CONDITION
Ventricular Septal Defect
A ventricular septal defect is an opening in the wall that normally separates the left and right pumping chambers of the heart. Blood flows through this opening in a direction and volume determined by the size of the defect and the pressure difference between the two sides, often creating a heart murmur that can be detected during routine examination. Many animals with small defects show no outward signs and live ordinary lives, while larger defects can lead to exercise intolerance, slower growth, or breathing changes as the heart and lungs work harder to compensate. Owners often arrive on this page after a murmur has been heard during a puppy or kitten check, or when an older animal begins to tire more easily than expected. This page explores what can be observed at home and during examination, what is happening inside the heart and circulation, how the defect is measured and characterised, and the range of approaches—from monitoring alone to surgical repair—that may be considered depending on the individual animal and the size of the opening.
Why this matters now
Ventricular septal defect is present from birth and tends to be detected during the first routine health examinations in puppies and kittens, often around six to eight weeks of age when early vaccinations are given. No breed predisposition for the condition is firmly established in dogs, though individual studies have noted patterns in particular breed populations, suggesting an inherited component may be present in some lines. In cats, ventricular septal defect is the most common congenital heart abnormality overall but occurs largely sporadically, without a well-established breed predisposition. The size and position of the opening vary between individuals and shape what an owner may observe throughout the animal's life.
The defect itself does not change in absolute size after birth, but the animal grows around it, which can alter the relative impact over time. Small openings may continue to allow a modest volume of blood to move between the chambers without producing noticeable effects, and many animals remain free of outward signs throughout their lives. Larger defects that permit substantial flow can lead to gradual changes in the heart's dimensions and the pressures within the lung circulation, particularly if the volume of blood passing through the lungs repeatedly exceeds what would normally occur. In rare cases, the opening may reduce over time as tissue accumulates around its edges, though true closure is uncommon in dogs and cats and has only occasionally been documented, rather than occurring reliably during early puppyhood.
Signals & patterns
Early signals
Heart murmur noticed by vet
A murmur is often the first sign, heard through a stethoscope during a routine check in a young puppy or kitten. The sound reflects blood moving through the opening in the wall between the ventricles, and its character can vary depending on the size and position of the defect.
Tiring during play or exercise
Some animals may show less stamina during active play compared to their littermates, pausing more often or seeking rest earlier. This pattern can reflect the heart working harder to maintain circulation when blood is recirculating through the lungs rather than moving efficiently forward.
Slower weight gain or growth
Puppies or kittens with larger defects may gain weight more slowly than expected or appear smaller than their siblings. The increased work of breathing and circulation can use energy that would otherwise support normal growth.
Faster breathing at rest
An owner may notice the animal's chest rising and falling more rapidly than usual, even when lying quietly. This can occur when extra blood volume passes through the lung circulation, prompting the body to adjust the breathing rate.
Later signals
Reduced ability to sustain activity
Over months or years, an animal may show a gradual decline in exercise tolerance, becoming reluctant to engage in activities that were previously manageable. This pattern can emerge as the heart and lung circulation adapt to sustained changes in blood flow and pressure.
Coughing or laboured breathing
Some animals develop a cough or more effortful breathing, particularly after exertion or when lying down. This may reflect fluid accumulation in or around the lungs as the heart's ability to manage increased blood volume becomes less efficient.
Blue-tinged gums or tongue
In uncommon cases where pressure changes in the lung circulation reverse the usual direction of blood flow through the defect, oxygen-poor blood may bypass the lungs and enter the body's main circulation, producing a dusky or bluish colour in the mucous membranes. This late pattern indicates a significant shift in the underlying mechanics.
Click to read about the biological mechanisms
How this is usually investigated
The investigation usually begins with history and observation—how the animal moves, breathes, and responds to activity—followed by careful listening to the chest. When a murmur is heard, the next layer of investigation focuses on confirming the presence of an opening in the ventricular septum, locating it, measuring how much blood is moving through it, and assessing the consequences for the heart chambers and pulmonary circulation. The approach is individualised, shaped by what is heard, what is seen, and what the animal is able to tolerate.
Physical examination
Echocardiography
Radiography
Blood pressure measurement
Complete blood count
Options & trade-offs
Management is shaped by the size of the opening, the volume and direction of blood flow, the presence or absence of clinical signs, and the degree of change in the heart chambers and pulmonary vessels. Many animals with small defects require no specific treatment and are monitored over time to detect any shift in the heart's response. Others benefit from interventions aimed at reducing the work the heart must do or addressing the consequences of altered flow. Each approach carries its own practical considerations.
Observation with periodic reassessment
For animals with small, restrictive defects and no signs of heart strain or exercise limitation, a common path is to monitor at intervals with physical examination and echocardiography. The defect itself does not change in size, but the heart's adaptation can evolve, and periodic review allows early detection of chamber enlargement, valve changes, or rising pulmonary pressures.
Trade-offs: This approach requires access to repeat imaging and an ability to recognise subtle changes in breathing pattern or stamina between appointments. It suits animals whose defects remain haemodynamically quiet but may not be appropriate if signs of volume overload or pulmonary hypertension emerge.
Medical support for heart remodelling or fluid accumulation
When the heart begins to enlarge significantly or fluid accumulates in the lungs as a result of sustained volume overload, medications such as diuretics may be used to reduce congestion and ease breathing effort. Other drugs may be introduced to support heart muscle function or reduce resistance in the pulmonary circulation if pressures are rising.
Trade-offs: Medications can improve comfort and activity tolerance but do not close the defect or reverse structural changes that have already occurred. They require ongoing administration, monitoring of kidney function and electrolyte balance, and adjustment over time as the heart continues to adapt.
Interventional or surgical closure
In selected cases, the defect can be closed using a device delivered via a catheter threaded into the heart, or through open-heart surgery with cardiopulmonary bypass. Closure may be considered when the defect is causing significant chamber enlargement or volume overload but before irreversible pulmonary vascular changes have occurred. Some muscular and perimembranous defects are more amenable to device placement than others.
Trade-offs: Both interventional and surgical approaches require specialised centres, carry procedural risks including bleeding, arrhythmia, and device malposition, and are not suitable for all defect locations or sizes. Closure is contraindicated once shunt reversal has occurred, as it removes the only route for blood to bypass the overloaded right ventricle. Cost and availability remain significant practical barriers for many owners.
Management of reversed shunt and pulmonary hypertension
When pulmonary vascular resistance has risen to the point that blood begins to flow from right to left through the defect—a condition sometimes called Eisenmenger syndrome—the focus shifts to reducing pulmonary artery pressure where possible and managing the consequences of reduced oxygen delivery to the tissues. This may involve drugs that relax pulmonary vessels, periodic removal of excess red blood cells if polycythaemia becomes severe, and adjustment of activity to the animal's tolerance.
Trade-offs: Shunt reversal represents an advanced stage in which closure of the defect is no longer an option, and the condition tends to progress despite intervention. Management is palliative, aiming to maintain quality of life and comfort rather than to reverse the underlying vascular changes.
Common misconceptions
"A loud heart murmur always means the defect is large and dangerous."
Murmur intensity reflects the velocity of blood flow through the opening and the pressure difference between the ventricles, not the size of the defect itself. Small, restrictive defects often produce loud murmurs because a high-pressure gradient drives blood through a narrow opening at speed. Conversely, some large defects produce softer murmurs if the pressures between the chambers have equalised, and murmurs may fade or disappear entirely if pulmonary vascular resistance rises significantly.
"Ventricular septal defects always get worse over time and will eventually cause heart failure."
The defect itself does not enlarge after birth—the opening is a fixed structural feature. Many small, restrictive defects remain haemodynamically insignificant throughout the animal's life, producing no clinical signs and requiring no treatment. Larger defects can lead to progressive chamber enlargement and pulmonary vascular changes as the heart adapts to sustained volume overload, but the time course and outcome vary widely between individuals, and some animals remain stable for years.
"If the defect can be closed, the heart will return to normal."
Closure—whether by device or surgery—stops the abnormal flow of blood between the ventricles and can prevent further progression of chamber enlargement or pulmonary vascular damage. However, structural changes that have already occurred, such as chamber dilation or pulmonary vessel remodelling, may persist to some degree. The younger the animal and the less advanced the remodelling at the time of closure, the greater the potential for the heart to return toward more normal dimensions and function.
Related conditions
Atrial Septal Defect
Atrial septal defect is another form of septal defect in which the opening lies between the upper chambers rather than the lower chambers of the heart. Both conditions involve abnormal blood flow between chambers, though the direction and volume of shunting, and the downstream effects on the heart and circulation, can differ depending on the location and size of the opening.
Patent Ductus Arteriosus
Patent ductus arteriosus is a congenital heart defect in which a vessel that should close after birth remains open, allowing abnormal blood flow between the aorta and pulmonary artery. Like ventricular septal defect, it can present with a murmur in young animals and may lead to volume overload on the left side of the heart, though the specific haemodynamic changes and affected structures differ.
Pulmonic Stenosis
Pulmonic stenosis can occur alongside ventricular septal defect in some animals as part of a broader spectrum of congenital heart abnormalities. When both are present, the narrowing at the pulmonic valve may influence the direction and volume of blood flow through the septal opening.
Tetralogy of Fallot
Tetralogy of Fallot is a complex congenital condition that includes a ventricular septal defect as one of its four component abnormalities. The presence of the defect in this context interacts with the other structural changes—particularly pulmonic stenosis and an overriding aorta—to produce a distinctive pattern of blood flow and clinical signs.
Pulmonary Hypertension
Pulmonary hypertension can develop over time in animals with a ventricular septal defect if a large volume of blood flows from the left ventricle into the right side and onward to the lungs. This sustained increase in pulmonary blood flow may eventually raise pressure in the lung vessels, which can alter the original flow pattern through the defect and affect the heart's workload.
Understanding the individual pattern of flow through the defect, the response of the heart chambers over time, and the behaviour of the pulmonary circulation provides a foundation for anticipating what may lie ahead and recognising early shifts in the animal's exercise tolerance, breathing pattern, or general vitality. For animals whose defects produce little haemodynamic consequence, the main consideration may be the timing and frequency of follow-up imaging. For those with more significant volume overload or pulmonary vascular changes, conversations about the role of medications, the possibility of intervention, and the practical realities of long-term monitoring tend to become more relevant as the picture evolves.