Continuous versus intermittent thermodilution for cardiac output measurement during alveolar recruitment manoeuvres in sheep

Effects of two alveolar recruitment manoeuvres (sustained inflation and stepwise) followed by positive end-expiratory pressure on cardiac output (measured with lithium dilution), invasive blood pressure and arterial oxygen tension in isoflurane-anaesthetised goats

Alveolar recruitment manoeuvres (ARM) performed during general anaesthesia improve oxygenation; however cardiovascular depression may be observed. The aim of the study was to compare the effects of sustained inflation (SI) and stepwise ARMs on cardiac output (CO), mean arterial blood pressure and arterial oxygen tension (PaO2) in ten mechanically ventilated goats anaesthetised with isoflurane. In the SI ARM, peak inspiratory presure (PIP) was increased to 30 cmH2O and sustained for 20 s. In the stepwise ARM, the PIP was increased by 5 cmH2O each minute for three minutes from 10 to 25 cmH2O. Both ARMs were followed by positive end-expiratory pressure of 5 cmH2O. Paired lithium dilution CO measurements and arterial blood samples were obtained before and after each ARM. The order of the ARM was randomised and each goat was subjected to both techniques. Data was reported as median and interquartile range (IQR). Significance was set at 0.05. The median change in CO (measured by subtracting values after and before ARM) was -0.15 L min-1 (IQR -0.51; 0.03) and - 0.90 L min-1 (IQR -1.69; -0.58) for SI and stepwise ARM respectively (p = 0.04). The median change in PaO2 was 3 kPa (IQR -2.7; 7.6) and 0.4 kPa (IQR -3.4; 5.5) for SI and stepwise ARM respectively (p = 0.03). In conclusion, SI ARM causes less impact on CO and provides a better improvement in PaO2 compared to stepwise ARM in goats.

Comparison of the efficacy of two alveolar recruitment manoeuvres in improving the lung mechanics and the degree of atelectasis in anaesthetized healthy sheep

The aim of the present study was to evaluate whether the application of two types of alveolar recruitment manoeuvres (ARMs) followed by a positive end-expiratory pressure (PEEP) improved lung mechanics and the degree of atelectasis caused by general anaesthesia. Twenty-one female Merino sheep were divided into three groups: sustained inflation ARM (ARMsust), stepwise ARM (AMRstep), and control (without ARM). Sheep received detomidine-morphine for premedication, propofol for induction, and isoflurane during general anaesthesia in a volume-controlled mode with 100% oxygen during the first 15 min of anaesthesia and 40% the rest of the study. The right jugular vein and metacarpal artery were catheterised for mixed venous and arterial blood sample collection, respectively. The quasistatic compliance (Cqst), oxygenation parameters, and shunt fraction (Qs/Qt) were monitored before ARM application (TpreARM), and at 10 (T10) and 60 min (T60) after ARM application. A pulmonary histopathological study was conducted on five animals from each group. A significant increase in Cqst was observed in both ARM groups at T10 compared to TpreARM (ARMsust: P = 0.001; ARMstep: P = 0.002), although only the ARMsust group showed significant differences compared to the control group. The ARMstep group presented a significant improvement in oxygenation parameters and Qs/Qt fraction (T10: 4.84 (3.26-16.48)%, P = 0.048; T60: 4.40 (4.31-14.16)%, P = 0.004) compared with TpreARM (21.48 (20.61-28.32)%). The ARMstep group had the highest percentage of alveolar area and the most homogeneous values. In conclusion, the application of a stepwise ARM followed by PEEP improved atelectasis caused by isoflurane anaesthesia in healthy sheep.

Corrective effect of diaphragm pacing on the decrease in cardiac output induced by positive pressure mechanical ventilation in anesthetized sheep

Positive pressure ventilation (PPV) is a fundamental life support measure, but it decreases cardiac output (CO). Diaphragmatic contractions produce negative intrathoracic and positive abdominal pressures, promoting splanchnic venous return. We hypothesized that: 1) diaphragm pacing alone could produce adequate ventilation without decreasing CO; 2) diaphragm pacing on top of PPV could improve CO. Of 11 anesthetized and mechanically ventilated ewes (39.6±5.9kg), 3 were discarded from analysis because of hemodynamic instability during the experiment, and 8 retained for analysis. Phrenic stimulation electrodes were inserted in the diaphragm (implanted phrenic nerve stimulation, iPS). CO was measured by the thermodilution technique (pulmonary artery catheter). CO during end-expiratory apnea served as reference. Median CO was 9.77 [6.25-11.25] lmin^-1 during end-expiratory apnea, 8.25 [5.06-9.25] lmin^-1 during "PPV" (-15%) (p<0.05), 9.19 [5.60-10.19] lmin^-1 during "PPV-iPS" (NS vs apnea) and 9.37 [6.12-10.48] lmin^-1 during "iPS" (NS vs. apnea). iPS-driven ventilation was comparable to its PPV counterpart (median 92% [74-97], NS). Diaphragm pacing alone can produce adequate ventilation without reducing CO. Superimposed onto PPV, diaphragm pacing can reduce the PPV-induced decrease in CO.

Lung recruitment manoeuvres do not cause haemodynamic instability or oxidative stress but improve oxygenation and lung mechanics in a newborn animal model: an observational study

BACKGROUND

Lung recruitment manoeuvres in neonates during anaesthesia are not performed routinely due to concerns about causing barotrauma, haemodynamic instability and oxidative stress.

OBJECTIVE

To assess the influence of recruitment manoeuvres and positive end-expiratory pressure (PEEP) on haemodynamics, oxidative stress, oxygenation and lung mechanics.

DESIGN

A prospective experimental study.

SETTING

Experimental Unit, La Paz University Hospital, Madrid, Spain.

ANIMALS

Eight newborn piglets (<48 h) with healthy lungs under general anaesthesia.

INTERVENTIONS

The recruitment manoeuvres in pressure-controlled ventilation (PCV) were performed along with a constant driving pressure of 15 cmH2O. After the recruitment manoeuvres, PEEP was reduced in a stepwise fashion to find the maximal dynamic compliance step (maxCDyn-PEEP). Blood oxidative stress biomarkers (lipid peroxidation products, protein carbonyls, total glutathione, oxidised glutathione, reduced glutathione and activity of glutathione peroxidase) were analysed.

MAIN OUTCOME MEASURES

Haemodynamic parameters, arterial partial pressure of oxygen (paO2), tidal volume (Vt), dynamic compliance (Cdyn) and oxidative stress biomarkers were measured.

RESULTS

The recruitment manoeuvres did not induce barotrauma. Haemodynamic instability was not detected either in the maximum pressure step (overdistension step 5) or during the entire process. No substantial differences were observed in blood oxidative stress parameters analysed as compared with their baseline values (with 0 PEEP) or the values obtained 180 min after the onset of the recruitment manoeuvres (optimal PEEP). Significant maximal values were achieved in step 14 with an increase in paO2 (32.43 ± 8.48 vs. 40.39 ± 15.66 kPa; P = 0.037), Vt (47.75 ± 13.59 vs. 73.87 ± 13.56 ml; P = 0.006) and Cdyn (2.50 ± 0.64 vs. 4.75 ± 0.88 ml cmH2O; P < 0.001). Maximal dynamic compliance step (maxCdyn-PEEP) was 2 cmH2O.

CONCLUSION

Recruitment manoeuvres in PCV with a constant driving pressure are a well tolerated open-lung strategy in a healthy-lung neonatal animal model under general anaesthesia. The recruitment manoeuvres improve oxygenation parameters and lung mechanics and do not cause barotrauma, haemodynamic instability or oxidative stress.

In vitro and in vivo evaluation of a new large animal spirometry device using mainstream CO2 flow sensors

REASONS FOR PERFORMING STUDY

A spirometry device equipped with mainstream CO2 flow sensor is not available for large animal anaesthesia.

OBJECTIVES

To measure the resistance of a new large animal spirometry device and assess its agreement with reference methods for volume measurements.

STUDY DESIGN

In vitro experiment and crossover study using anaesthetised horses.

METHODS

A flow partitioning device (FPD) equipped with 4 human CO2 flow sensors was tested. Pressure differences were measured across the whole FPD and across each sensor separately using air flows (range: 90-720 l/min). One sensor was connected to a spirometry monitor for in vitro volume (3, 5 and 7 l) measurements. These measurements were compared with a reference method. Five anaesthetised horses were used for tidal volume (VT) measurements using the FPD and a horse-lite sensor (reference method). Bland-Altman analysis, ANOVA and linear regression analysis were used for data analysis.

RESULTS

Pressure differences across each sensor were similar suggesting equal flow partitioning. The resistance of the device increased with flow (range: 0.3-1.5 cmH2 O s/l) and was higher than that of the horse-lite. The limits of agreement for volume measurements were within -1 and 2% in vitro and -12 and 0% in vivo. Nine of 147 VT measurements in horses were outside of the ± 10% limits of acceptance but most of these erroneous measurements occurred with VTs lower than 4 l. The determined correction factor for volume measurements was 3.97 ± 0.03.

CONCLUSIONS

The limits of agreement for volume measurements by the new device were within ± 10% using clinically relevant range of volumes. The new spirometry device can be recommended for measurement of VT in adult Warmblood horses.