Sequential gas delivery provides precise control of alveolar gas exchange

Of the factors determining blood gases, only alveolar ventilation (VA) is amenable to manipulation. However, current physiology text books neither describe how breath-by-breath VA can be measured, nor how it can be precisely controlled in spontaneously breathing subjects. And such control must be effected independent of minute ventilation (VE) and the pattern of breathing. Control of VA requires the deliberate partition of inhaled gas between the alveoli and the anatomical deadspace. This distribution is accomplished by sequential gas delivery (SGD): each breath consists of a chosen volume of 'fresh' gas followed by previously exhaled gas. Control of VA through SGD is a simple, inexpensive, yet powerful tool with many applications. Here we describe how to implement SGD, how it precisely controls VA, and consequently how it controls arterial blood gases.

A high-flow nasal cannula system with relatively low flow effectively washes out CO2 from the anatomical dead space in a sophisticated respiratory model made by a 3D printer

Background

Although clinical studies of the high-flow nasal cannula (HFNC) and its effect on positive end-expiratory pressure (PEEP) have been done, the washout effect has not been well evaluated. Therefore, we made an experimental respiratory model to evaluate the respiratory physiological effect of HFNC.

Methods

An airway model was made by a 3D printer using the craniocervical 3D-CT data of a healthy 32-year-old male. CO₂ was infused into four respiratory lung models (normal-lung, open- and closed-mouth models; restrictive- and obstructive-lung, open-mouth models) to maintain the partial pressure of end-tidal CO₂ (P_ETCO₂) at 40 mmHg. HFNC flow was changed from 10 to 60 L/min. Capnograms were recorded at the upper pharynx, oral cavity, subglottic, and inlet sites of each lung model.

Results

With the normal-lung, open-mouth model, 10 L/min of HFNC flow decreased the subglottic P_ETCO₂ to 30 mmHg. Increasing the HFNC flow did not further decrease the subglottic P_ETCO₂. With the normal-lung, closed-mouth model, HFNC flow of 40 L/min was required to decrease the P_ETCO₂ at all sites. Subglottic P_ETCO₂ reached 30 mmHg with an HFNC flow of 60 L/min. In the obstructive-lung, open-mouth model, P_ETCO₂ at all sites had the same trend as in the normal-lung, open-mouth model. In the restrictive-lung, open-mouth model, 20 L/min of HFNC flow decreased the subglottic P_ETCO₂ to 25 mmHg, and it did not decrease further. As HFNC flow was increased, PEEP up to 7 cmH₂O was gradually generated in the open-mouth models and up to 17 cmH₂O in the normal-lung, closed-mouth model.

Conclusions

The washout effect of the HFNC was effective with relatively low flow in the open-mouth models. The closed-mouth model needed more flow to generate a washout effect. Therefore, HFNC flow should be considered based on the need for the washout effect or PEEP.

A novel cause of rebreathing carbon dioxide related to removed CLIC-seal on the Dräger Apollo© anesthesia machine

We present a case report involving two sequential, surgically uneventful, laparoscopic cholecystectomies using the same anesthesia machine (Drager Apollo©) for which the level of inspired carbon dioxide was noted to be elevated following various diagnostic interventions including replacing the sodalime, increasing fresh gas flows, and a full inspection of equipment for malfunction. Eventually it was discovered that a rubber ring seal connecting the Dragersorb CLIC system© to the sodalime canister was inadvertently removed during the initial canister exchange resulting in an apparent bypassing of the absorbent and thus an inability of the exhaled gas to contact the sodalime. To our knowledge this is the first such description of this potential cause of elevated inspired carbon dioxide and should warrant consideration when other conventional interventions have failed.

Cerebrovascular and blood pressure responses during voluntary apneas are larger than rebreathing

Both voluntary rebreathing (RB) of expired air and voluntary apneas (VA) elicit changes in arterial carbon dioxide and oxygen (CO₂ and O₂) chemostimuli. These chemostimuli elicit synergistic increases in cerebral blood flow (CBF) and sympathetic nervous system activation, with the latter increasing systemic blood pressure. The extent that simultaneous and inverse changes in arterial CO₂ and O₂ and associated increases in blood pressure affect the CBF responses during RB versus VAs are unclear. We instrumented 21 healthy participants with a finometer (beat-by-beat mean arterial blood pressure; MAP), transcranial Doppler ultrasound (middle and posterior cerebral artery velocity; MCAv, PCAv) and a mouthpiece with sample line attached to a dual gas analyzer to assess pressure of end-tidal (P_ET)CO₂ and P_ETO₂. Participants performed two protocols: RB and a maximal end-inspiratory VA. A second-by-second stimulus index (SI) was calculated as P_ETCO₂/P_ETO₂ during RB. For VA, where P_ETCO₂ and P_ETO₂ could not be measured throughout, SI values were calculated using interpolated end-tidal gas values before and at the end of the apneas. MAP reactivity (MAPR) was calculated as the slope of the MAP/SI, and cerebrovascular reactivity (CVR) was calculated as the slope of MCAv or PCAv/SI. We found that compared to RB, VA elicited ~ fourfold increases in MAPR slope (P < 0.001), translating to larger anterior and posterior CVR (P ≤ 0.01). However, cerebrovascular conductance (MCAv or PCAv/MAP) was unchanged between interventions (P ≥ 0.2). MAP responses during VAs are larger than those during RB across similar chemostimuli, and differential CVR may be driven by increases in perfusion pressure.

Arterial to end-tidal CO2 gradients during isocapnic hyperventilation

Isocapnic hyperventilation (ICHV) is occasionally used to maintain the end-expired CO₂ partial pressure (P_ETCO₂) when the inspired CO₂ (P_ICO₂) rises. Whether maintaining P_ETCO₂ with ICHV during an increase of the P_ICO₂ also maintains arterial PCO₂ (P_aCO₂) remains poorly documented. 12 ASA PS I-II subjects undergoing a robot-assisted radical prostatectomy (RARP) (n = 11) or cystectomy (n = 1) under general endotracheal anesthesia with sevoflurane in O₂/air (40% inspired O₂) were enrolled. P_ICO₂ was sequentially increased from 0 to 0.5, 1.0, 1.5 and 2% by adding CO₂ to the inspiratory limb of the circle system, while increasing ventilation to a target P_ETCO₂ of 4.7-4.9% by adjusting respiratory rate during controlled mechanical ventilation. P_a-ETCO₂ gradients were determined after a 15 min equilibration period at each P_ICO₂ level and compared using ANOVA. Mean (standard deviation) age, height, and weight were 66 (6) years, 171 (6) cm, and 75 (8) kg, respectively. Capnograms were normal and hemodynamic parameters remained stable. P_ETCO₂ could be maintained within 4.7-4.9% in all subjects at all times except in 1 subject with 1.5% P_ICO₂ and 5 subjects with 2.0% P_ICO₂; data from the one subject in whom both 1.5 and 2.0% P_ICO₂ resulted in P_ETCO₂ > 5.1% were excluded from analysis. P_a-ETCO₂ gradients did not change when P_ICO₂ increased. The effect of a modest rise of P_ICO₂ up to 1.5% on P_ETCO₂ during RARP can be readily overcome by increasing ventilation without altering the P_a-ETCO₂ gradients. At higher P_ICO₂, airway pressures may become a limiting factor, which requires further study.

Comparison of minimal-flow sevoflurane versus desflurane anesthesia: randomized clinical trial

BACKGROUND AND OBJECTIVES

Minimal-flow anesthesia provides various advantages, such as reduced environmental pollution, proper humidification and warming of anesthetic gases, and reduced costs. The aim of this study was to compare the cost-effectiveness of minimal-flow sevoflurane and desflurane anesthesia and their effects on hemodynamics, postoperative recovery, respiratory parameters, and liver and kidney functions.

METHODS

A total of 60 ASA I-II patients aged 18-70 years who underwent posterior spinal instrumentation were included in the study. The patients were divided into Group S (sevoflurane) and Group D (desflurane). After anesthesia induction, the gas flow was initiated at a rate of 4 L.min^-1 using a concentration of 8% in Group D and 3.5% in Group S, and the time to reach 0.8 MAC was recorded. The gas flow was then switched to minimal flow. Patient hemodynamic and respiratory parameters, body temperatures and arterial blood gas levels were recorded. The integrated pulmonary index (IPI) was monitored postoperatively. Biochemical findings were recorded 12 hours after the operation. The amount of bleeding and blood transfused, and the costs involved were calculated.

RESULTS

The patients' demographic characteristics, duration of surgery, hemodynamic parameters, IPI values, body temperatures, and arterial blood gas levels were similar at all time points. Biochemical findings, amount of bleeding and amount of blood transfused were similar between the two groups. The mean cost was lower in Group S than in Group D (p = 0.007).

CONCLUSION

The study found no significant difference in terms of reliability between minimal-flow sevoflurane and desflurane anesthesia. Furthermore, the procedure was found to be more cost-effective for Group S than for Group D.

Comparison of Mild Physiological Effects of Surgical Masks and N95 Respirators over 60 Minutes at Rest

Dead-space associated rebreathing of expired air and heat-trapping with use of surgical masks and N95 respirators may underlie anecdotal reports of adverse symptoms associated with medical face barriers. Limited data exists directly comparing the physiological effects of masks and respirators at rest. We assessed the short-term physiological effects of both barrier types over 60-min at rest, including face microclimate temperature, end-tidal gases, and venous blood acid-base variables. We recruited 34 participants into two trials: surgical masks (n=17) and N95 respirators (n=17). In a seated position, participants underwent a 10-min baseline without a barrier, and then wore a standardized surgical mask or dome-shaped N95 respirator for 60-min, followed by a 10-min washout. We instrumented participants with a peripheral pulse oximeter (SpO₂) and a nasal cannula connected to a dual gas analyzer for measurement of the pressure of end-tidal (P_ET)CO₂ and P_ETO₂, with an associated temperature probe for face microclimate temperature. Venous (v) blood samples were obtained at baseline and following 60-min mask/respirator wearing to assess PvCO₂, [HCO₃^-]v and pHv. Compared to baseline during/following 60-min, temperature, P_ETCO₂, PvCO₂, and [HCO₃^-]v were mildly but significantly higher, and P_ETO₂ and PvO₂ were significantly lower, but SpO₂ was unaffected. The magnitude of effects was similar between barrier types. Temperature and P_ETCO₂ returned to baseline levels within 1-2 min following removal of the barrier. These mild physiological effects may underly reports of qualitative symptoms while wearing masks or respirators. However, the magnitudes were mild, not physiologically-relevant and reversed immediately with the removal of the barrier.

[Rebreathing method for measuring CO transfer factor in children]

INTRODUCTION

The reference technique to measure the diffusing capacity of the lung for carbon monoxide (DLco) is the single-breath method (sb). For patients unable to perform this method, the rebreathing method (rb) can be used. However, the clinical relevance of DLCOrb has not been evaluated. The aim of this study was to assess the feasibility of the rb method in children seen in a clinical setting and its relationships with sb method.

SUBJECTS AND METHOD

We prospectively included children referred for 1) a suspected or confirmed interstitial lung disease (ILD group) (DLCOsb and DLCOrb measurements) ; 2) controlled asthma with normal lung function (DLCOrb measurements to derive DLCOrb/KCOrb expected values). DLCOrb was computed from the decrease in CO and Helium concentrations during tidal breathing in a rebreathing bag.

RESULTS

Data on DLCOrb measurements were available for 53 (91%) children in the ILD group and 48 (91%) control children (mean (range) 11.5 (4.3-18.2) and 9.5 (4-17) years ; respectively). In the ILD group, high or moderate correlations were found between raw DLCOrb and DLCOsb values (rhô=0.82 ; P<0.0001) and between KCOrb and KCOsb (rhô=0.62 ; P<0.0001), respectively. Results expressed as percentage predicted were moderately correlated (rhô=0.55 ; P=0.0003 for DLCO ; rhô=0.51 ; P=0.001 for KCO).

CONCLUSION

DLCOrb is easy to perform in children and gives values that are highly correlated to DCLOsb. Our preliminary results are in favour of a possible clinical use after further validation.