Susana Canfrán

Effects of a stepwise lung recruitment manoeuvre and positive end-expiratory pressure on lung compliance and arterial blood oxygenation in healthy dogs

This study was performed to evaluate the effects of a stepwise lung recruitment manoeuvre (RM) on dynamic lung compliance (Cdyn) and gas exchange in mechanically ventilated healthy dogs. Fourteen healthy adult dogs, scheduled for elective surgery in dorsal recumbency were employed. After anaesthetic induction, dogs were mechanically ventilated in a volume-controlled mode (tidal volume, VT=10 mL/kg); positive end-expiratory pressure (PEEP)=0 cm H(2)O; oxygen inspired fraction (FiO(2))=0.4 for 30 min (baseline). The dogs were then randomly allocated into two groups, control and RM. The ventilatory mode was maintained during the whole surgical procedure in the control group without any intervention, as in general practice. The RM was performed in a pressure-controlled mode, with progressive increases of PEEP and end-inspiratory pressure of 5 cm H(2)O until 15 cm H(2)O and 30 cm H(2)O, respectively. After RM, PEEP was decreased to 4 cm H(2)O, and the ventilatory mode was returned to volume-control. Arterial blood gases and Cdyn were determined at baseline, 20 and 60 min afterwards. Students t test and the one-way ANOVA test were employed to compare data. Cdyn increased in the RM group (183 ± 30% and 165 ± 24% at 20 and 60 min, respectively; P=0.000). The baseline partial pressure of arterial oxygen to FiO(2) ratio (PaO(2)/FiO(2) ratio) did not change in the control group, but was higher in the RM group (527 ± 41 mm Hg and 511 ± 46 mm Hg at 20 and 60 min, respectively; baseline 371 ± 34 mm Hg, P<0.001). In conclusion, a stepwise RM followed by the use of PEEP improves Cdyn and oxygenation in mechanically ventilated healthy dogs.

Pressure safety range of barotrauma with lung recruitment manoeuvres: A randomised experimental study in a healthy animal model

CONTEXT Recruitment manoeuvres aim at reversing atelectasis during general anaesthesia but are associated with potential risks such as barotrauma. OBJECTIVE To explore the range of pressures that can be used safely to fully recruit the lung without causing barotrauma in an ex-vivo healthy lung rabbit model. DESIGN Prospective, randomised, experimental study. SETTING Experimental Unit, La Paz University Hospital, Madrid, Spain. ANIMALS Fourteen healthy young New Zealand rabbits of 12 weeks of age. INTERVENTIONS Animals were euthanised, the thorax and both pleural spaces were opened and the animals were allocated randomly into one of two groups submitted to two distinct recruitment manoeuvre strategies: PEEP-20 group, in which positive end-expiratory pressure (PEEP) was increased in 5-cmH2O steps from 0 to 20 cmH2O and PEEP-50 group, in which PEEP was increased in 5-cmH2O steps from 0 to 50 cmH2O. In both groups, a driving pressure of 15 cmH2O was maintained until maximal PEEP and its corresponding maximal inspiratory pressures (MIPs) were reached. From there on, driving pressure was progressively increased in 5-cmH2O steps until detectable barotrauma occurred. Two macroscopic conditions were defined: anatomically open lung and barotrauma. MAIN OUTCOME MEASURES We measured open lung and barotrauma MIP, PEEP and driving pressure obtained using each strategy. A pressure safety range, defined as the difference between barotrauma MIP and anatomically open lung MIP, was also determined in both groups. RESULTS Open lung MIP was similar in both groups: 23.6 ± 3.8 and 23.3 ± 4.1 cmH2O in the PEEP-50 and PEEP-20 groups, respectively (P = 0.91). However, barotrauma MIP in the PEEP-50 group was higher (65.7 ± 3.4 cmH2O) than in the PEEP-20 group (56.7 ± 5 0.2 cmH2O) (P = 0.003) resulting in a safety range of pressures of respectively 33.3 ± 8.7 and 42.1 ± 3.9 cmH2O (P = 0.035). CONCLUSION In this ex-vivo model, we found a substantial difference between recruitment and barotrauma pressures using both recruitment strategies. However, a higher margin of safety was obtained when a higher PEEP and lower driving pressure strategy was used for recruiting the lung.

Comparison of sedation scores and propofol induction doses in dogs after intramuscular administration of dexmedetomidine alone or in combination with methadone, midazolam, or methadone plus midazolam.

The objectives of this study were to determine: (1) the sedative effects of dexmedetomidine in combination with methadone, midazolam, or both, and (2) the propofol dose required to achieve endotracheal intubation in healthy dogs. Seven healthy Beagle dogs were included in a prospective experimental, crossover, randomised and masked design. All dogs received four treatments IM, with at least 1 week between sessions, as follows: dexmedetomidine 5 µg/kg (D) alone, or combined with methadone 0.3 mg/kg (DMe), midazolam 0.3 mg/kg (DMi), or both (DMeMi). The degree of sedation was evaluated using a numerical scale (maximum 15 points). The dose of propofol required for intubation was also calculated for each group. Recovery time and quality were determined. Statistical analysis was performed using parametric (ANOVA) and nonparametric tests (Friedman, Cochran Q), as appropriate. The degree of sedation obtained with DMe and DMeMi (13, [7-14]; 13, [6-14], respectively) was significantly higher than in the control group (2, [1-4]; P = 0.023, P = 0.006, respectively). The required dose of propofol was lower in all groups (DMi, 1.5 ± 0.5 mg/kg, P = 0.002; DMe, 1.2 ± 0.5 mg/kg, P <0.001; DMeMi, 0.9 ± 0.3 mg/kg) than in the control group (2.9 ± 0.9 mg/kg; P <0.001). Recovery quality was not different between groups (P = 0.137). In healthy dogs, the addition of midazolam did not enhance the sedative effects of dexmedetomidine or a dexmedetomidine-methadone combination at the doses studied, and propofol requirements were reduced. The sedative effect of dexmedetomidine was enhanced with methadone, and the required dose of propofol was reduced.

Evaluation of an oesophageal Doppler device for monitoring cardiac output in anaesthetised healthy normotensive dogs

OBJECTIVES To compare cardiac output measured by oesophageal Doppler and by thermodilution monitoring and to correlate the Doppler cardiac output-generated minute distance with thermodilution cardiac output in healthy anaesthetised beagle dogs. MATERIALS AND METHODS Prospective experimental study. Six healthy adult beagle dogs were pre-medicated with intramuscular acepromazine (0 · 05 mg/kg) and methadone (0 · 3 mg/kg). Anaesthesia was induced with intravenous propofol (dose-effect) and maintained with isoflurane in oxygen. Simultaneously, a constant rate infusion of dopamine (3 µg/kg/minute) was administered to the dogs to prevent hypotension. The minute distance, Doppler and thermodilution cardiac outputs were assessed at three different end-tidal concentrations of isoflurane (1 · 0, 1 · 3 and 2 · 0%). RESULTS Correlation between Doppler and thermodilution cardiac output (r(2)  = 0 · 582) and between minute distance and thermodilution cardiac output (r(2)  = 0 · 658) were moderately good, but the limits of agreement between Doppler and thermodilution cardiac outputs were above the recommended values (±39%, for a recommended value up to 30%). CLINICAL SIGNIFICANCE Doppler and minute distance cannot be considered as an alternative method to thermodilution to monitor cardiac output in the healthy anaesthetised dog.

Neonatal Pneumothorax Pressures Surpass Higher Threshold in Lung Recruitment Maneuvers : An In Vivo Interventional Study

BACKGROUND: Causing pneumothorax is one of the main concerns of lung recruitment maneuvers in pediatric patients, especially newborns. Therefore, these maneuvers are not performed routinely during anesthesia. Our objective was to determine the pressures that cause pneumothorax in healthy newborns by a prospective experimental study of 10 newborn piglets (<48 h old) with healthy lungs under general anesthesia. METHODS: The primary outcome was peak inspiratory pressure (PIP) causing pneumothorax. Animals under anesthesia and bilateral chest tube catheterization were randomly allocated to 2 groups: one with PEEP and fixed inspiratory driving pressure of 15 cm H2O (PEEP group) and the second one with PEEP = 0 cm H2O and non-fixed inspiratory driving pressure (zero PEEP group). In both groups, the ventilation mode was pressure-controlled, and PIP was raised at 2-min intervals, with steps of 5 cm H2O until air leak was observed through the chest tubes. The PEEP group raised PIP through 5-cm H2O PEEP increments, and the zero PEEP group raised PIP through 5-cm H2O inspiratory driving pressure increments. RESULTS: Pneumothorax was observed with a PIP of 90.5 ± 15.7 cm H2O with no statistically significant differences between the PEEP group (92 ± 14.8 cm H2O) and the zero PEEP group (89 ± 18.2 cm H2O). The zero PEEP group had hypotension, with a PIP of 35 cm H2O; the PEEP group had hypotension, with a PIP of 60 cm H2O (P = .01). The zero PEEP group presented bradycardia, with PIP of 40 cm H2O; the PEEP group presented bradycardia, with PIP of 70 cm H2O (P = .002). CONCLUSIONS: Performing recruitment maneuvers in newborns without lung disease is a safe procedure in terms of pneumothorax. Pneumothorax does not seem to occur in the clinically relevant PIPs of <50 cm H2O. Hemodynamic impairment may occur with high driving pressures. More studies are needed to determine the exact hemodynamic impact of these procedures and pneumothorax PIP in poorly compliant lungs.

The sedative effects of intramuscular low-dose medetomidine in combination with butorphanol or methadone in dogs.

OBJECTIVE To compare the sedative effects of an intramuscular (IM) low dose of medetomidine in combination with butorphanol or methadone in dogs. STUDY DESIGN Prospective, blinded, randomized clinical trial. ANIMALS Forty-eight healthy adult dogs that required sedation for diagnostic or surgical elective procedures. METHODS Dogs were sedated IM with medetomidine (2.5 μg kg(-1)) and either butorphanol (0.4 mg kg(-1)) or methadone (0.4 mg kg(-1)). The degree of sedation was assessed every 10 minutes, for 30 minutes, using a numeric descriptive scale. Data on heart rate (HR), respiratory rate, capillary refill time, temperature and response to a toe pinch were recorded. The response to venous catheterization at minute 30 was also evaluated. RESULTS Both combinations produced moderate to deep sedation with a maximal effect at 20-30 minutes without significant differences in the degree of sedation between the treatments at any studied time-point. HR decreased from minute 10 to minute 30 with both opioid combinations (p<0.05); this reduction did not differ between groups (p>0.05). No differences between groups were detected in any of the other variables. CONCLUSIONS AND CLINICAL RELEVANCE Combinations of a low dose of medetomidine with butorphanol or methadone, respectively, provide similar degrees of sedation.

Effects of fluid load on cardiovascular function during stepwise lung recruitment manoeuvre in healthy dogs.

The aim of this study was to evaluate the effects of a stepwise lung recruitment manoeuvre (RM) on cardiac output (CO) in mechanically ventilated dogs, with or without a previous fluid load. Eight healthy adult Beagle dogs were enrolled in a prospective crossover study. Following sedation with dexmedetomidine and methadone, anaesthesia was induced with propofol and maintained with isoflurane. CO (thermodilution method) and direct arterial blood pressure were monitored. The dogs were mechanically ventilated in a volume-controlled mode (tidal volume, VT = 10 mL/kg; positive end-expiratory pressure [PEEP] = 0 cm H2O) until normocapnia was achieved (end tidal CO2 35-45 mmHg). The RM was then performed in a pressure-controlled mode, with progressive increases of the PEEP and end-inspiratory pressure of 5 cm H2O, until 15 cm H2O and 30 cm H2O were reached, respectively. After the RM, the ventilatory mode was returned to volume-control, and the PEEP was sequentially decreased to 10, 5 and 0 cm H2O. Baseline ventilation was maintained for 30 min. Next, 10 mL/kg of lactated Ringers solution was administered within 10 min, prior to a second RM. The CO was determined before each RM (baseline) and at each pressure step. A repeated measures ANOVA test was used to compare data. Compared to baseline, CO decreased during the RM in both groups. However, there was a significantly higher CO during the second RM at the highest pressure step (P<0.05) and during all decreasing pressure steps (P<0.05). In conclusion, a previous crystalloid fluid load could reduce the impact of a RM on CO in healthy dogs.

Effects of dexmedetomidine administered at acupuncture point GV20 compared to intramuscular route in dogs

OBJECTIVE To compare the sedative effects produced by dexmedetomidine in dogs, administered either intramuscularly or into the Governing Vessel 20 acupuncture point. MATERIALS AND METHODS Six dogs were sedated with 125 µg/m2 dexmedetomidine injected either intramuscularly in the gluteal muscles or subcutaneously into the acupuncture point and in random order. Sedation and analgesia were assessed blindly before and after treatments at regular intervals for 90 minutes or until the dogs fully recovered. Duration and quality of sedation were assessed with a numerical sedation rating scale and a dynamic and interactive visual analogue scale. Analgesia was also assessed with a numerical rating scale. Heart and respiratory rates and rectal temperatures were recorded. RESULTS Sedative and analgesic scores were significantly increased when dexmedetomidine was administered at the Governing Vessel 20 acupuncture point compared with the routine intramuscular route. Duration of sedation was longer in the acupuncture site injection group compared to the intramuscular group (93 ±38 and 41 ±16 minutes). Bradycardia was significantly more pronounced in the acupuncture site group than the intramuscular group, whereas respiratory rates and rectal temperatures did not differ between administration routes. CLINICAL SIGNIFICANCE Administration at the Governing Vessel 20 acupuncture point increased the duration and degree of sedation and analgesic effects of dexmedetomidine compared with the intramuscular route.

Clinical comparison of the effects of isoflurane or propofol anaesthesia on mean arterial blood pressure and ventilation in dogs undergoing orthopaedic surgery receiving epidural anaesthesia

The aim of this study was to compare the effects on mean arterial pressure (MAP) and ventilation of propofol total IV anaesthesia (TIVA) and isoflurane as anaesthetic maintenance in healthy dogs undergoing orthopaedic surgery, with epidural anaesthesia. Dogs were premedicated IM with dexmedetomidine (4μg/kg) and methadone (0.3mg/kg), induced with IV propofol (0.65-5mg/kg) and randomly assigned to be maintained with isoflurane (group I) or propofol (group P). Isoflurane end-tidal concentration (EtISO) and propofol infusion rate were adjusted during the surgery to maintain a suitable anaesthetic depth. All dogs received bupivacaine (1mg/kg) and morphine (0.1mg/kg) in the lumbosacral epidural space (total volume 0.2mL/kg). MAP was recorded every 5min during the procedure. Statistical analysis was performed using parametric (Students t test) and nonparametric (Mann-Whitney U-test, chi-square) tests, as appropriate. Anaesthetic maintenance in groups I and P was accomplished by providing a mean EtISO of 1.12±0.15% and a mean propofol infusion rate of 15.0±4.7mg/kg/h, respectively. MAP was significantly higher in group P than in group I (92±17mmHg versus 78±10mmHg; P=0.021). Eleven dogs in group P and two dogs in group I reached an EtCO2>7.3kPa, requiring mechanical ventilation (P=0.001). In combination with epidural anaesthesia, propofol TIVA improved MAP and is a suitable alternative to isoflurane in orthopaedic surgery of the hind limb in healthy dogs. Nevertheless, since it was associated with increased respiratory depression, mechanical ventilation should be available.

Metamizole (dipyrone) effects on sevoflurane requirements and postoperative hyperalgesia in rats

Unlike non-steroidal anti-inflammatory drugs (NSAIDs), metamizole has poor anti-inflammatory effects; and is suitable for models where analgesia, but not anti-inflammatory effects, is desirable. Like opioids, these drugs produce perioperative analgesia while reducing anaesthetic requirements, but it remains unclear whether they may develop tolerance or hyperalgesia, and thus decrease in analgesic efficacy. The aim was to determine whether tolerance or hyperalgesia to metamizole occurred in rats, and whether the sevoflurane minimum alveolar concentration (MAC) was affected. In a randomized, prospective, controlled study, male Wistar rats (n = 8 per group) were administered metamizole (300 mg/kg, day 4). Previously, the following treatments were provided: daily metamizole for four days (0–3), morphine (10 mg/kg; positive control, day 0 only) or saline (negative control). The main outcome measures were mechanical (MNT) and warm thermal (WNT) nociceptive quantitative sensory thresholds. The baseline sevoflurane MAC and the reduction produced by the treatments were also determined. The mean (SD) baseline MAC [2.4(0.2)%vol] was decreased by morphine and metamizole by 45(11)% and 33(7)% (P = 0.000, both), respectively. Baseline MNT [35.4(4.5) g] and WNT [13.2(2.4) s] were decreased by morphine and metamizole: MNT reduction of 22(6)% (P = 0.000) and 22(7)% (P = 0.001), respectively and WNT reduction of 34(14)% (P = 0.000) and 24(13)% (P = 0.001). The baseline MAC on day 4 was neither modified by treatments nor the MAC reduction produced by metamizole (days 0 and 4; P > 0.05). In conclusion, repeated metamizole administration may produce hyperalgesia, although it may not modify its anaesthetic sparing effect. The clinical relevance of this effect in painful research models requiring prolonged analgesic therapy warrants further investigation.