Ventilation In Horses Research Articles

Evaluation of histamine-provoked changes in airflow using electrical impedance tomography in horses.

Electrical impedance tomography(EIT) generates thoracic impedance images of the lungs and has been used to assess ventilation in horses. This technique may have application in the detection of changes in airflow associated with equine asthma. The objective was to determine if histamine-induced airflow changes observed with flowmetric plethysmography (Δflow) could also be explained using global and regional respiratory gas flow signals calculated from EITsignals. Experimental in vivo study. Six horses, sedated using detomidine were fitted with a thoracic EITbelt and flowmetric plethysmography hardware. Saline (baseline=BL) and increasing concentrations of histamine (C1-4) were nebulised into the face mask until a change in breathing pattern was clinically confirmed and Δflow increased greater or equal to 50%. After nebulisation Δflow and EIT images were recorded over 3minutes and peak global inspiratory (InFglobal ) and expiratory (ExFglobal ) flow as well as peak regional expiratory and inspiratory flow for the dorsal and the ventral area of the right and left lungs were evaluated. Delta flow, InFglobal and ExFglobal at subsequent concentrations were indexed to baseline (yi =Ci /BL-1). Indexed and nonindexed variables were evaluated for a difference from baseline at sequential histamine doses (time). Multiple linear regression assessment of variance in delta flow was also investigated. Consistent with histamine-provoked increases in Δflow, the global flow indices increased significantly. A significant increase in regional inspiratory flow was seen in the right and left ventral lung and dorsal right lung. Multiple regression revealed that the variance in ExFglobal , and right and left ventral expiratory flow best explained the variance in Δflow (r2 =.82). Low number of horses and horses were healthy. Standardised changes in airflow during histamine challenge could be detected using EIT gas flow variables.

Maintenance of equine anaesthesia over the last 50years: Controlled inhalation of volatile anaesthetics and pulmonary ventilation.

In the first edition of this journal, Barbara Weaver wrote a review titled 'Equine Anaesthesia', stating that, at that time, it was quickly becoming accepted practice that many horses were being anaesthetised 'by essentially similar procedures, i.e. premedication, induction and then maintenance by controlled inhalation'. To celebrate the 50th anniversary of the first edition of this journal, this review covers the development of understanding and practice of inhalational anaesthesia and controlled ventilation in horses over the last 50years. We review how the perceived benefits of halothane led to its widespread use, but subsequently better understanding of halothane's effects led to changes in equine anaesthetic practice and the utilisation of different inhalation agents (e.g. isoflurane and sevoflurane). We discuss how more recently, better understanding of the effects of the 'newer' inhalation agents' effects has led to yet more changes in equine anaesthetic practice, and while, further new inhalation agents are unlikely to appear in the near future, further enhancements to anaesthetic practice may still lead to improved outcomes. We review advances in our understanding of the anatomy and pathophysiology of the equine lung as well of the effects of anaesthesia on lung function and how these predispose to some of the common problems of gas exchange and ventilation during anaesthesia. We identify the aims of optimal mechanical ventilation for anaesthetic management and whether the various methods of ventilatory support during equine anaesthesia achieve them. We also highlight that further developments in equipment and optimal ventilator modes are likely in the near future.

Regional distribution of ventilation in horses in dorsal recumbency during spontaneous and mechanical ventilation assessed by electrical impedance tomography: a case series

ObjectiveTo evaluate the regional distribution of ventilation in horses during spontaneous breathing and controlled mechanical ventilation (CMV) using electrical impedance tomography (EIT). Study designProspective, experimental case series. AnimalsFour anaesthetized experimental horses. MethodsHorses were anaesthetized with isoflurane in an oxygen-air mixture and medetomidine continuous rate infusion, placed in dorsal recumbency with an EIT belt around the thorax, and allowed to breathe spontaneously until PaCO2 reached 13.3 kPa (100 mmHg), when volume CMV was started. For each horse, the EIT signal was recorded for at least 2 minutes immediately before (T1), and at 30 (n = 3) or 60 (n = 1) minutes after the start of CMV (T2). The centre of ventilation (CoV), dependent silent spaces (DSS) (likely to represent atelectatic lung areas), non-dependent silent spaces (NSS) (likely to represent lung areas with low ventilation) and total ventilated area (TVA) were evaluated. Cardiac output (CO) was measured and venous admixture and oxygen delivery (DO2) were calculated at T1 and T2. Data are presented as median and range. ResultsAfter the initiation of CMV, the CoV moved ventrally towards the non-dependent lung by 10% [from 57.4% (49.6–60.2%) to 48.3% (41.9–54.4%)]. DSS increased [from 4.1% (0.2–13.9%) to 18.7% (7.5–27.5%)], while NSS [21.7% (9.4–29.2%) to 9.9% (1.0–20.7%)] and TVA [920 (699–1051) to 837 (662–961) pixels] decreased. CO, venous admixture and DO2 also decreased. Conclusions and clinical relevanceIn spontaneously breathing anaesthetized horses in dorsal recumbency, ventilation was essentially centred within the dependent dorsal lung regions and moved towards non-dependent ventral regions as soon as CMV was started. This shows a major lack of ventilation in the dependent lung, which may be indicative of atelectasis.

Arterial oxygen tension and pulmonary ventilation in horses placed in theAndersonSling suspension system after a period of lateral recumbency and anaesthetised with constant rate infusions of romifidine and ketamine

Some controversy exists over whether or not horses' recovery and cardiopulmonary function are affected by suspension in slings. To measure arterial oxygen tension and pulmonary ventilation in anaesthetised horses placed in a standing position in an Anderson Sling (AS) after a period of right lateral recumbency (RLR). Randomised crossover experimental study. Six Standardbred horses were anaesthetised twice. Catheters were inserted into the right jugular vein and the left carotid artery. After premedication with romifidine, anaesthesia was induced with diazepam and ketamine. Following 50 min in RLR, horses were maintained in either RLR or AS for an additional 60 min through to recovery. Anaesthesia was maintained i.v. with a constant rate infusion of romifidine and ketamine. Heart rate, respiratory rate, mean arterial pressure, expiratory tidal volume, minute volumes and end tidal CO2 were monitored continuously. Venous and arterial bloods were sampled for lactate concentration, creatine kinase activity and blood gas analysis before premedication, after induction, every 20 min for 100 min, as soon as the horse was standing (TR), and 24 h later. The data were averaged within 2 anaesthetic periods: P1, 0-20 min; and P2, 40-100 min. During P2, horses in the RLR group had lower arterial oxygen tension (P = 0.001), higher alveolar-arterial oxygen tension gradient (P = 0.005), higher respiratory rate (P = 0.04) and higher minute volumes (P = 0.04) than horses in the AS group. Arterial CO2 tension and mean arterial pressure increased in the AS group during P2 (P = 0.01 and 0.02 respectively). The recoveries were judged better in the AS group than in the RLR group (P = 0.01). During TR, lactate were higher in the RLR group than in the AS group (P = 0.007). Creatine kinase activities were higher in the AS group at 24 h vs. TR (P = 0.02). Anderson Sling suspension after a period of recumbency improves cardiopulmonary function and recovery quality in horses and therefore might be considered for use to recover hypoxic horses.

Controlled mechanical ventilation in horses under vecuronium blockage

The purpose of this study was to investigate the metabolic and cardiorespiratory effects of the administration of vecuronium under controlled mechanical ventilation versus spontaneous ventilation in horses. Twenty healthy horses scheduled to elective surgery were randomized and assigned in two groups. All animals were pre-medicated with romifidine (100 mu g/kg iv), anesthesia was induced with an association of tiletamine-zolazepam (2 mg/kg iv) and was maintained with halothane. Animals of group I remained in spontaneous ventilation without muscle relaxation while group II received vecuronium (0.1 mg/kg IV) and was submitted to controlled mechanical ventilation. Vecuronium administration did not cause any significant change of heart rate or rhythm, central venous pressure and arterial pressure. With the animals that remained under spontaneous ventilation, no differences were observed in these parameters. The animals that received vecuronium showed lower values of PaCO2 and normal values of pH in relation to the spontaneous ventilation group. Vecuronium duration was 12.83 ± 1.72 minutes. After halothane discontinuation the weaning time in group I I was 6.09 minutes with mean final PaCO2 values of 50.78 mmHg. There was no need of pharmacological reversion of vecuronium effect and recovery from anesthesia was similar in the two groups. In conclusion, the use of mechanical controlled ventilation and vecuronium in horses is easily performed, does not induce further cardiovascular depression and should be employed in equines undergoing major operations.

The effects of locomotor-respiratory coupling on the pattern of breathing in horses.

1. To investigate the effect of locomotor activity on the pattern of breathing in quadrupeds, ventilatory response was studied in four healthy horses during horizontal and inclined (7%) treadmill exercise at different velocities (1.4-6.9 m s(-1)) and during chemical stimulation with a rebreathing method. Stride frequency (f(s)) and locomotor-respiratory coupling (LRC) were also simultaneously determined by means of video recordings synchronized with respiratory events. 2. Tidal volume (V(T)) was positively correlated with pulmonary ventilation (V(E)) but significantly different linear regression equations were found between the experimental conditions (P < 0.0001), since the chemical hyperventilation was mainly due to increases in V(T), whereas the major contribution to exercise hyperpnoea came from changes in respiratory frequency (f(R)). 3. The average f(R) at each exercise level was not significantly different from f(S), although there was not always a tight 1:1 LRC. At constant speeds, f(S) was independent of the treadmill slope and hence the greater V(E) during inclined exercise was due to increased V(T). 4. At any ventilatory level, the differences in breathing patterns between locomotion and rebreathing or locomotion at different slopes derived from different set points of the inspiratory off-switch mechanism. 5. The percentage of single breaths entrained with locomotor rhythm rose progressively and significantly with treadmill speed (P < 0.0001) up to a 1:1 LRC and was significantly affected by treadmill slope (P < 0.001). 6. A LRC of 1:1 was systematically observed at canter (10 out of 10 trials) and sometimes at trot (5 out of 14) and it entailed (i) a 4- to 5-fold reduction in both V(T) and f(R) variability, and (ii) a gait-specific phase locking of inspiratory onset during the locomotor cycle. 7. It is concluded that different patterns of breathing are employed during locomotion and rebreathing due to the interference between locomotor and respiratory functions, which may play a role in the optimization and control of exercise ventilation in horses.

Effect of a mask and pneumotachograph on tracheal and nasopharyngeal pressures, respiratory frequency, and ventilation in horses

Abstract Objective To investigate the effect of a mask and pneumotachograph on ventilation, respiratory frequency, and tracheal and nasopharyngeal pressures in horses running on a treadmill. Design Six horses ran at 50, 75, and 100% of the speed that resulted in maximum oxygen consumption, with and without a mask and pneumotachograph. Tracheal and pharyngeal inspiratory and expiratory pressures, respiratory frequency, and arterial blood gases were measured. Animals Six Standardbred horses. Procedure Oxygen consumption was measured during an incremental exercise test to determine the speed that resulted in maximal oxygen consumption for each horse. Tracheal and pharyngeal pressures were measured, using transnasal tracheal and pharyngeal side-hole catheters connected to differential pressure transducers. Carotid arterial blood samples were collected and PaO2, PaCO2, and pH were measured with a blood gas analyzer. Results Peak tracheal and pharyngeal inspiratory pressures were significantly more negative, peak tracheal and pharyngeal expiratory pressures were significantly more positive and respiratory frequency was significantly lower (all P &lt; 0.05) at all speeds when horses wore a mask The PaCO2, was higher and arterial pH and PaO2, were lower (P &lt; 0.05) when horses wore a mask. Conclusions The mask and pneumotachograph altered upper airway pressures, respiratory frequency, and ventilation in horses running on a treadmill.(Am J Vet Res 1996; 57: 250-253)

Effects of xylazine on ventilation in horses

Summary The effects of 3 commonly used dosages (0.3, 0.5, and 1.1 mg/kg of body weight, iv) of xylazine on ventilatory function were evaluated in 6 Thoroughbred geldings. Altered respiratory patterns developed with all doses of xylazine, and horses had apneic periods lasting 7 to 70 seconds at the 1.1 mg/kg dosage. Respiratory rate, minute volume, and partial pressure of oxygen in arterial blood () decreased significantly (P &lt; 0.001) with time after administration of xylazine, but significant differences were not detected among dosages. After an initial insignificant decrease at 1 minute after injection, tidal volume progressively increased and at 5 minutes after injection, tidal volume was significantly (P &lt; 0.01) greater than values obtained before injection. Partial pressure of carbon dioxide in arterial blood () was insignificantly increased. After administration of xylazine at a dosage of 1.1 mg/ kg, the mean maximal decrease in was 28.2 ± 8.7 mm of Hg and 22.2 ± 4.9 mm of Hg, measured with and without a respiratory mask, respectively. Similarly, the mean maximal increase in PaCO2 was 4.5 ± 2.3 mm of Hg and 4.2 ± 2.4 mm of Hg, measured with and without the respiratory mask, respectively. Significant interaction between use of mask and time was not detected, although the changes in PaO2 were slightly attenuated when horses were not masked. The temporal effects of xylazine on ventilatory function in horses should be considered in selecting a sedative when ventilation is inadequate or when pulmonary function testing is to be performed.

Hemodynamic effects of carbon dioxide during intermittent positive-pressure ventilation in horses

SUMMARY The hemodynamic effects of high arterial carbon dioxide pressure (PaCO2) during anesthesia in horses were studied. Eight horses were anesthetized with xylazine, guaifenesin, and thiamylal, and were maintained with halothane in oxygen (end-tidal halothane concentration = 1.15%). Baseline data were collected while the horses were breathing spontaneously; then the horses were subjected to intermittent positive-pressure ventilation, and data were collected during normocapnia (PaCO2, 35 to 45 mm of Hg), moderate hypercapnia (PaCO2, 60 to 70 mm of Hg), and severe hypercapnia (PaCO2, 75 to 85 mm of Hg). Hypercapnia was induced by adding carbon dioxide to the inspired gas mixture. Moderate and severe hypercapnia were associated with significant (P &lt; 0.05) increases in aortic blood pressure, left ventricular systolic pressure, cardiac output, stroke volume, maximal rate of increase and decrease in left ventricular pressure (positive and negative dP/dtmax, respectively), and median arterial blood flow, and decreased time constant for ventricular relaxation. These hemodynamic changes were accompanied by increased plasma epinephrine and norepinephrine concentrations. Administration of the β-blocking drug, propranolol hydrochloride, markedly depressed the response to hypercapnia. This study confirmed that in horses, hypercapnia is associated with augmentation of cardiovascular function.

High frequency jet ventilation in horses: an experimental study

High frequency jet ventilation (HFJV) is a recently developed mode of ventilation that delivers small tidal volumes at frequencies greater than 60 cycles per min via an injection catheter to the animal's airway. The construction of a high frequency jet ventilator suitable for use in adult horses is described. The effectiveness of this ventilator in maintaining normal arterial blood-gas tension was evaluated in five healthy adult horses. The horses were anaesthetised with intravenous acetylpromazine, guaifenesin, and thiamylal, positioned in lateral recumbency and baseline measurements were made during spontaneous ventilation. The horses were then paralysed with succinylcholine and ventilated for at least 20 mins with HFJV. Air was delivered from the ventilator to the animal by a polyethylene tube. The tip of this tube remained within and approximately 30 cm from the cuffed end of a standard 30 mm internal diameter large animal orotracheal tube. Frequency of flow interruption was 3 Hz with a constant source pressure of 275 kPa and an inspiratory to expiratory ratio of approximately 1:2.6. Gas delivery to the horse, as estimated with a resonator system was approximately 2 litres/breath. During HFJV, arterial carbon dioxide tension was significantly reduced and arterial oxygen tension significantly increased above measurements made when the horses were spontaneously breathing air.

Ventilation and Cardiovascular Studies during Mechanical Control of Ventilation in Horses

Eleven out of 12 horses were underventilating while breathing spontaneously during halothane anaesthesia with high arterial carbon dioxide tensions. In addition, large alveolar to arterial oxygen tension gradients were found to be present. Mechanically, controlled ventilation with an intermittent positive pressure of 20-30 cm H2O reduced arterial carbon dioxide levels to normal. The alveolar to arterial oxygen gradients did not increase and in some cases decreased. These (A - a) Po2 gradients were due mainly to true shunt of the order of 30 per cent and not to ventilation perfusion inequality.