] vs. 12.1 [1.9] ml, P <0.001). A 40% reduction was observed in induction dose of propofol (83.0 [19.1] vs. 127.6 [24.8] mg, P <0.001). Mean Riker sedation-agitation score, visual analog score for pain and Aldrete's score were significantly lower in dexmedetomidine group as compared to control group. None of the patients experienced any significant side effects.Conclusion:A 41% reduction in sevoflurane consumption was observed in patients receiving IV dexmedetomidine as an adjuvant in patients undergoing laparoscopic cholecystectomy under GA.

Impact de la diminution d’utilisation du protoxyde d’azote sur la consommation d’agents halogénés

ObjectiveNitrous oxide (N2O) toxicity and its impact on pollution lead to restrict its use. A decrease of N2O consumption should increase the hypnotic inhaled consumption. This monocentric study estimated consumptions and costs of halogenated agents (HA) and N2O over 5years when the N2O consumption was reduced. Study designRetrospective from a computerized database. PatientsBetween 2006 and 2010, 34,097 procedures were studied after two meetings exposing the risks of the N2O. MethodsAt the end of anesthesia, consumptions of hypnotic agents (millilitres transmitted by the injectors and the blender) were archived in the database. The annual consumption of agents was obtained by adding the individual consumptions, then divided by the annual number of cases. The costs were given by the hospital pharmacy from invoices. ResultsN2O consumption per anesthesia constantly decreased during the study, from 75.1L by act to 22.7L. The sum of the annual consumptions of N2O and air did not change suggesting that total fresh gas flow remained stable. Between 2006 and 2010, the sevoflurane consumption by act increased by 25%, from 16.5 to 20.6mL, and desflurane consumption by 37%, from 46.1 to 63.1mL by patient. The costs of the administration of hypnotic agents remained stable. ConclusionN2O consumption decrease had an impact on the consumption of HA. The cost reduction of the N2O was counterbalanced by the increase of halogenated vapor cost. The profit of the ecological impact of the reduction in N2O use could be quantified.

The use of a double‐lumen central venous catheter for airway management in pediatric patients undergoing laryngeal papillomatosis surgery

To evaluate the efficacy and safety of a spontaneous ventilation anesthesia technique with insufflation of oxygen and volatile agent through a double-lumen central venous catheter (DLCVC) in pediatric patients undergoing suspension laryngoscopic surgery for laryngeal papillomatosis. Thirty-six pediatric patients with laryngeal papillomatosis undergoing suspension laryngoscopic surgery were anesthetized with oxygen and volatile anesthetic insufflation while spontaneously breathing. Anesthesia was induced by inhalation of 8% sevoflurane in oxygen by mask. Atropine, dexamethasone, lidocaine, and midazolam were administered intravenously. The tip of a 7Fr DLCVC was inserted below the glottis after placement of the laryngoscope and establishing suspension. Anesthesia was maintained with insufflation of 4-6% sevoflurane and oxygen with a total fresh gas flow of 6 l·min(-1) through the 14G (larger lumen) of the DLCVC. Endtidal carbon dioxide tension (PetCO2) was monitored using the other lumen of the DLCVC, which was connected to the CO2 sampling line. Duration of the procedure as well as total anesthesia time was recorded. Electrocardiography (ECG), heart rate (HR), mean arterial pressure (MAP), oxygen saturation (SpO2), and PetCO2 were also monitored. Arterial blood was sampled for blood gas analysis including pH, PaO2 , PaCO2 , and actual base excess (ABE). Complications, including intraoperative patient movement, hypoxemia (SpO2 < 95% during oxygen insufflation), nausea, vomiting, bronchospasm, and arrhythmias, were recorded. There was a significant increase in PetCO2 and PaCO2 (P values <0.05) as well as a decrease in ABE, pH, and PaO2 (P values <0.05) in samples collected before and after surgery. MAP, HR, and SpO2 after surgery were not significantly different from after induction values (P values >0.05). During surgery, SpO2 < 95% in three cases and body movements in three cases were observed. No patient had any other of the complications previously described. Furthermore, no postsurgical endotracheal intubation was needed in any patient. After establishing an adequate depth of anesthesia, a spontaneous ventilation anesthesia technique with insufflation of oxygen and volatile agent through a DLCVC is feasible in pediatric patients undergoing suspension laryngoscopic surgery for laryngeal papillomatosis.

Expiratory Limb Ventilation during Unique Failure of the Anesthesia Machine Breathing Circuit

Expiratory Limb Ventilation during Unique Failure of the Anesthesia Machine Breathing Circuit

Comparison of bispectral index (BIS) and entropy in patients with cerebral palsy during sevoflurane induction.

Demand of anesthesia for patients with cerebral palsy is more increasing. But there is still lacking in clinical research regarding how BIS and entropy reflect well on sedative and hypnotic state in patients with cerebral palsy. Fifteen patients with cerebral palsy (Group CP) and fifteen patients without cerebral palsy (Group NL) scheduled for elective orthopedic surgery were included in the study. Induction of anesthesia was done by having the patient inhale 1 vol% sevoflurane and 100% oxygen using a total fresh gas flow of 8 L/min. Simultaneously BIS, state entropy (SE), response entropy (RE), end-tidal sevoflurane concentration were recorded every 15 seconds till there was no self respiration. When end-tidal sevoflurane concentration had not risen any more for 30 seconds, we increased inhaled sevoflurane concentration in 1 vol% increments. End point of recording was when self respiration was lost or the time sevoflurane concentration reached 8 vol%. No significant differences in RE, SE, BIS at baseline and end point were found between the two groups. No significant difference in the time reach end point was found between the two groups. BIS, SE and RE correlated with end-tidal sevoflurane concentration in the two groups. The authors found no significant difference in the entropy values between patients with CP and normal patients. Also, the entropy values could be interpreted like BIS in patients with CP. And BIS showed a stronger correlation with end tidal sevoflurane concentrations than entropy.

The ideal oxygen/nitrous oxide fresh gas flow sequence with the Anesthesia Delivery Unit machine

Study Objective To determine whether early reduction of oxygen and nitrous oxide fresh gas flow from 6 L/min to 0.7 L/min could be accomplished while maintaining end-expired nitrous oxide concentration ≥50% with an Anesthesia Delivery Unit anesthesia machine. Study Design Prospective, randomized clinical study. Setting Large teaching hospital in Belgium. Patients 53 ASA physical status I and II patients requiring general endotracheal anesthesia and controlled mechanical ventilation. Interventions Patients were randomly assigned to one of 4 groups depending on the duration of high oxygen/nitrous oxide fresh gas flow (two and 4 L/min, respectively) before lowering total fresh gas flow to 0.7 L/min (0.3 and 0.4 L/min oxygen and nitrous oxide, respectively): one, two, three, or 5 minutes (1-minute group, 2-minute group, 3-minute group, and 5-minute group), with n = 10, 12, 13, and 8, respectively. The course of the end-expired nitrous oxide concentration and bellows volume deficit at end-expiration was compared among the 4 groups during the first 30 minutes. Results At the end of the high-flow period the end-expired nitrous oxide concentration was 35.6 ± 6.2%, 48.4 ± 4.8%, 53.7 ± 8.7%, and 57.3 ± 1.6% in the 4 groups, respectively. Thereafter, the end-expired nitrous oxide concentration decreased to a nadir of 36.1 ± 4.5%, 45.4 ± 3.8%, 50.9 ± 6.1%, and 55.4 ± 2.8% after three, 4, 6, and 8 minutes after flows were lowered in the 1- to 5-minute groups, respectively. A decrease in bellows volume was observed in most patients, but was most pronounced in the 2-minute group. The bellows volume deficit gradually faded within 15 to 20 minutes in all 4 groups. Conclusions A 3-minute high-flow period (oxygen and nitrous oxide fresh gas flow of 2 and 4 L/min, respectively) suffices to attain and maintain end-expired nitrous oxide concentration ≥50% and ensures an adequate bellows volume during the ensuing low-flow period.

Predictive Performance of a Model of Anaesthetic Uptake with Desflurane

We have previously shown that a model of anaesthetic uptake and distribution, developed for use as a teaching tool, is able to predict end-tidal isoflurane and sevoflurane concentrations at least as well as commonly used propofol models predict blood levels of propofol. Models with good predictive performance may be useful as part of real-time prediction systems. The aim of this study was to assess the performance of this model with desflurane. Twenty adult patients undergoing routine anaesthesia were studied. The total fresh gas flow and vaporizor settings were collected at 10-second intervals from the anaesthetic machine. These data were used as inputs to the model, which had been initialized for patient weight and desflurane. Output of the model is a predicted end-tidal value at each point in time. These values were compared with measured end-tidal desflurane using a standard statistical technique of Varvel and colleagues. Data was analysed from 19 patients. Median performance error was 78% (95% CI 8-147), median absolute performance error 77% (6-149), divergence 10.6%/h (-80-101) and wobble 8.9% (-6-24). The predictive performance of this model with desflurane was poor, with considerable variability between patients. The reasons for the difference between desflurane and our previous results with isoflurane and sevoflurane are not obvious, but may provide important clues to the necessary components for such models. The data collected in this study may assist in the development and evaluation of improved models.

Proper Respiratory Rate Determination during Controlled Ventilation in Infants under General Anesthesia

Background: We experienced unintentional hyperventilation during mechanical ventilation in infants under general anesthesia. It is very difficult to decide upon respiratory rate or tidal volume for adequate ventilation without respiratory gas monitoring. During pulmonary ventilation using a Mapleson D circuit, the utilization of high fresh gas flow dilutes the expired gas and causes an underestimation of end-tidal . We undertook to find a proper respiratory rate (RR) with a fixed tidal volume during controlled ventilation in infant general anesthesia. Methods: We studied 50 infants weighing below 10 kg during general anesthesia. An uncuffed endotracheal tube was selected 3.5 or 4.0 mm (inner diameter). After intubation with midazolam, thiopental sodium and vecuronium, controlled ventilation was applied: total fresh gas flow 3 L/minute, peak inspiratory pressure 15-20 , and RR 19 or 20/minute. Arterial blood gas analysis was done 20 minutes later. We calculated the RR for a of 36 mmHg according to the equation: RR (applied) = estimated RR estimated /ideal . In addition, linear regression was performed to analyze the relation between age and RR. Results: The estimated regression equation for RR using Pearson's correlation coefficient was as follows: RR = 22.835 - 0.415 age (months). Conclusions: The equation (RR = 22.835 - 0.415 age [months]) could be used as an index for safe ventilatory management without severe hyper- or hypo-carbia in infants during general anesthesia.

Model-based administration of inhalation anaesthesia. 4. Applying the system model

We developed and tested a simple dosing strategy for rapid induction with isoflurane followed by maintenance under minimal-flow conditions, that is 0.5 litre min(-1) total fresh gas flow (FGF). An end-expired concentration was to be achieved within 5 min in a desired therapeutic window, that is 0.8-1.1 vol%, and to be maintained within it for at least 30 min. With our new model we computed a three-stage regimen using one fixed vaporizer setting: 3 vol% isoflurane in a FGF of 3 and 1.5 litre min(-1), each for 3 min, and 0.5 litre min thereafter. The ratio of nitrous oxide:oxygen was, consecutively, 2:1, 2:1, and 2:3. We evaluated this scheme in 58 adult patients (body mass 74 (SD 13) kg), mostly during eye and ear, nose, and throat surgery. Measured oxygen (33-45 vol%) and nitrous oxide concentrations (66-50 vol%) evolved in accordance with those computed. In five patients with a median of body mass 92 kg (range 76-126 kg), inspired oxygen concentrations decreased to less than 30 vol%. End-expired isoflurane concentration entered the window after 2 min (range 1.0-5.67 min) and attained its maximum, that is 0.96 vol% (0.8-1.2 vol%), after 3.45 min (1.67-6.33 min). The mean end-expired concentration was in the desired window from 3-60 min and an average of 72% of individual measurements were within the window from 3-30 min. The scheme was adapted in six patients (excluded from analysis) because of hypotension. The regimen is easily remembered, reliable, and lends itself to alternative strategies, but must be guided by the monitoring of gas and vapour concentrations and haemodynamic variables.

A comparison of the ‘Maxima’ and the modified Ayre's T-piece breathing systems in spontaneously breathing cats

To compare the fresh gas flow requirements of the 'Maxima' and Jackson-Rees modified Ayre's T-piece (JRMATP) in spontaneously breathing anaesthetized in cats. Prospective randomized clinical study. Fifteen adult cats (6 male, 9 female, 3.1 ± 0.4 kg [ x¯ ± SD]). After pre-anaesthetic medication with acepromazine and pethidine, anaesthesia was induced using thiopentone and the trachea was intubated with a cuffed endotracheal tube. This was attached to either a 'Maxima' or a JRMATP breathing system; allocation was randomized. Anaesthesia was maintained with halothane delivered in a 1 : 1 oxygen : nitrous oxide mixture. Initial total fresh gas flow (FGF) was set at 600 mL kg-1 min-1. After 20 minutes, FGF was reduced in increments of 200 mL min-1 until rebreathing (inspired CO2 concentration >0.2%) occurred. At this point, FGF was increased to 600 mL kg-1 and the process was repeated with the other breathing system. The respiratory rate and airway pressure at the endotracheal tube connector were monitored throughout anaesthesia. The mean fresh gas flow that prevented rebreathing with the Maxima system (164 ± 39 mL kg-1) was significantly less (p < 0.0001) than that required in the modified T-piece (455 ± 0.77 mL kg-1). Respiratory rates and airway pressures at the endotracheal tube connector were not significantly affected by breathing system employed. In terms of the gas flow requirements that prevent rebreathing, the 'Maxima' breathing system is more efficient than the modified Ayre's T-piece in spontaneously breathing cats anaesthetised with halothane.

Onset Time, Recovery Duration, and Drug Cost with Four Different Methods of Inducing General Anesthesia

We compared two conventional induction techniques (thiopental and propofol), an inhaled induction with sevoflurane using a circle system, and a rebreathing method.Fentanyl 1 [micro sign]g/kg was given to women undergoing 10- to 20-min procedures. Anesthesia was induced (n = 20 each) with one of the following: 1) sevoflurane and N2 O from a rebreathing bag (Sevo/Bag). A 5-L bag was prefilled with a mixture of sevoflurane 7% and N2 O 60% in oxygen. The bag was connected between the normal circle system, separated by a spring-loaded valve; 2) sevoflurane 8% and N (2) O 60% from a circle system on a conventional anesthesia machine with a total fresh gas flow of 6 L/min (Sevo/Circle); 3) propofol 3 mg/kg as an IV bolus; 4) thiopental sodium 5 mg/kg as an IV bolus. Postoperative nausea and vomiting was treated with ondansetron. Induction times were comparable with each method. Recovery duration was shortest with sevoflurane, intermediate with propofol, and longest with thiopental. Induction drug costs were lowest with Sevo/Bag and thiopental, intermediate with Sevo/Circle, and highest with propofol. However, sevoflurane (by either method) caused considerable nausea and vomiting that required treatment. Consequently, total drug cost was least with thiopental, intermediate with Sevo/Bag and propofol, and greatest with Sevo/Circle. Thus, no single technique was clearly superior. Implications: Anesthetic induction techniques influence awakening time, recovery duration, and drug costs. We tested two IV methods and two inhaled techniques. However, none of the four tested methods was clearly superior to the others. (Anesth Analg 1999;88:930-5)

Model for the administration of low-flow anaesthesia

A general (multiple-gas) three-compartment mass-balance model of the circle-absorber breathing circuit with intermittent positive-pressure ventilation has been developed. We propose it as a tool to determine flowmeter and vapourizer settings for inhalation anaesthesia by low-flow methods (less than 1 litre min-1 total fresh gas flow). This model reproduces the results of various previously-published mass-balance models, but it appears to underestimate slightly the delivery of fresh gases to the inspired limb of the breathing circuit when tested with clinical data from surgical cases. Low-flow dosing schedules for 35% inspired oxygen and 1% inspired halothane were computed with the model and tested in vitro with a circle-absorber breathing circuit and an active gas-exchange lung at nine values of simulated patient gas exchanges. The mean inspired oxygen concentration over all trials was 32.8% (SD = 1.9%), while the mean inspired halothane concentration was 1.2% (SD = 0.3%). The flow meter and vapourizer settings calculated from the model appear to have sufficient accuracy to be useful in the clinical setting in conjunction with active oxygen and anaesthetic agent monitoring.

Compound A concentrations during sevoflurane anesthesia in children.

Sevoflurane is a new inhalation agent that should be useful for pediatric anesthesia. Sevoflurane undergoes degradation in the presence of carbon dioxide absorbents; however, quantification of the major degradation product (compound A) has not been evaluated during pediatric anesthesia. This study evaluates sevoflurane degradation compound concentrations during sevoflurane anesthesia using a 2-1 fresh gas flow and a circle system with carbon dioxide absorber in children with normal renal and hepatic function. The concentrations of compound A were evaluated during sevoflurane anesthesia in children using fresh soda lime as the carbon dioxide absorbent. Nineteen patients aged 3 months-7 yr were anesthetized with sevoflurane (2.8% mean end-tidal concentration) using a total fresh gas flow of 21 in a circle absorption system. Inspiratory and expiratory limb circuit gas samples were obtained at hourly intervals, and the samples were analyzed using a gas chromatography-flame ionization detection technique. Carbon dioxide absorbent temperatures were measured in the soda lime during anesthesia for hepatic and renal function studies. Venous blood samples were obtained before anesthesia, at the end of anesthesia, and 2h after anesthesia for plasma inorganic fluoride ion concentration. The maximum inspiratory concentration of compound A was 5.4 +/- 4.4 ppm (mean +/- SD), and the corresponding expiratory concentration was 3.7 +/- 2.7 ppm (mean +/- SD). The maximum inspiratory compound A concentration in any patient was 15 ppm. Mean concentrations of compound A peaked at intubation and remained stable, declining slightly after 120 min of anesthesia. The duration of anesthesia was 240 +/- 139 min (mean +/-SD). Maximum soda lime temperature ranged between 23.1 degrees C and 40.9 degrees C. There was a positive correlation between maximum absorbent temperature and maximum compound A concentration (r2 = 0.58), as well as between the child's body surface area and maximum compound A concentration (r2 = 0.59). Peak plasma inorganic fluoride ion concentration was 21.5 +/- 6.1 microgmol/1. There were no clinically significant changes in hepatic or renal function studies performed 24 h postanesthesia. Sevoflurane anesthesia of 4 h in normal children using a 2-1 flow circle system produced concentrations of compound A of 15 ppm or less. There was no evidence of abnormality of renal or hepatic function up to 24 h after anesthesia; however, larger studies will be required to confirm the absence of organ toxicity.

Arterial carbon dioxide tensions during anaesthesia with manual ventilation. A descriptive study of the effects of various non-polluting circuits.

In 660 supine, intubated and anaesthetized, healthy patients scheduled for various elective surgical procedures, the distribution of arterial carbon dioxide tension (PaCO2) was investigated during manual non-monitored ventilation. The study comprised six equal groups: group 1: ventilation with a circle circuit absorber system; group 2: ventilation with the Hafnia A circuit using a total fresh gas flow (FGF) of 100 ml . kg-1 . min-1; groups 3-6: ventilation with a Hafnia D circuit with fresh gas flows of 100, 80, 70 and 60 ml . kg-1 . min-1, respectively. The mean PaCO2's of the first three groups were situated in the lower range of normocapnia (the observations in the first group having the greatest total range), whereas the rebreathing (Hafnia A and D) circuits resulted in a clustering of observed data. Employing the rebreathing circuits, protection against hypocapnia can be achieved by lowering the fresh gas flow. The most satisfying result was obtained with the Hafnia D circuit with a fresh gas flow of 70 ml . kg-1 . min-1 resulting in normocapnia with a modest and limited spread towards hypo- and hypercapnia. FGF in excess of this level must be considered as wasted. The study indicates that corrections of fresh gas flows for age are superfluous. Use of relaxants and type of surgery had no influence on the observations.

The regulation of $$Pa_{CO_2 } $$ during controlled ventilation of children with a T-piece

A lung model study described the factors determining the carbon dioxide tension during controlled ventilation with a T-piece system (either a Bain circuit or the classical Ayre’s T-piece). When minute ventilation was large with respect to fresh gas flow and a standard ventilatory pattern was utilized the important variables were fresh gas flow (FGF) and carbon dioxide production $$\left( {\dot V_{CO_2 } } \right)$$ . Under these circumstances the influence of minute ventilation $$\left( {\dot V_E } \right)$$ , dead space to tidal volume ratio (VD/VT), waveform and $$a - AD_{CO_2 } $$ on carbon dioxide tension were minimized. Eight healthy patients (11–64 kg) were studied during controlled ventilation to determine how much of the fresh gas coming from the flowmeters actually participates in gas exchange. With a T-piece system when minute ventilation to fresh gas flow ratio is at least 1.5 the patient’s effective alveolar ventilation equals 72 per cent of the total fresh gas flow. This value and levels of carbon dioxide production from the literature were used to derive a simple two part formula of fresh gas flow requirements for two levels of arterial carbon dioxide in children. Children weighing between 10 and 30 kg require fresh gas flows of 1000 + 100 ml · kg-1 · min-1 and 1600 + 100 ml · kg-1 · min-1 to achieve arterial carbon dioxide tensions of 4.9 kPa (37 torr) and 4.0 kPa (30 torr) respectively. When body weight is between 30 and 70 kg the requirements increased to 2000 + 50 ml · kg-1 · min−1 to achieve a $$Pa_{CO_2 } $$ of 4.9 kPa (37 torr) and 3200 + 50 ml · kg-1 · min-1 to achieve a $$Pa_{CO_2 } $$ of 4.0 kPa (30 torr). In all situations Ve must be at least twice the fresh gas flow requirements. p ]The formula was validated in a heterogeneous group of 26 patients. Mean values of $$Pa_{CO_2 } $$ were very close to those predicted, although individual values differed considerably from the mean. These deviations cannot be eliminated because of patient and equipment variability. Therefore blood gas analysis must be performed whenever the exact regulation of $$Pa_{CO_2 } $$ is required.

Normocapnic Ventilation Using the Circle System

A formula is derived for maintaining normocapnia during controlled ventilation using a circle system without carbon dioxide absorption. In a series of 70 patients, unselected in terms of age, sex, obesity, ASA status, body position during operation, type of anaesthetic administered or type of circle system used, it was found that a total fresh gas flow of 50 ml/kg body weight/min and a minute ventilation of 120 to 150 ml/kg body weight at a rate of 10 to 12/min achieved normocapnia. For moderate hypocapnia a total fresh gas flow of 60 ml /kg body weight/min and a minute ventilation of 120 ml/kg at a rate of 10–12/min is suggested.

Carbon dioxide output and elimination in children under anaesthesia.

The requirements for fresh gas inflow with the Bain breathing circuit in children was examined by determining the PaCO2 in 46 children during controlled ventilation with a total fresh gas inflow of 3.5 l/min and by measuring the carbon dioxide output in 83 children under anaesthesia. It could be shown that all children below 40 kg body weight had a PaCO2 below 40 torr (5.32 kPa) and the PaCO2 paralleled the body weight, i.e., the lowest carbon dioxide tension was seen in children under 10 kg. As expected, the highest carbon dioxide output was found in children below 5 kg body weight; the carbon dioxide output per kilogram decreased with increasing body weight up to 30-35 kg and remained at that level in larger children. Children in their teens, although they may have attained adult body weight, had a higher carbon dioxide output than adults. Based on these findings, our recommendation of a total fresh gas inflow of 3.5 1/min for all children would appear adequate for a body weight up to 35 kg on controlled ventilation. In children under 10 kg body weight, a reduction of the total fresh gas flow to two litres per minute will avoid marked respiratory alkalosis. For children over 35 kg, a fresh gas flow of 100 ml/kg/min should be satisfactory during controlled ventilation.

Normocapnic ventilation using the circle system.

A formula is derived for maintaining normocapnia during controlled ventilation using a circle system without carbon dioxide absorption. In a series of 70 patients, unselected in terms of age, sex, obesity, ASA status, body position during operation, type of anaesthetic administered or type of circle system used, it was found that a total fresh gas flow of 50 ml/kg body weight/min and a minute ventilation of 120 to 150 ml/kg body weight at a rate of 10 to 12/min achieved normocapnia. For moderate hypocapnia a total fresh gas flow of 60 ml/kg body weight/min and a minute ventilation of 120 ml/kg at a rate of 10-12/min is suggested.