Parturition is defined as expulsion of the foetus(es) and foetal adnexae from the maternal uterus and occurs, in normal conditions, at the physiological completion of the pregnancy when the foetus(es) is(are) mature. The process of parturition is divided into three stages: the first stage is dilatation, characterized by progressive dilatation of the cervix and unapparent uterine contractions; the second stage is expulsion, in which the foetus(es) is(are) expelled through the cervix; the third stage is the afterbirth which concludes with expulsion of the placenta or, in animals that give birth to more than one foetus, expulsion of the last placenta.
The physiological duration of each stage of delivery has been defined for various species of animals. In the bitch and the queen the stage of dilatation lasts about 6-12 hours, while the expulsive stage has a very variable duration (usually 3-6 hours, but can be more than 24 hours) and comprises the interval from the start of the expulsion and the birth of the first foetus (within 4 hours in the bitch, within 1 hour in the queen) and the interval between the birth of one foetus and another, which ranges from 15 to 120 minutes in the bitch (and only becomes a source of concern if it exceeds 4-5 hours) and from 10 to 60 minutes in the queen. However, it should be noted that these data are only indicative and there are reports of normal deliveries taking up to 48 hours in queens. The dilatation stage in the bitch is characterized by loss of appetite, restlessness, breathlessness, nest building and a drop of about 1°C in body temperature and, in the queen, by vocalisation and restlessness or, contrariwise, extreme calm (the animal is relaxed and purrs).
DEFINITION
Dystocia means difficulty in giving birth (from the Greek dus, difficult, painful, abnormal and τοκίας, partus, birth) and includes every event that can alter the normal process of parturition.
CAUSES
Dystocia occurs in 5% of births in bitches and 3.3-5.8% in queens (18% in the Devon Rex). Small and miniature breeds of dogs are more predisposed to dystocia (about 60%): breeds particularly affected include: Chihuahua, Pekinese, Dachshund, Miniature Poodle, Yorkshire Terrier and Pomeranian, as well as all brachycephalic breeds, even though a certain predisposition has also been observed in many other breeds (Boxer, Greyhound, Scottish Terrier, Welsh Corgi, Border Terrier, Aberdeen Terrier).
There are numerous causes of dystocia which, for educational purposes, can be divided into those of maternal origin (more frequent in bitches aged >4-6 years), those of foetal origin, and those caused by both maternal and foetal factors. The maternal causes include both systemic factors, such as malnutrition, severe parasitic infestation and debilitating diseases, and more specifically reproductive factors, such as insufficiently developed pelvis, abnormalities of the birth canal (both bone and soft tissue components) and primary or secondary uterine inertia. The breeds most predisposed to dystocia due to “obstruction” are the French Bulldog, King Charles Spaniel and Terriers. The foetal causes include foetal death, foetal monstrosities, abnormal presentation, position or posture of the foetus, contemporaneous engagement of two foetuses, absolute foetal macrosomia and partial macrosomia of regions of the foetus which are too large to pass through the maternal pelvis. The typical example of this last occurrence is represented by both feline and canine brachycephalic breeds and feline dolichocephalic breeds (Siamese). Foeto-pelvic incompatibility causes dystocia of both maternal and foetal origin, in that the foetus is larger than the diameter of the maternal pelvis.
Maternal factors are the main cause of dystocia in both bitches (69% in one study of dystocia in Boxers, 38% in a retrospective study carried out on 530 cases of dystocia) and queens, with primary and secondary uterine inertia being among the most common causes (60% in the same study of Boxers, 35% in the study of 530 cases of dystocia). Primary uterine inertia, which is observed at higher frequencies in some dog breeds (Beagle, Bullmastiff, Irish Wolfhound, Labrador Retriever and Terriers) and is the most frequent cause of dystocia in the queen, is characterized by a lack of initiation of the expulsive stage of parturition at the physiological term of pregnancy, considered indicatively to be 65 days after ovulation in the bitch and after 71 days of pregnancy in the queen.
There are numerous causes of primary uterine inertia in dogs and cats, including advanced age at first delivery, obesity, uterine disorders, hypocalcaemia, overstimulation of the uterus by too many foetuses or lack of uterine stimulation by a single foetus (Fig. 1). External factors can also interfere with the process of giving birth. Secondary uterine inertia or inertia due to “exhaustion” of oxytocin develops more readily in the case of a large litter or when the stage of expulsion is prolonged excessively, for example because of obstruction of the birth canal.
DIAGNOSIS
Dystocia should be suspected in all cases in which labour does not start within the expected term of the gestation, when expulsion of the first foetus has not occurred within 4 hours of the completion of the stage of cervical dilatation, when the interval between the expulsion of consecutive foetuses exceeds 2-4 hours (it should be noted that in some queens, delivery of all live kittens may take as long as 2 to 3 days), when foetal expulsion does not occur after 30 minutes of strong, unproductive uterine contractions, when haemorrhages occur during the birth, when blackish-green discharges are not followed by expulsion of a foetus or when the parturient shows signs of malaise or distress.
The clinical examination involves an evaluation of the general condition of the parturient, transabdominal palpation of the uterus, inspection of the vulva and vaginal exploration in bitches of sufficient size and when possible in queens and bitches of toy breeds. After having ascertained the absence of evident signs of maternal pathologies, transabdominal palpation enables evaluation of the size of the uterus and any foetal movements. The observation of a green or blackish discharge (uteroverdin, green lochia), which is normal during delivery, indicates that placental separation has occurred and should be followed by foetal expulsion within 2-4 hours at most; abundant aqueous discharges are associated with expulsion of allantoid or amniotic fluid, but may be confused with urine, whereas haemorrhagic discharges indicate trauma and must be distinguished from normal uterine losses that may contain a modest amount of blood. Digital vaginal exploration is aimed at determining the possible presence and presentation of a foetus. With regards to this, it is worth remembering that a posterior presentation of a puppy, although observed in more than 40% of cases, is associated with a higher risk of neonatal mortality and more difficult neonatal adaptation.
Beyond the sixth week of pregnancy, radiological examinations can show the number, size and position of the foetuses and appearance of the foetal structures as well as the morphology of the maternal pelvis (Fig. 2). The appearance of gas in foetal cavities or surrounding the skeleton, and overlapping or collapse of cranial bones or alterations in the spatial relationships between skeletal segments are indicators of foetal death.
Ultrasound studies can confirm foetal vitality as well as show foetal malformations and placental anomalies and the amount of foetal fluids. Foetal vitality is assessed ultrasonographically by observing foetal movements, but above all by measuring the foetal heart rate, which is influenced by the state of foetal oxygenation. In the dog, the foetal heart rate is about 200-220 beats per minute (bpm); values between 170 and 200 bpm indicate foetal distress, requiring obstetric intervention within 12 hours, while the obstetric intervention must be immediate if the values are below 150 bpm. The normal foetal heart rate in cats is about 200-250 bpm. A tocodynamometer, applied before the start of labour, is necessary to make a certain diagnosis of primary uterine inertia, since this instrument can monitor the strength and frequency of uterine contractions.
MEDICAL TREATMENT
Depending on the cause of the dystocia and the conditions of the parturient and foetuses, the dystocia may be managed by manual obstetric assistance or medical or surgical interventions.
Manual obstetric assistance should be limited to those cases in which the dystocia is mild and the foetuses of normal dimensions in which the correction of slight alterations in foetal presentation can easily resolve the problems with the delivery (Fig. 3).
Medical treatment, only indicated when maternal and foetal conditions are good, is limited to the treatment of uterine inertia, after having excluded the presence of obstructions in the birth canal (uterine torsion, foetal macrosomia, cephalo-pelvic disproportion, etc.), when the foetuses are not distressed and under close medical surveillance. The medical treatment is based on potentiating the uterine contractions and carries the risk of tetanic spasms of the uterine muscles or even rupture of the uterus and must, therefore, be carried out with great care. Medical treatment is contraindicated when, at the end of the process of parturition, the uterus has lost sensitivity to uterine stimulants. The medical management of uterine inertia is based on the use of oxytocin and calcium gluconate.
Oxytocin is a peptide hormone produced by the hypothalamus and stored in the posterior part of the pituary gland (neurohypophysis). Once released into the circulation, oxytocin modifies the transport of ions across cell membranes, increasing the permeability of myofibrils to sodium, inducing their contraction and modifying the mobilisation of cellular calcium and its entry into some types of cells. Although implicated in numerous physiological processes, at the time of delivery, oxytocin acts mainly on the myometrium, stimulating uterine contractions in the stage of expulsion, and on breast tissue, where it is responsible for ejection of milk. The action of oxytocin on the myometrium is induced by a preceding sensitisation through a complex hormonal mechanism at the end of pregnancy. The progressive increase in the plasma concentration of oxytocin is associated with the initiation of the expulsive stage of parturition.
Various formulations of oxytocin are commercially available and the hormone can be administered intravenously, intramuscularly, subcutaneously or through the oral mucosa, but cannot be administered per os because it is inactivated in the stomach. Oxytocin, which has a half-life of 5 minutes and is rapidly metabolised in the liver and kidneys, must be given by bolus injections, starting with a dose of 0.1 U/kg up to a maximum of 2 IU/kg, every 20-40 minutes (a dose of 20 IU/animal should never be exceeded, regardless of the breed or size) to avoid prolonged contractions of the myometrium which could compromise the foetus by interrupting the foeto-placental blood flow. High doses of oxytocin are responsible for myometrial hypertonus and the risk of rupture of the uterus, precocious detachment of the placenta, obstruction of the umbilical cord blood vessels, foetal death, and maternal vasodilatation and hypotension. It has been observed that only a third of bitches with uterine inertia respond to a single dose of oxytocin and that combining this treatment with calcium can be of help and also reduce the risk of hypocalcaemia. It is plausible that this pharmacological management leads to an increase in the frequency of uterine contractions through the effect of oxytocin and an increase in the strength of the contractions through the effect of calcium. However, the level of calcium in the blood of bitches with dystocia is often in the normal range and a more accurate evaluation of this factor can only be achieved by measuring ionic calcium.
From among the various pharmaceutical formulations of calcium, calcium gluconate 10% is the most commonly used in bitches; the dose is 0.2 ml/kg intravenously or 1-5 ml/animal subcutaneously. This product is much less widely used in queens because of the frequent, strong contractions it causes; when it is used, the dose is 0.5 ml/animal intravenously. Given the side effects of calcium gluconate on cardiac activity (arrhythmias), intravenous administration of the product must be slow and cardiac activity must be monitored.
One protocol used for the management of dystocia in bitches is administration of oxytocin every 30 minutes until complete expulsion of the foetuses and the use of calcium gluconate when the interval between the expulsion of each foetus exceeds 30 minutes. If, following administration of calcium gluconate, a subsequent dose of oxytocin is ineffective, surgical intervention is indicated. Most authors advise surgery if two or more injections of oxytocin, administered at intervals of 20-30 minutes, do not induce expulsion of the foetus.
In queens, the protocol is initial intravenous administration of 2-4 IU of oxytocin followed, if unsuccessful, after 20 minutes by slow intravenous infusion of 0.5-2 ml calcium gluconate 10% and another dose of oxytocin after a slow intravenous infusion of 2 ml of dextrose 50% solution. If this pharmacological management fails, immediate surgery is necessary.
Some authors have suggested that hypoglycaemia could be a primary cause of uterine inertia, particularly in toy breeds, whereas other consider that hypoglycaemia is rare during dystocia in bitches. One study even reported a state of hyperglycaemia in bitches with dystocia, probably secondary to high concentrations of cortisol.
PROGNOSIS
The prognosis depends on various factors including the condition of the parturient, the cause of the dystocia, the time passed between the start of labour and the intervention of the veterinarian and the methods of treating the dystocia. In queens the time between the start of labour and the obstetric intervention is strongly associated with survival of the parturients: the survival rate is 90% in cases treated within 30 hours of the start of labour, 75% in cases treated within 40 hours and 30% in cases treated from 40 to 60 hours after the start of labour. In a retrospective study carried out on 530 cases of dystocia in bitches, the mortality rate among the parturients was 0.01%.
Suggested readings
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