How does experimental pulmonary embolism decrease CO2 elimination?

Pressure-controlled versus manual facemask ventilation for anaesthetic induction in adults: A randomised controlled non-inferiority trial

BACKGROUND

Pressure-controlled face mask ventilation (PC-FMV) with positive end-expiratory pressure (PEEP) after apnoea following induction of general anaesthesia prolongs safe apnoea time and reduces atelectasis formation. However, depending on the set inspiratory pressure, a delayed confirmation of a patent airway might occur. We hypothesised that by lowering the peak inspiratory pressure (PIP) when using PC-FMV with PEEP, confirmation of a patent airway would not be delayed as studied by the first return of CO2 , compared with manual face mask ventilation (Manual FMV).

METHODS

This was a single-centre, randomised controlled non-inferiority trial. Seventy adult patients scheduled for elective day-case surgery under general anaesthesia with body mass index between 18.5 and 29.9 kg m-2 , American Society of Anesthesiologists (ASA) classes I-III, and without anticipated difficult FMV, were included. Before the start of pre-oxygenation and induction of general anaesthesia, participants were randomly allocated to receive ventilation with either PC-FMV with PEEP, at a PIP of 11 and a PEEP of 6 cmH2 O or Manual FMV, with the adjustable pressure-limiting valve set at 11 cmH2 O. The primary outcome variable was the number of ventilatory attempts needed until confirmation of a patent airway, defined as the return of at least 1.3 kPa CO2 .

RESULTS

The return of ≥1.3 kPa CO2 on the capnography curve was observed after mean ± SD, 3.6 ± 4.2 and 2.5 ± 1.9 ventilatory attempts/breaths with PC-FMV with PEEP and Manual FMV, respectively. The difference in means (1.1 ventilatory attempts/breaths) had a 99% CI of -1.0 to 3.1, within the accepted upper margin of four breaths for non-inferiority.

CONCLUSION

Following induction of general anaesthesia, PC-FMV with PEEP was used without delaying a patent airway as confirmed with capnography, if moderate pressures were used.

Transcutaneous carbon dioxide monitoring as a predictive tool for all-cause 6-month mortality after acute pulmonary embolism

BACKGROUND

Pulmonary embolism (PE) frequently remains undiagnosed. The partial pressure of carbon dioxide (Pa_CO2)_, a surrogate of dead-space ventilation, is useful in the evaluation of the degree of pulmonary artery occlusion. At present, there is no knowledge about the prognostic role of Pa_CΟ2 variations during the first hours of an acute PE. Transcutaneous measurement of CO₂ (Ptc_CO2) is a simple, non-invasive method that correlates well with Pa_CO2 levels, evaluated in this study for the first time in patients with PE.

PURPOSE

To assess the correlation between Ptc_CO2 and Pa_CO2 levels in the acute phase of PE and the role of Ptc_CO2 in predicting 6-months mortality.

METHODS

This was a pilot study including 53 patients with acute PE who hospitalized in Respiratory Medicine Department at University Hospital of Larissa in central Greece during 15 months. Ptc_CO2 was constantly monitored for four hours after PE diagnosis with the TCM40 monitoring system (SmartCal). Simultaneous arterial blood gas measurements were performed. Each patient was prospectively recorded for six months via standard telephone calls.

RESULTS

Pa_CO2 and Ptc_CO2 values were well-correlated in the acute phase of PE. Decreased Ptc_CO2 levels in the first monitoring hour were associated with a higher risk of mortality. In the PE subgroup who died, the lower Ptc_CO2 level in the first hour of PE was a predictor of shorter survival time independently of gender, age, comorbidities, and smoking status.

CONCLUSION

Ptc_CO2 measurement, especially in the first hour after PE, seemed to be a valid tool in predicting all-cause 6-month mortality.

Biologically variable ventilation improves gas exchange and respiratory mechanics in a model of severe bronchospasm*

OBJECTIVE

Mechanical ventilation can be lifesaving for status asthmaticus, but how best to accomplish mechanical ventilation is unclear. Biologically variable ventilation (mechanical ventilation that emulates healthy variation) and conventional control mode ventilation (monotonously regular) were compared in an animal model of bronchospasm to determine which approach yields better gas exchange and respiratory mechanics.

DESIGN

A randomized prospective trial of biologically variable ventilation vs. control mode ventilation in swine.

SETTING

University research laboratory.

SUBJECTS

Eighteen farm-raised pigs.

INTERVENTIONS

Methacholine was administered as a nebulized aerosol to initiate bronchospasm, defined as doubling of peak inspiratory pressure and respiratory system resistance, and then randomized (n = 9 each group) to either continue control mode ventilation or switch to biologically variable ventilation at the same minute ventilation. Over the next 4 hrs, hemodynamics, blood gases, respiratory mechanics, and carbon dioxide expirograms were recorded hourly. At end-experiment, tracheobronchial lavage was undertaken to determine interleukin-6 and -10 concentrations.

MEASUREMENTS AND MAIN RESULTS

Measurements of physiologic variables and inflammatory cytokines showed that biologically variable ventilation significantly improved gas exchange, with greater arterial oxygen tensions (p = .006; group x time interaction), lower arterial carbon dioxide tensions (p = .0003; group effect), lower peak inspiratory pressures (p = .0001; group x time), greater static compliance (p = .0001; group x time), greater dynamic compliance (p = .0001; group x time), and lower total respiratory system resistance (p = .028; group x time), compared with conventional ventilation. The appearance of inflammatory cytokines in bronchoalveolar lavage fluid (interleukin-6 and -10) was not affected by mode of ventilation.

CONCLUSIONS

In this experimental model, biologically variable ventilation was superior to control mode ventilation in terms of gas exchange and respiratory mechanics during severe bronchospasm.

Effects of alveolar dead-space, shunt andV˙/Q˙distribution on respiratory dead-space measurements

BACKGROUND

Respiratory dead-space is often increased in lung disease. This study evaluates the effects of increased alveolar dead-space (Vd(alv)), pulmonary shunt, and abnormal ventilation perfusion ratio (/) distributions on dead-space and alveolar partial pressure of carbon dioxide (Pa(co(2))) calculated by various methods, assesses a recently published non-invasive method (Koulouris method) for the measurement of Bohr dead-space, and evaluates an equation for calculating physiological dead-space (Vd(phys)) in the presence of pulmonary shunt.

METHODS

Pulmonary shunt, / distribution and Vd(alv) were varied in a tidally breathing cardiorespiratory model. Respiratory data generated by the model were analysed to calculate dead-spaces by the Fowler, Bohr, Bohr-Enghoff and Koulouris methods. Pa(co(2)) was calculated by the method of Koulouris.

RESULTS

When Vd(alv) is increased, Vd(phys) can be recovered by the Bohr and Bohr-Enghoff equations, but not by the Koulouris method. Shunt increases the calculated Bohr-Enghoff dead-space, but does not affect Fowler, Bohr or Koulouris dead-spaces, or Vd(phys) estimated by the shunt-corrected equation if pulmonary artery catheterization is available. Bohr-Enghoff but not Koulouris or Fowler dead-space increases with increasing severity of / maldistribution. When alveolar Pco(2) is increased by any mechanism, Pa(co(2)) calculated by Koulouris' method does not agree well with average alveolar Pco(2).

CONCLUSIONS

Our studies show that increased pulmonary shunt causes an apparent increase in Vd(phys), and that abnormal / distributions affect the calculated Vd(phys) and Vd(alv), but not Fowler dead-space. Dead-space and Pa(co(2)) calculated by the Koulouris method do not represent true Bohr dead-space and Pa(co(2)) respectively, but the shunt-corrected equation performs well.

Severe Hypercapnia Due to Pulmonary Embolism of Polymethylmethacrylate During Vertebroplasty

Pulmonary polymethylmethacrylate embolism is a rare but potentially fatal complication of percutaneous vertebroplasty. Clinical signs are typical for pulmonary embolism: they include respiratory distress, hypotension, and decreases in end-tidal CO(2). We report a case of fatal pulmonary polymethylmethacrylate embolism during percutaneous vertebroplasty that initially presented with hypertension (arterial blood pressure 190/90 mm Hg), normocardia, and hypercapnia (PaCO(2) 96 mm Hg), along with loss of consciousness. Several pieces of polymethylmethacrylate were found in the pulmonary vasculature at autopsy.

IMPLICATIONS

Osteoporotic spine fractures are increasingly treated by injection of bone cement into the vertebral body. Polymethylmethacrylate embolism is a rare but potentially fatal complication. We report on a case of polymethylmethacrylate embolism that was at first unrecognized because of uncharacteristic signs and symptoms.

Non-steady state monitoring by respiratory gas exchange

Traditionally, the study of CO2 and O2 kinetics in the body has been mostly confined to equilibrium conditions. However, the peri-anesthesia period and the critical care arena often involve conditions of non-steady state. The detection and explanation of CO2 kinetics during non-steady state pathophysiology have required the development of new methodologies, including the CO2 expirogram, average alveolar expired PCO2, and CO2 volume exhaled per breath. Several clinically relevant examples of non-steady state CO2 kinetics perturbations are examined, including abrupt decrease in cardiac output, application of positive end-expiratory pressure during mechanical ventilation, and occurrence of pulmonary embolism. The lesser known area of non-steady state O2 kinetics is introduced, including the measurement of pulmonary O2 uptake per breath. Future directions include the study of the respiratory quotient per breath, where the anaerobic threshold during anesthesia is identified by increasing respiratory quotient.

Carbon dioxide kinetics and capnography during critical care

Greater understanding of the pathophysiology of carbon dioxide kinetics during steady and nonsteady state should improve, we believe, clinical care during intensive care treatment. Capnography and the measurement of end-tidal partial pressure of carbon dioxide (PETCO2) will gradually be augmented by relatively new measurement methodology, including the volume of carbon dioxide exhaled per breath (VCO2,br) and average alveolar expired PCO2. Future directions include the study of oxygen kinetics.

Bymixer provides on-line calibration of measurement of CO2 volume exhaled per breath

The measurement of CO2 volume exhaled per breath (VCO2.br) can be determined during anesthesia by the multiplication and integration of tidal flow (V) and PCO2. During side-stream capnometry, PCO2 must be advanced in time by transport delay (TD), the time to suction gas through the sampling tube. During ventilation, TD can vary due to sample line connection internal volume or flow rate changes. To determine correct TD and measure accurate VCO2.br during actual ventilation. TD can be iteratively adjusted (TDADJ) until VCO2-br/tidal volume equals PCO2 measured in a mixed expired gas collection (PECO2) (J Appl. Physiol. 72:2029-2035, 1992). However. PECO2 is difficult to measure during anesthesia because CO2 is absorbed in the circle circuit. Accordingly, we implemented a bypass flow-mixing chamber device (bymixer) that was interposed in the expiration limb of the circle circuit and accurately measured PECO2 over a wide range of conditions of ventilation of a test lung-metabolic chamber (regression slope = 1.01: R2 = 0.99). The bymixer response (time constant) varied from 18.1 +/- 0.03 sec (12.5 l/min ventilation) to 66.7 +/- 0.9 sec (2.5 l/min). Bymixer PECO2 was used to correctly determine TDADJ (without interrupting respiration) to enable accurate measurement of VCO2.br over widely changing expiratory flow patterns.