Yvette Lovis is a human obstetric sonographer, undertaking both NHS and private work. She also teaches canine pregnancy scanning to dog breeders and veterinarians. The following article is a guide to the fetal heart in the canine and human fetus. A basic understanding of the cardiovascular system of the developing fetus is important, and certainly, achieving a basic four-chamber view of the beating fetal heart is a must when confirming viability of a pregnancy.
The circulatory system before birth
Within the uterus, the canine and human fetal heart circulation is different to that following birth.
Within the uterus, the fetus relies on oxygen from the mother’s circulation, which is delivered from the placenta through the umbilical vein – one of the vessels in the umbilical cord. The other vessels within the cord – two arteries – carry deoxygenated and nutrient-depleted blood from the fetus back to the placenta.
Above: Umbilical cord in the canine (left) and human (right). The image on the right is also using colour Doppler, to indicate blood flow.
In normal development, both canine and human fetal hearts develop into four chambers; left atrium and left ventricle, right atrium and right ventricle.
Above: The four chambers of the fetal heart, shown in the canine (left) and the human (right).
The great vessels – pulmonary artery and aorta
The pulmonary artery and the aorta arise from a single structure. The aorta and pulmonary artery are initially spiralled together and connected through the ductus arteriosis, because the fetal circulation obtains its oxygen from the placental circulation. After birth, blood is sent to the lungs to gain oxygen from respiration, and the connection between the two great vessels closes. In normal dogs and humans, the ductus arteriosis begins to close within a few hours of birth.
The pulmonary artery / pulmonary trunk
The pulmonary artery arises from the right ventricle, dividing into the left and right pulmonary artery branches.
Above: Right ventricular outflow tract view in a canine fetus, through which blood is pumped through the pulmonary valve and into the pulmonary artery.
The aorta arises from the left ventricle, arching over the top of the heart and running down through the body. It transports oxygenated blood to the vital organs via various branches.
Above: Left ventricular outflow tract in the human fetus, through which blood is pumped through the aortic valve and into the aorta.
The right atrium
The inferior vena cava (IVC) / caudal vena cava delivers oxygenated blood from the placenta via the umbilical vein and the fetal liver into the right atrium. The superior vena cava / cranial vena cava delivers deoxygenated blood from the upper body in the right atrium, which mixes with the oxygenated blood from the IVC. The foramen ovale (a flap between the right and left atria) allows some of the blood from the IVC and SVC to enter the left atrium from the right atrium, where it is then pumped into the aorta – and hence, oxygenated blood circulates the fetal body.
Above: The two atria and foramen ovale in a canine fetal heart.
The remainder of the blood from the right atrium is pumped into the right ventricle, and out through the pulmonary artery. This blood then rejoins the mostly oxygenated blood in the aorta (sent through the foreman ovale to the left atrium and left ventricle), via the ductus arteriosus.
Above: Ductus arteriosus in the human fetus, which allows blood to pass from the pulmonary artery into the aorta. When this duct fails to close after birth, the human or animal is left with a residual shunt between the two great vessels. This is referred to as a “patent ductus arteriosus,” and can range in severity, from immediately life-threatening to being well-tolerated and even undetected in some individuals.
The fetal lungs as yet have little function, since oxygenated blood is delivered via the placenta into the IVC. The right side of the fetal heart is exercised and allowed to develop normally because of the presence of the foreman ovale and the ductus arteriosis.
During the first day of a neonate’s life, the first breaths are taken and pressures within the fetal circulation change, causes the foramen ovale to close, and the ductus arteriosus and ductius venosus to become ligaments. The right atrium is then full of deoxygenated blood from all over the body (IVC and SVC), which is pumped through the right ventricle into the pulmonary artery. The pulmonary artery divides, as previously mentioned, into the left and right pulmonary artery, where blood is then pumped into the left and right lungs respectively. Oxygen collection then takes place at the lung surfaces, and oxygenated blood then enters the left atria via the pulmonary veins, where it is pumped into the left ventricle and out to the rest of the body through the aorta.
Fetal Heart Rate
The canine fetal heart rate can be monitored during delivery using ultrasound M-mode, the presence of fetal heart motions signifies viability.
- Fetal stress due to hypoxia is indicated if the fetal heart rate at term drops to less than 180bpm. The normal fetal heart rate is between 220 and 240bpm (Zone and Wanke, 2001). In comparison, during labour of the human fetus, a reassuring base line heart rate is between 110 and 160bpm (NICE Guideline: Interpretation of intrapartum CTG).
- The fetus is often monitored using a cardiotocograph (CTG machine). This registers the fetal heart rate, and detects accelerations and decelerations relative to the mother’s well being and the uterine contractions.
- In humans, decelerations occur with 50% or more contractions in any 20 minute segment (Perinatology.com Intrapartum Fetal Heart Rate Monitoring). This compares directly to the canine fetus in labour; intermittent uterine contractions over a fetus can cause a temporary, substantial reduction in heart rate which should return to normal in 1-2 minutes and then remain in normal range if there is no fetal distress (Lopate, 2008).
Above: The final third of the image shows cardiac motion. The green line in the 2D image above the M-mode shows where the M-mode line transects the image.