Korean Journal of
Veterinary Service

In the context of anesthetic respiration, positive ventilation is often required instead of spontaneous ventilation in several clinical scenarios. For instance, in cases of respiratory distress in which the patient is unable to breathe spontaneously due to severe respiratory failure, positive ventilation becomes necessary. In addition, during surgical procedures that require muscle relaxation, spontaneous breathing may be inhibited, thereby necessitating the use of positive pressure ventilation. In trauma patients, injuries that impair the respiratory muscles or nerves can make artificial ventilation essential when spontaneous breathing is compromised. Furthermore, the administration of anesthetics or other medications that depress the central nervous system can lead to a reduction or cessation of spontaneous respiration, making positive ventilation critical. Finally, individuals with compromised lung function, such as those with chronic obstructive pulmonary disease (COPD) or asthma, may also require positive ventilation support to ensure effective gas exchange (Naimark and Cherniack, 1960; Emirgil and Sobol, 1973; Luce, 1980; Nagels et al., 1980; Suratt et al., 1984; Manens et al., 2012; Ary et al., 2016; Monte et al., 2018; Aroas et al., 2021). Overall, in these scenarios, positive ventilation is utilized to support the patient’s breathing, thereby ensuring adequate oxygen delivery and carbon dioxide removal. Guidelines for intermittent positive pressure ventilation (IPPV) found in various studies and textbooks are quite extensive (Blumenthal et al., 1998; Hartsfield, 2007; Hartsfield, 2010). The suggested tidal volume (Vt) for ventilation is typically in the range of 10 to 20 mL per kilogram of body weight (BW), with recommended respiratory rates varying from 8 to 30 breaths per minute. The inspiratory to expiratory (I:E) ratios are usually set between 1:3 and 1:2. Furthermore, an inspiratory pressure of approximately 15 cmH₂O is often advised. While arterial blood gas analysis is recognized as the gold standard for evaluating ventilation, capnography serves as a useful alternative for estimating paCO₂ levels (Hartsfield, 2007). Despite these monitoring recommendations, such tools are not accessible in all clinical settings implementing IPPV. In situations where these assessments are unavailable, the existing guidelines may offer limited assistance to less experienced practitioners tasked with configuring a ventilator. Some studies have scrutinized these recommended ranges and found that the guidelines concerning inspiratory pressure and Vt are inconsistent and challenging to apply for effective ventilation (Blumenthal et al., 1998). Using inappropriate IPPV settings, such as an excessively large Vt or high inspiratory pressures, can lead to diminished venous return, decreased cardiac output, and hypotension (Hartsfield, 2010). Animals that are hypovolemic or have cardiovascular issues may be particularly vulnerable to these adverse effects. Consequently, improved recommendations for IPPV could enhance patient outcomes.

Obesity is known to cause mechanically impaired respiration, which is often accompanied by gas exchange abnormalities (Fraoise and Hubert, 1993). Differences in lung volume and function are evident: underweight dogs typically have relatively large lungs for their BW, necessitating a higher Vt for effective ventilation. In contrast, overweight or obese dogs often experience compression of their lungs due to excessive fatty tissue, resulting in reduced lung volume and limited capacity for expansion, which requires a lower Vt. Furthermore, variations in body composition play a significant role; underweight dogs have less muscle mass and minimal fatty tissue, allowing their lungs to move more freely during ventilation. In contrast, obese dogs tend to accumulate excess fat in the abdomen and thorax, which interferes with lung expansion and contraction and further leads to decreased Vt requirements. The efficiency of the respiratory muscles can also differ based on the dog’s physical condition: obese dogs are more likely to have suppressed or inefficient respiratory muscles due to fat accumulation, while underweight dogs may require a higher Vt due to their respiratory muscles functioning with greater ease.

This study investigated factors to be considered when applying IPPV for appropriate ventilation according to obesity level.

Animals

This retrospective study enrolled dogs that underwent anesthesia and were maintained using MV at the Time Animal Medical Center between August 2022 and April 2023. Among them, only dogs without suspicion of respiratory disease were included in the study. The inclusion criteria were as follows: 1) dogs without any respiratory clinical signs before, during, or after anesthesia; 2) dogs without abnormalities on pre-anesthetic thoracic radiographs; and 3) dogs without abnormalities on lung auscultation before anesthesia. Dogs with incomplete clinical or anesthetic records were excluded from the study.

Data analysis

The clinical records of the dogs with data including age, breed, sex, BW, body condition score (BCS), clinical signs, and purpose of the anesthesia were analyzed. The BCSs of the dogs were scored from 1 to 9. After reviewing the clinical records, the dogs were divided based on their BCSs into Group 1 (BCS=1 to 3), Group 2 (BCS=4 to 6), and Group 3 (BCS=7 to 9) (Kimberly et al., 2010).

The following parameters from the anesthetic records were analyzed: ventilation machine, ventilator settings including inspiratory pressure (Pinsp), inspiratory rise time, time slope, and positive end-expiratory pressure (PEEP), and Vt. The Vt of each dog was automatically measured by the machine and recorded.

The K value, a dimensionless parameter expressing Vt per kilogram BW, was determined (K value= Vt value). The K value was monitored continuously under driving pressure mode.

Pressure-controlled ventilation

All dogs in this study were ventilated using a microprocessor-controlled mechanical ventilator (Dräger Primus, Drägerwerk AG & Co. KGaA, Lübeck, Germany) set to pressure-controlled ventilation mode. In this mode, two pressure levels are maintained: the lower pressure level, PEEP, and the upper pressure level, Pinsp. The ventilator settings, including the PEEP, Pinsp, and number of mandatory breaths per minute (respiratory rate, RR), were adjustable to accommodate variations in the patient’s lung mechanics, which influenced the resulting Vt and minute volume (MV). During ventilation, the slope adjustment allowed for control over the pressure rise to the upper pressure level based on individual patient needs. This adjustment was particularly utilized in neonatal ventilation to determine the pressure increase. The inspiratory phase duration was defined by the inspiratory time (Ti), with the upper pressure level (Pinsp) maintained throughout this phase. The initiation of the next mandatory breath was determined by the RR and Ti. Notably, this time control is not employed in pressure-controlled synchronized ventilation (PC-PSV). Variations in lung mechanics, which include changes in resistance (R) and compliance (C), may alter the Vt applied during treatment. However, the pressures Pinsp and PEEP remained constant and were maintained even in cases of leakage.

Ventilator settings

The ventilator settings for all subjects were standardized with a Pinsp of 10 cmH₂O and a set inspired-to-expired time ratio along with monitoring of Vt. All animals were positioned in dorsal recumbency during ventilation.

Statistical analysis

Statistical analysis was performed using SPSS version 27.0 (IBM SPSS Statistics). The Kolmogoros-Smirnov test was used to assess data for normality. Only K value and Vt in Group 3 were normally distributed; therefore, non-parametric analyses were performed in this study. To evaluate for differences in the age, BW, Vt, and K value according to the BCS group, Kruskal-Wallis tests were performed, and post-hoc analyses were performed using Bonferroni correction with the Mann-Whitney U test. Spearman correlation analysis was performed to evaluate the correlation between BCS and K value. Values of P<0.05 were considered significant.

Study population

A total of 90 dogs were included in this study. None of the dogs showed any respiratory clinical signs or abnormalities on thoracic radiography or auscultation, and none were excluded from the analysis. The study included 90 dogs of various breeds including 18 Maltese, 21 Poodles, 8 Bichon Frises, 10 Pomeranians, and 33 mixed breed dogs. Their ages ranged from 0.5 to 15 years, with an average age of 5.6 years. The sex distribution was 33 intact males, 12 neutered males, 28 intact females, and 17 spayed females. The average BW was 4.6 kg (±2.3 kg), and the average blood pressure was 146.0 mmHg (±22.1 mmHg). The purpose of anesthesia included 20 ovariohysterectomies, 28 castrations, and 42 medial patellar luxation treatments. Based on their BCSs, 26 dogs (28.8%), 48 dogs (53.3%), and 16 dogs (17.7%) were categorized into Groups 1, 2, and 3 respectively. There were no significant differences in age, sex, and BW among the BCS groups (Table 1).

Table 1 . Clinical data of dogs included in the study.

Clinical data	Dogs (n=90)
Breed	Maltese (18), Poodle (21), Bichon Frise (8), Pomeranian (10), Mix (33)
Age (yr)	5.6 (0.5∼15)
Sex	Intact male (n=33), neutered male (n=12), intact female (n=28), spayed female (n=17)
Body weight (kg)	4.6±2.3
Blood pressure (mmHg)	146.0±22.1
Purpose of anesthesia	Ovariohysterectomy (20), castration (28), medial patella luxation (42)

Anesthetic record

As a single-institution retrospective analysis, all dogs included in this study received the same anesthetic protocol. Maintaining and recording of the dogs during anesthesia were performed using a controlled ventilation machine (Drager primus, Drägerwerk AG & Co. KGaA. Lübeck, Germany). All dogs were anesthetized ith the following settings: Pinsp=10 hPA, inspiratory rise time=1.5 sec, time slope=0.0 sec, and PEEP=0 hPA.

K value according to the BCS groups

There were significant differences in K value among the BCS groups (P<0.001). The higher BCS group had a lower K value (Table 2). In the post-hoc analysis, the K value of Group 3 (10.9 mL/kg, 6.2∼13.1 mL/kg) was significantly lower than those of Group 2 (17.7 mL/kg, 10.0∼36.6 mL/kg, P<0.001) and Group 1 (23.6 mL/kg, 9.1∼44.7 mL/kg, P<0.001). The K value of Group 2 was significantly lower than that of Group 1 (P<0.001). The BCSs of dogs were significantly negatively correlated with the K value (P=0.001, r=−0.629).

Table 2 . The K value determined in dog groups with different body condition score.

	Group 1 (n=26)	Group 2 (n=48)	Group 3 (n=16)
Age (yr)	3.75 (0.5∼12)	5.77 (0.6∼15)	7.87 (1∼13)
Body weight (kg)	3.68 (1.75∼16.5)	5.56 (2.1∼13.7)	7.13 (4.3∼16.6)
Tidal volume (mL)	78.73 (43∼150)	94.7 (32∼304)	77.69 (45∼186)
Body condition score	2.9 (2∼3)	4.93 (4∼6)	7.69 (7∼9)
K value (mL/kg)	23.6 (9.1∼44.7)*	17.7 (10∼36.6)#	10.9 (6.2∼13.1)

*P<0.001 compared to the Group 3; ^#P<0.001 compared to the Group 3..

This study examined the variability in Vt requirements among mechanically ventilated dogs with differing BCSs. Our findings indicate a significant variation in the Vt and Vt per kilogram (K value) across the BCS groups, with leaner dogs requiring higher Vts than their heavier counterparts. These results align with those of previous research suggesting that BW alone does not reliably predict Vt (Doris, 2012), emphasizing the necessity of considering body condition when administering MV (Fraoise and Hubert, 1993).

The K value analysis revealed that the higher BCS group had significantly lower Vt/kg than both the ideal and underweight groups. The observed decrease in Vt with an increasing BCS can be attributed to the mechanical impairments imposed by obesity, such as reduced lung volume and diminished respiratory muscle efficiency (Lazarus et al., 1997). These findings underscore the need for tailored ventilation strategies in overweight dogs to avoid potential complications such as decreased venous return and cardiac output, which may occur when inappropriate ventilation settings are applied (Naimark and Cherniack, 1960; Luce, 1980; Nagels et al., 1980).

Interestingly, the results of this study also highlight the potential for using body condition rather than BW alone as a more refined metric for setting ventilation parameters. This approach may enhance the precision of ventilation management, particularly in practices dealing with a wide range of breed sizes and physical conditions (McKiernan and Johnson, 1992; Balakrishnan and King, 2014; Ary et al., 2016; Bumbacher et al., 2017).

The discrepancies observed between the Vt needs of underweight and overweight dogs highlight the mechanical challenges imposed by body composition on respiratory function (Manens et al., 2012). In underweight dogs, the relatively larger lung capacity relative to body size requires a higher Vt to maintain effective alveolar ventilation. In contrast, overweight dogs are prone to lung compression from excess adipose tissue, necessitating lower Vts to maintain ventilation within safe limits (Emirgil and Sobol, 1973; Suratt et al., 1984; Monte et al., 2018).

One limitation of this study is that it was conducted on patients requiring specific surgeries, which led to a non-uniform distribution of BCSs among the subjects. In our clinical setting, there was a higher prevalence of dogs with normal BCSs than of those categorized as underweight or overweight, potentially introducing bias in the results. Furthermore, age-based classification was not performed, which may have underestimated age-related variations in respiratory and metabolic rates that could influence Vt requirements.

In addition, while the BCS was used in this study to evaluate Vt requirements, the inherent subjectivity of the BCS system may result in evaluator variability, potentially affecting the consistency of findings across different clinical settings. Lean BW, which could provide further insights into respiratory mechanics given its potential impact on Vt, was also not measured in this study.

The retrospective design and single-institution focus of this study limit the generalizability of the findings to broader canine populations, especially since small to medium-sized breeds primarily under 15 kg were represented. Furthermore, the application of a uniform Pinsp of 10 cmH₂O may not account for individual physiological differences or variations necessary in dogs with comorbid conditions that were not included in this sample.

For more accurate results, future research could benefit from conducting experiments on more homogeneous groups or measuring changes in Vt in the same subjects as their BCS varies. Expanding the demographic and clinical diversity of the study population would enhance the applicability and robustness of the outcomes across various clinical scenarios and breeds.

In conclusion, the study reinforces the importance of considering body condition in the management of MV in dogs. By tailoring ventilation strategies according to the BCS, veterinary practitioners can achieve more effective and safer outcomes, minimizing the risk of adverse events associated with improper ventilation settings. Future studies should aim to extend these findings across larger populations and different clinical settings to validate and refine predictive models for better integration into practice.

No potential conflict of interest relevant to this article was reported.