Comparative study of ventilation techniques with supraglottic airway devices in cats: volume-controlled vs pressure-controlled techniques

Abstract

Objectives

This study compared the effectiveness of a new supraglottic airway device (SGAD) in cats undergoing anaesthesia using two types of mechanical ventilation: volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV).

Methods

A total of 13 healthy cats (five male, eight female; median age 2 years [range 1–3]) were randomly allocated to either VCV or PCV. Five tidal volumes (6, 8, 10, 12 and 14 ml/kg) and five peak inspiratory pressures (4, 5, 6, 7 and 8 cmH₂O) were randomly applied with a minute ventilation of 100 ml/kg/min. Various parameters, such as blood pressure, gas leakage, end-tidal CO₂ (ETCO₂) and work of breathing (WOB), were measured while using VCV or PCV.

Results

The occurrence of hypotension (mean arterial blood pressure <60 mmHg) was slightly less frequent with VCV (38 events, 65 ventilating sessions) than with PCV (40 events, 65 ventilating sessions), but this difference did not reach statistical significance (P = 0.429). The number of leakages did not differ between the VCV group (3 events, 65 ventilating sessions) and the PCV group (3 events, 65 ventilating sessions) (P = 1.000). Hypercapnia was identified when using VCV (10 events, 65 ventilating sessions) less frequently than when using PCV (17 events, 65 ventilating sessions), but this difference did not reach statistical significance (P = 0.194). The study found a significantly higher WOB in the PCV group compared with the VCV group (P <0.034).

Conclusions and relevance

The present results suggested that both VCV and PCV can be used with an SGAD during anaesthesia, with VCV preferred for prolonged mechanical ventilation due to its lower workload. Adjusting tidal volume or inspiratory pressure corrects hypercapnia.

Keywords

Hypotension pressure-controlled ventilation respiratory work supraglottic airway device volume-controlled ventilation

Introduction

Maintenance of the upper airway is a major concern during the administration of general anaesthesia.¹ The preferred technique for achieving airway control in small animals is the insertion of an endotracheal tube (ETT), recognised as the gold standard method for general anaesthesia.² However, it has been associated with various complications in cats, such as soft-tissue swelling, laryngeal oedema,^3,4 arytenoid tears,⁵ tracheal mucosal damage⁶ and even tracheal wall compression due to high cuff pressure⁷ causing ischemia and leading to tracheal tears.

A supraglottic airway device (SGAD) was developed and designed specifically for the anatomy of cats for assisting with airway patency. The SGAD is available in various sizes to accommodate cats of different sizes during general anesthesia.^8,9 The SGAD features a specialised cuff that covers the laryngeal inlet and an inflatable component that can enhance the sealing pressure. It is worth noting that a previous study revealed a higher risk of peri-anaesthetic mortality in cats with an ETT, even when considering confounding factors such as American Society of Anesthesiologists (ASA) status and surgical procedure.¹⁰ Using an SGAD instead of an ETT has the potential to reduce the complications^2,11 associated with tracheal intubation in cats. A recent study in cats showed that while placement of the SGAD was easy to execute, approximately 7% of cases required device replacement due to mispositioning or dislocation.¹² A laryngeal mask airway (LMA), which is one type of SGAD, can be effectively utilised for mechanical ventilation in humans with normal airway resistance and compliance.¹³ Comparisons have been made between the application of an SGAD and ETT in cats, employing either pressure-controlled ventilation (PCV) or volume-controlled ventilation (VCV).^2,9 Nevertheless, a definitive direct comparison between PCV and VCV when utilising an SGAD in cats has not been established, leaving the distinct effects of different ventilation modes with an SGAD in cats uncertain.

Modern anaesthetic machines are furnished with an advanced mechanical ventilator with various ventilatory modes.¹⁴ VCV and PCV are traditionally used to support patients’ ventilation, and patients can take spontaneous breaths between the mandatory breaths.¹⁵ VCV has been a preferred mode of ventilation during the intraoperative period.¹⁶ It is also a commonly used ventilatory mode in veterinary anaesthesia; the ventilator delivers the preset tidal volume with a constant air flow at the preset respiratory rate, regardless of the associated pressure required.¹⁷ The advantage of VCV is that it provides not only a constant tidal volume but also a controllable minute volume.¹⁸ In comparison, a peak inspiratory pressure (PIP) determines the pressure gradient from the initiation and the end of inspiration, thus influencing the tidal volume and minute ventilation delivered to patients using PCV.⁹ PCV offers mandatory PCV with predetermined pressure levels. Both PCV and VCV are advantageous for patients that do not exhibit spontaneous respiration, especially in cases of emergency and respiratory distress.¹⁹ PCV has been shown to improve arterial oxygenation by promoting the uniform distribution of pulmonary gas and reducing peak airway pressure, thereby decreasing the incidence of barotrauma.^20,21 The effects of VCV and PCV on respiratory function in feline patients using an SGAD remain elusive.

The aims of the present study were to test the hypothesis that comparing the effects of VCV at different tidal volume settings and PCV at different pressure settings with a fixed minute ventilation in cats undergoing general anaesthesia using an SGAD would reveal differences in the work of breathing (WOB) and its components between VCV and PCV. In addition, we also investigated whether other variables, such as blood pressure, occurrence of gas leakage and end-tidal CO₂ (ETCO₂) measured during general anaesthesia, would differ between VCV and PCV.

Materials and methods

Animals

This study was approved by the Kasetsart University Institutional Animal Care and Use Committee (approval number ACKU61-VET-046) and by the Ethical Review Board of the Office of National Research Council of Thailand (NRCT licence U1-07457-2561). The study involved 13 cats visiting the dental unit at the Kasetsart University Veterinary Teaching Hospital in Bangkok, Thailand, between June 2018 and June 2019. The cats’ owners were informed about the procedure and provided informed consent.

All cats recruited for this study underwent a physical examination, a complete blood count and serum biochemistry. Only cats classified according to the ASA physical status as ASA I, with body condition scores ranging between 2/5 and 4/5, were included in the study. All cats in the present study underwent professional dental examinations to assess oral health.²² Cats with severe pre-existing conditions were excluded from the study. These evaluations, performed by experienced veterinarians specialising in veterinary dentistry, were not study-specific but part of routine comprehensive assessments. Food was withheld for 8–12 h and water for 4–6 h before anaesthesia was administered. The VCV and PCV modes of ventilation were randomly applied to each cat in our study. The SGAD, sizes C3–C4 (the classic V-gel Supraglottic Airway Device; Docsinnovent), were used for all 13 cats.

Anaesthesia

All anaesthetic protocols were executed by the same veterinarian (NN). Before undergoing general anaesthesia, the cats’ health status was assessed through a physical examination, and their body temperature, heart rate and electrocardiogram (EKG) results were noted. An intravenous (IV) catheter was placed in a cephalic vein, and a 100 ml normal saline bag (0.9% NSS; General Hospital Products) was initiated at a rate of 5 ml/kg/h. The cats were then preoxygenated for 5 mins before induction. Anaesthesia induction involved IV propofol (1% w/v; Troikaa Pharmaceuticals) at a dose of 2 mg/kg, followed by additional propofol administered at a slow rate of 2 mg/kg over 45 s. The depth of anaesthesia was assessed in all cats based on four criteria: palpebral reflex; jaw tone; protraction of tongue; and reaction of the larynx.²³ A moderate depth of anaesthesia was confirmed before endotracheal intubation or V-gel insertion by ensuring the absence of palpebral and pedal reflexes, and observing slackness in jaw tone and minimal or no gagging. The amount of propofol required for induction in each cat was then recorded in milligrams per kilogram (mg/kg).

A puff of lidocaine 10% spray (Lidocaine 10 mg/puff; AstraZeneca) was applied to desensitise the larynx, and a water-based lubricant (VetLube; Docsinnovent) was applied to the SGAD before insertion to allow a tight seal between the SGAD and the larynx. Before inserting the SGAD, the tongue was pulled outward. The device was inserted without a laryngoscope, and the dorsal cuff was inflated with 1–2 ml of air. A tie gauze was wrapped around the animal’s mouth and securely fastened. This helps to secure the SGAD in place, prevent tube slippage and minimise leakage during mechanical ventilation. Capnography was measured by placing the airway connector between the airway device and the Y-piece of the anaesthesia machine (FLOW-i; Maquet Critical Care). An infant circle rebreathing system was used when cats were under general anaesthesia, and the general anaesthesia was maintained with sevoflurane (SEVO; Singapore Pharmawealth Lifesciences) with an oxygen/air mixture (fraction of inspired oxygen, FiO₂, targeted at 90%) flow at 2 l/min. The end-tidal concentration of sevoflurane in each cat was initially set at 2.5% (approximately 1 minimum alveolar concentration [MAC]). If a cat developed spontaneous ventilation, the anaesthetic concentration was increased up to 1.2 MAC (3.0% sevoflurane) until the cat no longer exhibited spontaneous ventilation at the time of measurement. Throughout the study, the cat’s body temperature was maintained at 37.78ºC (100ºF). A water-circulating blanket (Warm Pad TP700; Soar Medical-Tech) and a Bair Hugger body-warming blanket (Breeze; Be Hos Group) were used throughout the period of anaesthesia to prevent hypothermia.

Controlled mechanical ventilation and monitoring

After inserting the SGAD, and while the cats had spontaneous breathing, pulmonary and cardiovascular parameters (respiratory rate, heart rate calculated from EKG, mean arterial blood pressure (MABP), oxygen saturation (SpO₂), ETCO₂, tidal volume, inspiratory tidal volume [VTi], expiratory tidal volume [VTe] and PIP) were measured and recorded to establish baseline values. When the depth of anaesthesia was stable, controlled mechanical ventilation was initiated by the anaesthesia machine (FLOW-i; Maquet Critical Care). Cats were permitted to breathe spontaneously and recuperate during general anaesthesia for 20 mins before starting the VCV or PCV protocol. This timeframe was implemented to mitigate the effects associated with the use of propofol for induction in the study. Mechanical ventilation was randomly assigned, either VCV (starting at 10 ml/kg) or PCV (starting with PIP at 6 cmH₂O). Leaks were checked, and cats with leaks during the recuperation period or with a dislodged V-gel during the experiment were not enrolled in the study.

In all cats, the inspiratory-to-expiratory time ratio (I:E ratio) was set at 1:2. Five VTis (6, 8, 10, 12 and 14 ml/kg) were equally and randomly applied during the study to assess the efficacy of VCV, and the respiratory rates (6–20 breaths/min) were adjusted to achieve a minute ventilation of 100 ml/kg/min. For PCV protocol, five PIPs (4, 5, 6, 7 and 8 cmH₂O) were equally and randomly applied to assess the efficacy of PCV, and the respiratory rates (6–20 breaths/min) were adjusted to achieve a minute ventilation of 100 ml/kg/min. Tidal volumes (VTi) in the PCV protocol were recorded in accordance with the machine’s report. The order of receiving VCV and PCV was randomised. Within the VCV and PCV groups, the VTi and PIP were randomly altered every 3 mins, respectively. Vital signs, including SpO₂, heart rate, EKG, body temperature and oscillometric non-invasive blood pressure, were recorded every minute using a multi-parameter monitor (CARESCAPE Monitor B650; GE Healthcare). In addition, the ETCO₂, respiratory rate, VTi, VTe, PIP, sevoflurane concentration and gas leakage were recorded every minute using the anaesthesia machine (FLOW-i; Maquet Critical Care).

A small polyethylene tubing (PE90) with a length of 4 cm was attached to the end of the CO₂ sampling line, allowing us to obtain a gas sample at the junction of the SGAD and the elbow gas sampling connector. To ensure accurate measurements, the FLOW-i anaesthetia machine performed self-checks for ETCO₂, tidal volume and airway pressure at the start of each experiment. The accuracy verification of these measurements is crucial, ensuring the validity of the study results and supporting reliable conclusions. The respiratory system’s static compliance (Crs) and airway resistance (Rrs) were quantified through the assessment of end-inspiratory hold during VCV or PCV, employing the static method with the use of the FLOW-i anaesthesia machine. The WOB was determined by the product of pressure and volume (WOB = P × V) for each breath.²⁴ To quantify the WOB, we utilised screenshots of pressure-volume loops. The total observed area (80 ml × 20 cmH₂O = 1600 ml.cmH₂O) served as the benchmark. The quantification of WOB involved measuring the enclosed area within the pressure-volume loop and then comparing it with the total observed area, enabling the calculation of WOB under VCV or PCV for each cat.

A MABP <60 mmHg was considered to be hypotension. The measurements were taken at the end of each 3 min step, after allowing sufficient time for parameter stabilisation. The VTi and VTe were reported in real time by the anaesthetia machine. The leakage was detected by measuring the difference between VTi and VTe. Hypercapnia was defined as an ETCO₂ >45 mmHg. Body temperature was monitored every 15 mins by inserting the temperature probe into the oesophagus.

Recovery

After the removal of the SGAD, all cats were monitored for 1 h for breathing patterns and upper respiratory airway discomfort, including stridor, coughing, retching and hoarse voice. After full recovery from the general anaesthesia, cats were returned to their owners. The owners were told to observe and record any abnormal signs in the first 24 h at home. Clinical signs including dry cough, voice changes, gagging and stridor may indicate problems associated with the use of the SGAD, such as laryngitis and tracheitis.

Statistical analysis

STATA12 (StataCorp) was employed to estimate the required sample size based on preliminary data. A paired t-test with a power of 80% and an alpha error of 0.05 was utilised to detect the frequency of hypercapnia occurrences during mechanical ventilation in both VCV and PCV modes. The data normality was evaluated using a Shapiro–Wilk test. Since the VCV and PCV were conducted in the same cats, all static respiratory measurements, work of breathing, resistive inspiratory work and resistive expiratory work of cats in the VCV and PCV groups were analysed using a paired t-test. A Fisher’s exact test was used to identify whether overall number of leakage and overall number of hypercapnia were different between the VCV and PCV groups. Cardiovascular-respiratory parameters (tidal volume, respiratory rate, heart rate, ETCO₂, SpO₂, body temperature and MABP) were compared within the group to identify the effects of tidal volume variations (6, 8, 10, 12 and 14 ml/kg) in VCV or PIP variations (4, 5, 6, 7 and 8 cmH₂O) in PCV using one-way ANOVA with Tukey’s multiple comparisons test. The significance level was set at P <0.05.

Results

A total of 13 client-owned cats (five male, eight female; all domestic shorthair cats; median age 2 years [range 1–3]; median weight 3.6 kg [range 2.7–4.6]; median values for body condition score 3 [range 2.5–4]) were included in the study. The mean amount of propofol required for induction anaesthesia in each cat was 7.10 ± 1.4 mg/kg. Tidal volume or inspiratory pressure as well as respiratory rates were adjusted in each cat to allow a minute ventilation of 100 ml/kg/min, and there was no significant difference in respiratory minute volume between the VCV and PCV groups (P >0.05) (Figure 1). The respiratory rates were also adjusted for each tidal volume of controlled ventilation to achieve a minute ventilation of 100 ml/kg/min, and there was no significant difference in respiratory rates between the VCV and PCV groups (P >0.05) (Figure 1). Airway resistance was compared between the VCV group (at a tidal volume of 10 ml/kg) and the PCV group (at an inspiratory pressure of 6 cmH₂O) with a minute ventilation of 100 ml/kg/min (Table 1). The static compliance, static resistance and static elastance of the VCV and PCV groups were comparable (Table 1).

Figure 1

Comparison of average minute ventilation (MVe) in cats during mechanical ventilation with either volume-controlled ventilation (VCV) or pressure-controlled ventilation (PCV) using a supraglottic airway device. There was no significant difference between the VCV and PCV groups at the different volume or pressure settings

Table 1

Static respiratory measurements in cats (n = 13) undergoing volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) using a supraglottic airway device

Static respiratory measurement	VCV	PCV	P value
Peak inspiratory pressure (cmH₂O)	6.7 ± 1.4	6.0 ± 0.0	0.108
Static compliance (ml/cm H₂O)	6.5 ± 2.1	6.3 ± 2.8	0.814
Static resistance (cmH₂O/l/s)	24.9 ± 15.8	23.7 ± 9.1	0.719
Static elastance (cmH₂O/ml)	169.0 ± 54.0	197.2 ± 79.7	0.152

Data are mean ± SD. Static respiratory measurements were recorded using a minute ventilation of 100 ml/kg/min with either a tidal volume of 10 ml/kg or an inspiratory pressure of 6 cmH₂O

The number of airway leakages (>20% of the baseline tidal volume) in the present study protocol did not differ between VCV (three events, 65 ventilating sessions) and PCV groups (three events, 65 ventilating sessions, P = 1.000) (Table 2). There was no significant difference in the number of leakages between the VCV group (three events, 65 ventilating sessions) and the PCV group (three events, 65 ventilating sessions; P = 1.000). The occurrence of hypercapnia determined by capnography (ETCO₂ >45 mmHg) was also compared between the VCV and PCV groups. Hypercapnia was observed to occur less frequently during the use of VCV (10 events, 65 ventilating sessions) compared with PCV (17 events, 65 ventilating sessions). No significant difference was detected in the number of hypercapnia occurrences between the VCV and PCV groups (P = 0.194) (Table 2).

Table 2

Airway leakage and hypercapnia identified in cats (n = 13) undergoing volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) using a supraglottic airway device

VCV
Tidal volume (ml/kg)	6	8	10	12	14	Total
Leak >20% of baseline (number of cats)	1	0	0	1	1	3
Hypercapnia (CO₂ >45 mmHg) (number of cats)	4	3	1	1	1	10
PCV
Peak inspiratory pressure (cmH₂O)	4	5	6	7	8	Total
Leak >20% of baseline (number of cats)	0	0	0	1	2	3
Hypercapnia (CO₂ >45 mmHg) (number of cats)	5	6	2	2	2	17

The cardiorespiratory variables of the VCV and PCV groups are shown in Table 3. The mean heart rate, MABP, ETCO₂, SpO₂ and body temperature remained stable throughout the experiment, with no significant difference between the VCV and PCV groups. The respiratory rate varied, but this was due to the adjustment for a similar minute ventilation as indicated in Table 3. The non-invasive blood pressure measurement revealed a significant number of cats with hypotension (MABP <60 mmHg) in both the VCV and PCV groups (Table 4). However, there was no significant difference in hypotension between the different tidal volume settings of the VCV group and the different PIP settings of the PCV group. Hypotension was slightly less common in the VCV (38/65 events) vs the PCV (40/65 events) group, but this difference did not reach statistical significance (P = 0.858) (Table 4).

Table 3

Comparison of cardiorespiratory parameters recorded during tail clamping for determination of sevoflurane minimum alveolar concentration in cats (n = 13) undergoing volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) using a supraglottic airway device

Parameter	VCV
Tidal volume (ml/kg)	6	8	10	12	14
Respiratory rate (breaths/min)	16 ± 1*	12 ± 1^†	10 ± 1^‡	9 ± 1^‡§	8 ± 1^§
Heart rate (beats/min)	125 ± 22	123 ± 22	126 ± 26	124 ± 22	123 ± 23
ETCO₂ (mmHg)	44 ± 4	43 ± 4	41 ± 4	41 ± 5	42 ± 8
SpO₂ (%)	98 ± 1	98 ± 1	98 ± 1	98 ± 1	98 ± 1
Temperature (°F)	100.6 ± 0.7	100.5 ± 0.7	100.5 ± 0.7	100.5 ± 0.7	100.5 ± 0.7
MABP (mmHg)	54 ± 5	54 ± 8	62 ± 8	58 ± 9	58 ± 11
Parameter	PCV
Pressure (cmH₂O)	4	5	6	7	8
Respiratory rate (breaths/min)	14 ± 6*	12 ± 3*^†	11 ± 2*^†‡	9 ± 2^†‡	8 ± 2^‡
Heart rate (beats/min)	125 ± 20	126 ± 18	124 ± 20	123 ± 20	124 ± 19
ETCO₂ (mmHg)	44 ± 5	43 ± 5	43 ± 4	42 ± 5	40 ± 4
SpO₂ (%)	98 ± 1	98 ± 1	98 ± 1	98 ± 1	98 ± 1
Temperature (°F)	100.5 ± 0.6	100.6 ± 0.6	100.6 ± 0.6	100.5 ± 0.6	100.5 ± 0.6
MABP (mmHg)	58 ± 9	56 ± 7	58 ± 9	60 ± 8	58 ± 12

Data are mean ± SD. The symbols indicate a significant difference between groups (in the same row); P <0.05

ETCO₂ = end tidal CO₂; SpO₂ = oxygen saturation; MABP = mean arterial blood pressure

Table 4

Effects of ventilation on the occurrence of hypotension in cats (n = 13) undergoing volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) using a supraglottic airway device

VCV			PCV			P value
Tidal volume (ml/kg)	Hypotension		Pressure (cmH₂O)	Hypotension
Tidal volume (ml/kg)	Positive	Negative	Pressure (cmH₂O)	Positive	Negative
6	10	3	4	9	4	1.000
8	10	3	5	9	4	1.000
10	4	9	6	8	5	0.238
12	7	6	7	7	6	1.000
14	7	6	8	7	6	1.000
Total	38	27	Total	40	25	0.858

The WOB of the PCV group was significantly higher than that of the VCV group (P <0.034) (Figure 2). Higher respiratory work indicates that a greater amount of energy is required to overcome the elastic and resistive properties of the respiratory system. The resistive inspiratory work to resistive expiratory work ratio during respiration in cats in the VCV group was significantly lower than that of the PCV group (Figure 3a). This is due to the significant reduction in resistive work during inspiration in the VCV group compared with the PCV group (Figure 3b).

Figure 2

Comparison of the work of breathing (WOB) associated with volume-controlled ventilation (VCV) vs pressure controlled ventilation (PCV) in cats during mechanical ventilation using a supraglottic airway device. (a) Pressure-volume loop in a cat under mechanical ventilation. (b) WOB comparison between VCV and PCV

Figure 3

Analysis of the resistive works during inspiration and expiration in cats undergoing mechanical ventilation with volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV). (a) The resistive inspiratory work to resistive expiratory work ratio. (b) The resistive inspiratory work and the resistive expiratory work. *P <0.05 vs VCV, **P <0.01 vs VCV

Discussion

Studies on the use of SGADs in cats undergoing general anaesthesia with mechanical ventilation are limited. Although some studies have compared the use of ETTs with SGADs in cats, this study explored the use of SGADs with VCV and PCV in cats undergoing general anaesthesia. We compared the effects of VCV and PCV not only on cardiorespiratory parameters, but also on WOB and its components. Our results suggested that there is no significant difference in the effects of VCV and PCV on cardiorespiratory parameters. Nonetheless, there was significantly less respiratory work with the application of VCV compared with PCV.

During general anaesthesia, a secure airway is important to ensure patient safety.⁹ The use of an ETT is a standard procedure for intubation, although it may be challenging in cats with a small trachea and a tendency to laryngospasm. The SGAD has been designed based on the anatomy of a cat’s pharynx and larynx, with an inflatable cuff allowing a seal around the larynx;^2,8 thus, it is possible to use an SGAD instead of an ETT during mechanical ventilation.

An SGAD can help maintain airway patency for routine procedures, such as dental examinations, as well as during emergency procedures.^9,22 To prevent fluid aspiration, inflate the SGAD with a dorsal cuff using air, pack gauzes at the larynx and employ suction during dental procedures. In addition, the utilisation of an SGAD enables rapid airway securing for feline anaesthesia without the need for specialised equipment, such as a laryngoscope,^9,17 and has been shown to be as effective as the endotracheal intubation.⁹ In human medicine, SGADs should be avoided in non-fasting patients,²⁵ as well as in those with poor pulmonary compliance, high airway resistance, pharyngeal pathology, risk for aspiration or airway obstruction below the larynx.²⁶ Comparison of airway leakage was conducted between the VCV and PCV groups, revealing instances of leakage in both groups. Real-time measurements revealed leakage by assessing the difference between VTi and VTe, proving to be more effective than audible detection.³ There was no significant difference in leakage between the VCV and PCV groups. When leakage occurred, it had the potential to impact the accuracy of various respiratory parameters, including VTi, VTe, PIP, respiratory resistance and WOB.

In the present study, static resistance was compared between the VCV and PCV groups using ventilator-derived measurements. Static resistance in the PCV group was significantly higher than in the VCV group (Figure 2). These findings can be explained in part due to the differences in the flow and pressure patterns of gas delivery between VCV and PCV. The pressure waveform of PCV is square, leading to a rapid inspiratory flow rate, followed by a descending ramp pattern of inspiratory flow.²⁷ The inspiratory flow rate of VCV is constant, leading to a parabolic pressure waveform. The pressure waveform during the use of VCV is highly variable depending on the compliance and the resistance of the total respiratory system.²⁸ This variability is influenced by the relative contribution of the lung and the chest wall to the total respiratory system mechanics.

The interplay of tidal volume and ventilation rate impacts both alveolar ventilation and respiratory dead space.²⁹ In the present study, ETCO₂ was monitored as a measure of alveolar ventilation and respiratory dead space in cats undergoing mechanical ventilation with an SGAD using either VCV or PCV. Elevated levels of carbon dioxide (hypercapnia) serve as a noteworthy indicator of potential ventilation occurring in dead space. With comparable minute ventilation, hypercapnia was identified when using lower pressure settings in PCV (6–8 mmHg) or lower tidal volumes in VCV (6–8 ml/kg), despite the respiratory rate adjustment. A well-established principle of respiratory physiology indicates that positive pressure ventilation entails both benefits and negative hemodynamic effects.¹⁸ The beneficial effects include that it improves gas exchange, decreases WOB and rests the respiratory muscles.³⁰ Nevertheless, positive pressure ventilation can elevate thoracic pressure, potentially reducing systemic venous return to the heart and concurrently decreasing afterload to the left ventricle and cardiac output.^31–33 These effects will be more pronounced in hypovolaemic patients, which tend to have low central venous pressure.³⁴ Our study found that the SGAD had comparable cardiovascular effects in both the PCV and VCV groups, with a similar occurrence of hypotension. The potential cause of hypotension in both groups might be associated with a reduction in preload caused by increased intrathoracic pressure.³⁵ Moreover, we hypothesise that the activation of the vasovagal reflex due to SGAD application may induce hypotension by slowing down the heart rate.³⁶ Topical anaesthesia of the upper airway before application of the SGAD is important and may minimise vagus nerve stimulation and hypotension. The hypotensive effects of propofol were unlikely since we allowed a 20 min recovery period during general anaesthesia before commencing mechanical ventilation to mitigate the effects associated with the use of propofol for induction in the study.

Mechanical ventilation is recommended in cases where arterial blood gas analysis reveals severe hypoventilation (PCO₂ >60 mmHg) and significant hypoxemia persist despite oxygen therapy (PaO₂ <60 mmHg), severe circulatory shock or excessive WOB.^37,38 This intervention aims to alleviate tissue hypoxia associated with circulatory shock, optimising oxygenation, ventilation and overall respiratory function, contributing to the stabilisation of the patient in critical condition. WOB is determined based on the pressure-volume characteristics of the respiratory system, representing the effort required to overcome the tendency of the lung to collapse. Measuring WOB is crucial for assessing the status of patients during controlled mechanical ventilation. Therefore, WOB, calculated as the integral of the product of volume and pressure, was compared between the VCV and PCV groups. Generally, PCV is considered to have lower WOB than VCV.²⁸ However, our results revealed that VCV is associated with lower WOB and a more stable tidal volume compared with PCV, leading to a more stable minute volume. In contrast, in PCV, the ventilator delivers a constant pressure, which can result in variable tidal volumes and may require additional work by the patient to overcome the tendency for lung collapse. This finding is in line with our previous study using an ETT.³⁹ Chronic use of a ventilator is associated with increased WOB and may lead to respiratory failure in humans. It is helpful to use a ventilator mode with a lower WOB for prolonged use of a ventilator in unconscious patients or those in the intensive care unit. In patients with an airway obstruction, the application of VCV should be avoided because elevated PIP may occur, leading to a ventilator-induced lung injury.

The present study has certain limitations as it was designed to compare the immediate effects of VCV and PCV with the use of an SGAD. To obtain a comprehensive understanding of the impacts of VCV and PCV on patients with prolonged mechanical ventilation, further studies are needed, particularly those investigating the long-term effects of mechanical ventilation. It is important to recognise additional limitations, such as the small sample size and the inclusion of only clinically healthy animals without respiratory signs. Therefore, additional research is required to explore the effects of various ventilator modes on patients experiencing respiratory conditions or respiratory failure. Another limitation of the present study is the use of ETCO₂ as a marker for ventilation and the use of SpO₂ as a marker for low oxygen saturation levels, as it may not accurately reflect the presence of increased alveolar dead space caused by low MABP during anaesthesia. Arterial blood gas analysis for partial pressure of arterial carbon dioxide (PaCO₂) serves as an ideal marker of ventilation in the presence of alveolar dead space.

Conclusions

The present results indicated that both VCV and PCV can be used effectively with an SGAD to maintain airway control with minimal leakage in cats undergoing general anaesthesia. The occurrence of hypercapnia can be corrected by adjusting the tidal volume or inspiratory pressure. Hypotension is commonly found and may be due to the vasovagal reaction. VCV is recommended for use with an SGAD in cats experiencing respiratory distress and undergoing prolonged mechanical ventilation due to its lower respiratory workload in comparison to PCV.

Footnotes

Acknowledgements

The authors would like to thank the staff at Kasetsart University Veterinary Teaching Hospital, Bangkhen, and the cat owners who participated in this study.

Accepted: 20 December 2023

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The present study was financially supported by the Faculty of Veterinary Medicine, Kasetsart University.

Ethical approval

The work described in this manuscript involved the use of non-experimental (owned or unowned) animals and procedures that differed from established internationally recognised high standards (‘best practice’) of veterinary clinical care for the individual patient. The study therefore had prior ethical approval from an established (or ad hoc) committee as stated in the manuscript.

Informed consent

Informed consent (verbal or written) was obtained from the owner or legal custodian of all animal(s) described in this work (experimental or non-experimental animals, including cadavers) for all procedure(s) undertaken (prospective or retrospective studies). For any animals or people individually identifiable within this publication, informed consent (verbal or written) for their use in the publication was obtained from the people involved.

ORCID iD

Naris Thengchaisri

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