Indexing cardiovascular and respiratory variables: allometric scaling principles

Static respiratory compliance in anaesthetised and intubated brachycephalic dogs with and without brachycephalic obstructive airway syndrome

The impact of brachycephalic obstructive airway syndrome in dogs (BOAS) on respiratory mechanics is unclear and may affect the choice of ventilation strategies during anaesthesia. This prospective study included 56 client-owned brachycephalic dogs, allocated to be BOAS (n = 26) or non-BOAS dogs (n = 30) based on functional grading. All dogs were anaesthetised using a standardised anaesthetic protocol. Pressure-controlled ventilation was initiated around 30 min post-induction, maintaining peak inspiratory pressure at 7-12 cm H2O. Static respiratory compliance (Cstat) was recorded at predetermined time points in sternal, right and left lateral recumbency. Thorax dimensions were assessed with a tape measure. Body surface area (BSA) was calculated and the ratio Cstat/BSA used as the main outcome variable. Comparison of means/medians, analysis of proportions, the Spearman correlation coefficient and both logistic and linear regression were used for data analysis. P < 0.05 was considered statistically significant. Non-BOAS dogs showed significantly higher Cstat/BSA compared to BOAS dogs in sternal (41.6 (31.1-51.8) vs. 32.9 (24.4 - 39.2), respectively, P = 0.028), right lateral (36.2 (25.7 - 46.4) vs. 27.0 (22.7 - 35.6); P = 0.026) and left lateral (33.6 (22.6 - 45.5) vs. 24.6 (18.4 - 32.2); P = 0.020) recumbencies. For all dogs, the Cstat/BSA ratio was higher in sternal compared to lateral recumbencies. BOAS dogs had a significantly shorter distance between thoracic inlet and last rib compared to non-BOAS dogs (20 ± 4 vs. 23 ± 6 cm, respectively; P = 0.043). Reduced respiratory compliance in BOAS-affected dogs should be considered during mechanical ventilation.

Comparison Between Medetomidine and a Medetomidine-Vatinoxan Combination on Cardiorespiratory Variables in Dogs Undergoing Ovariectomy Anesthetized with Butorphanol, Propofol and Sevoflurane or Desflurane

The aim of this study was to compare the effects of a medetomidine-vatinoxan combination versus medetomidine alone on heart rate (HR) and mean arterial pressure (MAP) in a short-term surgery in dogs. Four groups of 10 dogs were administered as follows: medetomidine and sevoflurane; medetomidine and desflurane; medetomidine-vatinoxan and sevoflurane; and medetomidine-vatinoxan and desflurane. After administration, the increase in MAP soon stopped at 102-104 mmHg in the two groups administered medetomidine-vatinoxan, compared with significantly higher values of 143-126 mmHg achieved in the two groups administered medetomidine alone. The lowest MAPs in the two medetomidine-vatinoxan groups were 46-50 mmHg, while in the medetomidine groups, they were 58-79 mmHg. From 3 min onwards after administration, in the medetomidine-vatinoxan treatments, HR remained at values very close to those of pre-administration, between 83 and 118 beats min-1, while in the medetomidine treatments, it dropped to 36-43 beats min-1 and then slowly rose to reach 71-90 beats min-1. These results encourage the use of vatinoxan in clinical settings, particularly in anesthetic protocols for dogs when bradycardia and an increase in systemic pressure should be avoided. Further clinical studies are needed to manage the short periods of hypotension, as well as the slight reduction in sedative and pain-relieving medetomidine effects found, particularly when vatinoxan is in combination with desflurane rather than sevoflurane.

Effects of vatinoxan in dogs premedicated with medetomidine and butorphanol followed by sevoflurane anaesthesia: a randomized clinical study

OBJECTIVE

To investigate effects of vatinoxan in dogs, when administered as intravenous (IV) premedication with medetomidine and butorphanol before anaesthesia for surgical castration.

STUDY DESIGN

A randomized, controlled, blinded, clinical trial.

ANIMALS

A total of 28 client-owned dogs.

METHODS

Dogs were premedicated with medetomidine (0.125 mg m^-2) and butorphanol (0.2 mg kg^-1) (group MB; n = 14), or medetomidine (0.25 mg m^-2), butorphanol (0.2 mg kg^-1) and vatinoxan (5 mg m^-2) (group MB-VATI; n = 14). Anaesthesia was induced 15 minutes later with propofol and maintained with sevoflurane in oxygen (targeting 1.3%). Before surgical incision, lidocaine (2 mg kg^-1) was injected intratesticularly. At the end of the procedure, meloxicam (0.2 mg kg^-1) was administered IV. The level of sedation, the qualities of induction, intubation and recovery, and Glasgow Composite Pain Scale short form (GCPS-SF) were assessed. Heart rate (HR), respiratory rate (f_R), mean arterial pressure (MAP), end-tidal concentration of sevoflurane (Fe'Sevo) and carbon dioxide (Pe'CO₂) were recorded. Blood samples were collected at 10 and 30 minutes after premedication for plasma medetomidine and butorphanol concentrations.

RESULTS

At the beginning of surgery, HR was 61 ± 16 and 93 ± 23 beats minute^-1 (p = 0.001), and MAP was 78 ± 7 and 56 ± 7 mmHg (p = 0.001) in MB and MB-VATI groups, respectively. No differences were detected in f_R, Pe'CO₂, Fe'Sevo, the level of sedation, the qualities of induction, intubation and recovery, or in GCPS-SF. Plasma medetomidine concentrations were higher in group MB-VATI than in MB at 10 minutes (p = 0.002) and 30 minutes (p = 0.0001). Plasma butorphanol concentrations were not different between groups.

CONCLUSIONS AND CLINICAL RELEVANCE

In group MB, HR was significantly lower than in group MB-VATI. Hypotension detected in group MB-VATI during sevoflurane anaesthesia was clinically the most significant difference between groups.

Cardiovascular and Gas Exchange Effects of Individualized Positive End-Expiratory Pressures in Cats Anesthetized With Isoflurane

Objectives

To compare the effects of four levels of end-expiratory pressure [zero (ZEEP) and three levels of positive end-expiratory pressure (PEEP)] on the cardiovascular system and gas exchange of cats anesthetized with isoflurane and mechanically ventilated for 3 h with a tidal volume of 10 ml/kg.

Study Design

Prospective, randomized, controlled trial.

Animals

Six healthy male neutered purpose-bred cats.

Methods

Anesthesia was induced with isoflurane and maintained at 1.3 minimum alveolar concentration. PEEP of maximal respiratory compliance (PEEP_maxCrs) was identified in a decremental PEEP titration, and cats were randomly ventilated for 3 h with one of the following end-expiratory pressures: ZEEP, PEEP_maxCrs minus 2 cmH₂O (PEEP_maxCrs−2), PEEP_maxCrs, and PEEP_maxCrs plus 2 cmH₂O (PEEP_maxCrs+2). Cardiovascular and gas exchange variables were recorded at 5, 30, 60, 120, and 180 min (T5 to T180, respectively) of ventilation and compared between and within ventilation treatments with mixed-model ANOVA followed by Dunnet's and Tukey's tests (normal distribution) or Friedman test followed by the Dunn's test (non-normal distribution). Significance to reject the null hypothesis was considered p < 0.05.

Results

Mean arterial pressure (MAP—mmHg) was lower in PEEP_maxCrs+2 [63 (49–69); median (range)] when compared to ZEEP [71 (67–113)] at T5 and stroke index (ml/beat/kg) was lower in PEEP_maxCrs+2 (0.70 ± 0.20; mean ± SD) than in ZEEP (0.90 ± 0.20) at T60. Cardiac index, oxygen delivery index (DO₂I), systemic vascular resistance index, and shunt fraction were not significantly different between treatments. The ratio between arterial partial pressure and inspired concentration of oxygen (PaO₂/FIO₂) was lower in ZEEP than in the PEEP treatments at various time points. At T180, DO₂I was higher when compared to T5 in PEEP_maxCrs. Dopamine was required to maintain MAP higher than 60 mmHg in one cat during PEEP_maxCrs and in three cats during PEEP_maxCrs+2.

Conclusion

In cats anesthetized with isoflurane and mechanically ventilated for 3 h, all levels of PEEP mildly improved gas exchange with no significant difference in DO₂I when compared to ZEEP. The PEEP levels higher than PEEP_maxCrs−2 caused more cardiovascular depression, and dopamine was an effective treatment. A temporal increase in DO₂I was observed in the cats ventilated with PEEP_maxCrs. The effects of these levels of PEEP on respiratory mechanics, ventilation-induced lung injury, as well as in obese and critically ill cats deserve future investigation for a better understanding of the clinical use of PEEP in this species.

Computed Tomography Assessment of Tidal Lung Overinflation in Domestic Cats Undergoing Pressure-Controlled Mechanical Ventilation During General Anesthesia

Objective

This study aimed to evaluate lung overinflation at different airway inspiratory pressure levels using computed tomography in cats undergoing general anesthesia.

Study Design

Prospective laboratory study.

Animals

A group of 17 healthy male cats, aged 1.9–4.5 years and weighing 3.5 ± 0.5 kg.

Methods

Seventeen adult male cats were ventilated in pressure-controlled mode with airway pressure stepwise increased from 5 to 15 cmH₂O in 2 cmH₂O steps every 5 min and then stepwise decreased. The respiratory rate was set at 15 movements per min and end-expiratory pressure at zero (ZEEP). After 5 min in each inspiratory pressure step, a 4 s inspiratory pause was performed to obtain a thoracic juxta-diaphragmatic single slice helical CT image and to collect respiratory mechanics data and an arterial blood sample. Lung parenchyma aeration was defined as overinflated, normally-aerated, poorly-aerated, and non-aerated according to the CT attenuation number (−1,000 to −900 HU, −900 to −500 HU, −500 to −100 HU, and −100 to +100 HU, respectively).

Result

At 5 cmH₂O airway pressure, tidal volume was 6.7± 2.2 ml kg⁻¹, 2.1% (0.3–6.3%) of the pulmonary parenchyma was overinflated and 84.9% (77.6%−87.6%) was normally inflated. Increases in airway pressure were associated with progressive distention of the lung parenchyma. At 15 cmH₂O airway pressure, tidal volume increased to 31.5± 9.9 ml kg⁻¹ (p < 0.001), overinflated pulmonary parenchyma increased to 28.4% (21.2–30.6%) (p < 0.001), while normally inflated parenchyma decreased 57.9% (53.4–62.8%) (p < 0.001). Tidal volume and overinflated lung fraction returned to baseline when airway pressure was decreased. A progressive decrease was observed in arterial carbon dioxide partial pressure (PaCO₂) and end-tidal carbon dioxide (ETCO₂) when the airway pressures were increased above 9 cmH₂O (p < 0.001). The increase in airway pressure promoted an elevation in pH (p < 0.001).

Conclusions and Clinical Relevance

Ventilation with 5 and 7 cmH₂O of airway pressure prevents overinflation in healthy cats with highly compliant chest walls, despite presenting acidemia by respiratory acidosis. This fact can be controlled by increasing or decreasing respiratory rate and inspiratory time.

Retrospective evaluation of the use of quick Sepsis-related Organ Failure Assessment (qSOFA) as predictor of mortality and length of hospitalization in dogs with pyometra (2013-2019): 52 cases

OBJECTIVE

To evaluate the prognostic utility of quick Sepsis-related Organ Failure Assessment (qSOFA) for prediction of in-hospital mortality and length of hospitalization in dogs with pyometra.

DESIGN

Retrospective cohort study from February 2013 to April 2019 SETTING: Tertiary referral hospital ANIMALS: Fifty-two dogs referred with confirmed diagnosis of pyometra INTERVENTIONS: None MEASUREMENTS AND PRINCIPAL OUTCOMES: Sixty-five percent of dogs survived to discharge. A cut-off score of ≥2 for qSOFA was associated with in-hospital mortality (odds ratio 6.51 [95% CI: 1.35 - 31.3]) P = 0.019. The area under the receiver operator characteristic curve for a qSOFA score ≥ 2 for mortality was 0.72 (95% CI: 0.59-0.85), with a sensitivity of 77.8% and a specificity of 66.7%. The mean ± SD number of organs with dysfunction was significantly higher in dogs with a qSOFA score ≥2 1.76 ± 0.83 compared to dogs with a qSOFA score < 2 1.08 ± 1.09, P = 0.015. The presence of a qSOFA score ≥ 2 was associated with a longer time of hospitalization in survivors with a median (interquartile range) length of stay in qSOFA < 2 (48 [33]) hours versus qSOFA score ≥ 2 (78 [52]) hours, P = 0.027.

CONCLUSIONS

In dogs with pyometra, the qSOFA score was associated with mortality and length of hospitalization. This score might be useful to improve the risk stratification in dogs with pyometra. Further studies are necessary to evaluate the predictive capacity of qSOFA in other septic patient populations.

Effect of fentanyl, with or without treatment of bradycardia, on the minimum alveolar concentration of isoflurane and cardiovascular function in dogs

OBJECTIVE

To determine the effect of fentanyl on the minimum alveolar concentration of isoflurane (MAC_ISO) and cardiovascular variables in dogs, and how the treatment of bradycardia affects them.

STUDY DESIGN

Prospective, randomized crossover-controlled trial.

ANIMALS

A total of six male Beagle dogs weighing 9.9 ± 0.7 kg (mean ± standard deviation) and aged 13 months.

METHODS

To each dog, two treatments were assigned on different days: fentanyl (FENTA) or fentanyl plus glycopyrrolate (FENTA_glyco) to maintain heart rate (HR) between 100 and 132 beats minute^-1. Determinations of MAC_ISO were performed with 10 plasma fentanyl target concentrations ([Fenta]_Target (0, 0.16, 0.32, 0.64, 1.25, 2.5, 5.0, 10.0, 20.0 and 40.0 ng mL^-1) for FENTA and 5 [Fenta]_Target (0, 1.25, 2.5, 5.0, 10.0 ng mL^-1)) for FENTA_glyco. During each MAC_ISO determination, cardiovascular variables [mean arterial pressure (MAP), HR and cardiac index (CI)] were measured, and systemic vascular resistance index (SVRI) calculated. Pharmacodynamic models were used to describe the plasma fentanyl concentration [Fenta]-response relationship for the effect on MAC_ISO and cardiovascular variables. A mixed-model analysis of variance followed by Dunnett's or Tukey's test, and the Bonferroni adjustment were used for comparisons within and between each treatment, respectively. Significance was set as p < 0.05.

RESULTS

Fentanyl decreased MAC_ISO by a maximum of 84%. The [Fenta] producing 50% decrease in MAC, HR and CI were 2.64, 3.65 and 4.30 ng mL^-1 (typical values of population model), respectively. The prevention of fentanyl-mediated bradycardia caused no significant effect on MAC_ISO, but increased HR, MAP and CI, and decreased SVRI when compared with isoflurane alone.

CONCLUSIONS AND CLINICAL RELEVANCE

Fentanyl caused a plasma concentration-dependent decrease in MAC_ISO, HR and CI and an increase in SVRI. Cardiovascular improvements associated with fentanyl in isoflurane-anesthetized dogs only occurred when the fentanyl-mediated bradycardia was prevented.

Sizing the lung in dogs: the inspiratory capacity defines the tidal volume

Objective

To evaluate a novel physiological approach for setting the tidal volume in mechanical ventilation according to inspiratory capacity, and to determine if it results in an appropriate mechanical and gas exchange measurements in healthy and critically ill dogs.

Methods

Twenty healthy animals were included in the study to assess the tidal volume expressed as a percentage of inspiratory capacity. For inspiratory capacity measurement, the mechanical ventilator was set as follows: pressure control mode with 35cmH₂O of inspired pressure and zero end-expiratory pressure for 5 seconds. Subsequently, the animals were randomized into four groups and ventilated with a tidal volume corresponding to the different percentages of inspiratory capacity. Subsequently, ten critically ill dogs were studied.

Results

Healthy dogs ventilated with a tidal volume of 17% of the inspiratory capacity showed normal respiratory mechanics and presented expected PaCO₂ values more frequently than the other groups. The respiratory system and transpulmonary driving pressure were significantly higher among the critically ill dogs but below 15 cmH₂O in all cases.

Conclusions

The tidal volume based on the inspiratory capacity of each animal has proven to be a useful and simple tool when setting ventilator parameters. A similar approach should also be evaluated in other species, including human beings, if we consider the potential limitations of tidal volume titration based on the calculated ideal body weight.

Comparative Physiology of Fatigue

This review attempts to provide insights into factors associated with fatigue in human and nonhuman animals by using the two fundamental approaches of comparative physiology: determining common principles that govern structure and function in animals that are relatively invariant between animals and evaluating animals that have been highly adapted by natural selection to demonstrate extreme performance. In this review, I approach the topic of fatigue by considering factors that are associated with its reciprocal or inverse or duration of sustained performance before fatigue sets in to end the performance. The two general factors that I consider that affect endurance time more than any other are body temperature and body mass. The former affects endurance time because of thermodynamic effects on chemical reaction rates and metabolism; the latter acts through the mechanism of allometry or scaling. The examples of extreme animal performance that I discuss are two examples of bird migration, the diving performance of marine mammals, and the unique relationship that governs energy cost of locomotion in hopping kangaroos.