Anaesthesia and outcome
The mean (range) duration of anaesthesia was 208 (145–300) minutes for the colic horses and 230 (193–273) minutes for the healthy horses. The mean (range) time from discontinuation of anaesthesia until the standing position was regained was 52 (15–105) minutes in the colic horses and 53 (18–75) minutes in the healthy horses. Eight colic horses needed one or two attempts to stand. Two colic horses (C8, C14) never regained the standing position. The quality of recovery for those horses that regained the standing position was mostly good, it was violent in one horse (C13) and another horse (C15) did some paddling before regaining the standing position. Both of these horses had signs of slight hind limb dysfunction for one day. Seven of the ten colic horses survived at least 24 h after recovery to standing. One horse (C8) died from cardiovascular collapse and pulmonary oedema 65 min after termination of inhalation anaesthesia without ever making any attempts to stand or lie in the sternal position. One mare (C14) was in severe pain and had spontaneous reflux of gastric contents and metabolic acidosis (BE: -17) in the recovery box. She made one assisted, but unsuccessful, attempt to stand. This horse was nine months pregnant and was euthanised 3 h after discontinuation of inhalation anaesthesia. The third non-surviving horse (C19) was euthanised 14 h after standing due to progressive endotoxemia and bloody diarrhoea. Of the surviving colic horses four showed mild to moderate gait disturbances from the hind limbs during the study period. Clinical signs of myopathy (swollen, sore muscles) were not detected.
The healthy horses stood after one to four attempts (median 1.5). One healthy horse (H2) made several violent attempts to stand but without injuring itself. Two other horses were distressed during their attempts to stand and both of these showed symptoms of post-anaesthetic myopathy post anaesthesia; one had a slightly painful gracilis muscle (H10) and another developed a progressively worse triceps myopathy (H14). They were treated with flunixin after recovery. All healthy horses completed the study.
Dialysate was successfully collected for a mean of 10 h 59 min and 20 h 43 min after recovery to standing in the healthy and the colic horses respectively. With time the membrane of the microdialysis catheters broke or the catheters were pulled out and at 20 h after standing there are results from five colic horses but from no healthy horse. Therefore, the mean levels at the end of the graphs in Figures , , , were calculated from only a few samples.
Figure 2 Lactate concentrations in dialysate and plasma in colic and healthy horses. The mean (± SEM) lactate concentrations in gluteal muscle dialysate and plasma in 8 colic horses (a) and in 10 healthy horses (b) during anaesthesia, in response to regaining (more ...)
Figure 3 Glucose concentrations in dialysate and plasma in colic and healthy horses. The mean (± SEM) glucose concentrations in gluteal muscle dialysate and plasma in 8 colic (a) and 10 healthy horses (b) during anaesthesia, in response to regaining the (more ...)
Figure 4 Urea concentrations in dialysate and plasma in colic and healthy horses. The mean (± SEM) urea concentrations in gluteal muscle dialysate and plasma in 8 colic horses (a), gluteal muscle dialysate urea concentrations in 10 healthy horses and plasma (more ...)
Figure 5 Glycerol concentrations in dialysate and plasma in colic and healthy horses. The mean (± SEM) glycerol concentration in plasma in 8 colic horses and in gluteal muscle dialysate in 4 colic horses (a), mean (± SEM) plasma and gluteal muscle (more ...)
The concentration of lactate was always higher in dialysate than in plasma in both groups (Figure and ), but the concentration difference between dialysate and plasma varied greatly between groups, individuals and over time. In the colic horses the maximum dialysate-to-plasma difference occurred at time 0 (4.2 ± 1.3 mmol/L) while it occurred at 30 min after standing in the healthy horses (2.1 ± 0.3 mmol/L).
Dialysate lactate concentrations increased in all but one colic horse in response to the work of regaining the standing position and was significantly higher at 1 h (p = 0.02) and 2 h (p = 0.04) after standing compared to the end of anaesthesia. In the group of healthy horses there was no significant increase in dialysate lactate after regaining the standing position. The concentration of lactate in dialysate was significantly higher in the colic horses compared to the healthy horses at 1 h (C: 8.7 ± 1.8 and H: 3.1 ± 0.3 mmol/L, p = 0.02) and 2 h (C: 7.0 ± 1.2 and H: 2.8 ± 0.3 mmol/L, p = 0.04) after standing.
The general trends for the plasma lactate concentration changes were similar in colic and healthy horses but larger fluctuations were seen in the colic horses and the concentrations were higher in this group until 2 hours after standing. Plasma lactate increased from before anaesthesia to after one hour of anaesthesia in both colic horses (C: 2.2 ± 0.8 mmol/L to 3.4 ± 0.6 mmol/L, p < 0.001) and in the healthy horses (H: 0.5 ± 0.1 to 1.5 ± 0.1 mmol/L, p < 0.001). In the colic horses, the lactate concentration in plasma was significantly increased (p = 0.003) at 15 minutes after standing (6.2 ± 1.3 mmol/L), compared to the end of anaesthesia (3.1 ± 0.6 mmol/L) but decreased thereafter. In the healthy horses plasma lactate was significantly lower (p = 0.001) at two hours after standing (1.1 ± 0.1 mmol/L) compared to the end of anaesthesia (2.0 ± 0.2 mmol/L).
In the two most severely affected colic horses whose results are not included in the mean values (C8 and C14), lactate in both dialysate and plasma were above 15 mmol/L at all times and in C14 lactate in dialysate reached a maximum concentration of 42 mmol/L. In these horses, plasma lactate concentrations were 20.7 mmol/L and 15.4 mmol/L before anaesthesia and reached concentrations of 28.5 and 17.8 mmol/L at the end of anaesthesia. In horse C19, dialysate lactate increased post operatively, from 2.7 to 6.6 mmol/L when its condition deteriorated during the last hours before euthanasia. The healthy horse (H14) that developed a triceps myopathy had the highest concentrations of both dialysate and plasma lactate during anaesthesia (6 mmol/L and 4 mmol/L in dialysate and plasma respectively) and immediately after standing (8.1 mmol/L and 7.2 mmol/L in dialysate and plasma respectively) of all healthy horses. The concentrations decreased quickly thereafter.
Pyruvate in the dialysate was analysed in five colic horses, hence no statistical comparisons were performed on these data. The temporal changes in pyruvate basically followed the changes in lactate with an increase after standing, the maximum levels (0.3–0.5 mmol/L) being reached within 2–4 h after regaining the standing position and then a gradual decrease towards stable levels around 0.1 mmol/L.
The dialysate lactate-to-pyruvate ratio
The lactate-to-pyruvate ratio (La/Py ratio) reached its highest level at the beginning of sampling during anaesthesia with ratios varying from 38 to 75 and decreased thereafter. A short-lasting small increase was seen in association with the work of standing up. By 20 h after standing, in the three horses where samples still were obtained the ratio varied from 17 to 25. In the horse that was euthanised due to aggravating endotoxemia and diarrhoea 14 h after standing (C19), the La/Py ratio increased by more than 100% (from 15 to 43) during the last 2 h before euthanasia.
In the healthy horses the concentration of glucose was always lower in dialysate compared to that in plasma whereas in the colic horses the opposite situation was sometimes present, especially during anaesthesia and early after standing (Figure ). In some colic horses the glucose levels in the dialysate exceeded that in plasma by 5–8 mmol/L.
In the colic horses dialysate glucose was increased during the first hours after standing compared to during anaesthesia (p < 0.01), whereas in the healthy horses there was no change over time. The concentration of dialysate glucose was higher in the colic horses than in the healthy horses, the difference being significant at time 0 (C: 10.5 ± 1.3 mmol/L and H: 5.7 ± 0.4 mmol/L, p = 0.01) and 1 h after standing (C: 10.4 ± 1.3 mmol/L and H: 5.9 ± 0.4 mmol/L, p = 0.001) and a near significant difference at 2 h after standing (C: 10.0 ± 2.8 mmol/L and H: 5.6 ± 0.4 mmol/L, p = 0.06).
The plasma glucose concentration was significantly higher in the colic than in the healthy horses during anaesthesia (p = 0.002) but not after standing. Plasma glucose did not change significantly after standing in either group, but tended to decrease over the following 12 h in the colic horses.
The concentration of dialysate urea was significantly higher in the colic than in the healthy horses until at least 2 h after standing (p = 0.02) (Figure ). In the colic horses dialysate urea increased significantly after standing (p = 0.003) at time 0 compared to the last sample during anaesthesia) and decreased slowly thereafter. The plasma urea level did not change significantly but the trend over time was similar to that of dialysate urea. The relationship between the dialysate and plasma concentrations varied over time and between individuals in the group of colic horses. Higher concentrations in the dialysate than in plasma were sometimes present during anaesthesia and in the early recovery-to-standing period whereas in the later samples, similar levels in the dialysate and plasma were seen. In the healthy horses urea concentrations remained stable showing no dialysate-to-plasma differences.
In all healthy horses, the glycerol concentrations were always higher in dialysate than in plasma until immediately after or within a few hours after regaining the standing position, individual concentration differences being 2 to 10-fold. Thereafter, in those horses where dialysis continued to function, glycerol in dialysate was slightly lower or of similar concentration as in plasma (Figure ).
The plasma sample obtained in the healthy horses at 15 min after standing was significantly increased compared to all other sampling times (p = 0.04).
In the five colic horses in which dialysate glycerol was analysed, concentrations varied largely between individuals and over time (Figure ) and hence no statistical analysis was performed. The colic horse that died from pulmonary oedema and cardiovascular collapse during recovery (C8) had extremely high values (above 2200 mmol/L) during anaesthesia and early in recovery, but a decrease was seen in the last sample before the horse died. In this horse, the concentration of glycerol in plasma was approximately 50% of that in the dialysate.