Continuing Education Activity

A venous gas embolism is an abnormal collection of gas that forms a bubble in the systemic venous circulation, which can act as an embolus and affect blood flow. This activity examines when a venous gas embolus should be considered on differential diagnosis and how to properly evaluate it. This activity highlights the role of the interprofessional team in caring for patients with this condition.

Objectives:

• Identify the etiology of venous gas emboli.
• Review the presentation of venous gas embolism
• Outline the treatment options for venous gas emboli.
• Explain the importance of improving care coordination among interprofessional team members to improve outcomes for patients affected by venous gas emboli.

Introduction

Venous gas embolism is defined by an abnormal collection of gas that forms a bubble in the systemic venous circulation. This bubble can act as an embolus affecting blood flow. In most cases, venous gas embolism is iatrogenic in nature. It may occur during cannulation of the central veins, head and neck surgery, blunt and penetrating chest trauma, thoracentesis, hemodialysis, and high pressure during mechanical ventilation. Venous gas embolism is known to occur during diving and during the injection of radiocontrast for CT scans. Most cases of venous gas embolism are benign and are not even reported. However, when symptoms develop, the disorder does require immediate treatment as there is a risk of death.

Etiology

A venous gas embolism is usually caused by medical interventions which expose the venous circulation to outside air/gas. Examples of procedures and situations which have caused venous gas embolism include central venous catheter insertion/removal, CT injector, hemodialysis, penetrating chest injuries, lung biopsy, cardiovascular surgery, neurosurgery, angioplasty, arthroscopy, laparoscopic procedures, hysteroscopy, among many others.

Anytime the patient is with the head elevated or fowler position, air embolism is a risk during surgery. Air embolism has been reported after ENT procedures and neurosurgery. The reason is in many cases, the patients are in the Fowler position or placed prone. Both gynecologic and obstetric procedures have the potential to cause venous air embolism. The risk of air embolism is highest when the uterus is exteriorized during a cesarean section. A common cause of venous air embolism is central venous access. This may be due to failure to occlude the needle hub, detachment of the catheter connections, deep inspiration during removal or insertion of the line, the upright position of the patient during insertion, and hypovolemia.

Another common cause of venous air embolism is mechanical insufflation during hysteroscopy, laparoscopy, or during a urethral procedure. Finally, positive pressure ventilation also places the patient at risk of venous gas embolism. Overdistension and rupture of the alveoli with high PEEP can lead to free air.

The common factor in these procedures is the possibility of the introduction of air/gas into the venous circulation. Additionally, deep-sea diving can result in nitrogen gas coalescing in the venous system as the diver ascends. When ascending, the pressure decreases, and the nitrogen which was dissolved in the blood at a much higher temperature begins to form bubbles and is no longer kept in solution with the blood. The nitrogen boils out of the venous system in particular as this is a lower pressure system. The nitrogen gas will then form bubbles and coalesce into emboli. This is prevented by slowly ascending and ensuring the diver has adequate breathes as they ascend to wash out excess nitrogen via exhalation. [1][2][3][4][5]

Epidemiology

Due to an increase in invasive medical procedures, the incidence of gas embolism has been steadily increasing. Additionally, the use of end-tidal carbon dioxide and doppler monitoring has lead to an increase in gas embolus detection. The actual incidence is unknown, however, because most venous gas emboli are largely subclinical and do not result in any obvious symptoms in patients. Neurosurgical procedures have the highest risk of a venous gas embolus due to the patient being upright, the position of the brain relative to the heart, and the brain's noncompressed venous system.

Pathophysiology

The severity of venous gas embolism is dependent upon the rate of gas introduced into circulation as well as the volume of gas and patient position. Rapid gas entry and larger volumes of gas both increase the size of the venous gas embolism and increase the severity of symptoms. Assuming no right to left shunting is occurring, the venous gas embolus will lodge in the pulmonary artery in the lungs. This can cause pulmonary artery hypertension and subsequent right heart strain. Occasionally, the gas embolus can pass through the pulmonary artery and make its way to the left heart and into the systemic arterial circulation, possibly causing far more severe sequelae such as stroke. This can also happen with right-to-left shunts such as a fistulous tract or a patent foramen ovale which allows the embolus to travel to the systemic arterial circulation, thus bypassing the lungs. This is called an arterial gas embolus or paradoxical embolism.[6][7]

In most cases, small amounts of air are broken down in the capillary bed and absorbed into the systemic circulation without any sequelae. To produce symptoms, it is estimated that more than 5 ml/kg of air has to be introduced into the venous system. However, complications can occur with even 20 ml of air. Sometimes even injection of 1 to 2 ml of air into the CNS can be fatal. Further, as little as 0.5 ml of air introduced into the coronary arteries can initiate ventricular fibrillation. The closer the air is injected to the right heart, the higher the risk of complications.

Large amounts of air introduced into the right ventricle can result in outflow tract obstruction, especially if the pulmonary artery pressures are high. When venous air embolism occurs, both tachy and bradyarrhythmias can occur. In patients with a right to left shunt (e.g., VSD, ASD, PFO), venous air embolism can prove to be even more dangerous as the air can enter the arterial system and induce a stroke, mesenteric ischemia, or death.

History and Physical

A history of recent surgical procedures or trauma increases the suspicion of venous gas embolism. It can be difficult to diagnose at times due to the similarity to various other causes for the patient's symptoms. Clinical manifestations of venous gas embolism include the following: Right heart strain on EKG, tachyarrhythmias, hypotension, dyspnea, chest pain, and coughing. If the patient is being monitored, one may see a decrease in end-tidal CO2 and hypercapnia. Increased pulmonary artery pressure may also be seen if invasive monitoring is established. If severe hypotension due to pulmonary artery hypertension and right heart strain is present, an altered mental status may also be seen. Arterial gas emboli will not be discussed here but can lead to stroke and myocardial infarction, among other sequelae.

Cardiac signs of venous air embolism include:

• A mill-wheel murmur which is loud and machinery like
• Tachy or bradyarrhythmias
• Elevation in jugular venous pressure
• Hypotension
• Myocardial ischemia

Pulmonary features include:

• Tachypnea
• Rales, wheezing
• Cyanosis
• Mild hemoptysis
• Apnea

CNS features include:

• Altered mental status
• Seizures
• Coma
• Transient focal neurological deficits

An eye examination may reveal air bubbles in the retinal vessels.

With massive air embolism, palpation of the skin may reveal crepitus over the superficial veins.

Because there are no specific signs and symptoms, a high index of suspicion is required to make the diagnosis and initiate treatment.

Evaluation

The diagnosis of venous gas embolism is largely clinical and requires high suspicion. Sudden symptoms after or during an invasive procedure such as loss of consciousness or hemodynamic instability are highly suggestive of gas embolism. The temporal relationship between symptoms and invasive procedures is the biggest clue in making the diagnosis. Various monitors used during surgery that may already be in place can be helpful in making the diagnosis of venous gas embolism or arterial gas embolism.

The blood gas may reveal hypercapnia, hypoxemia, and metabolic acidosis.

Transesophageal echocardiography may visualize a gas bubble in the heart and is the most sensitive diagnostic test for venous gas emboli. Precordial Doppler monitoring is the most sensitive noninvasive test routinely used. As noted previously, end-tidal carbon dioxide and nitrogen monitoring are useful in detecting venous gas emboli that have lodged into the pulmonary vasculature. Additionally, a drop in tidal volume is more specific for venous gas embolism than many other tests.

CT scans can detect small amounts of air in the central venous system. The technique may also reveal intracerebral air or cerebral edema.

End-tidal nitrogen is a very sensitive method of detecting venous air embolism. The response is much faster than with ETC02 measurements. However, the technique does not always identify minute amounts of air and may falsely indicate resolution.

If the patient has a central venous catheter, aspiration may reveal the presence of air bubbles.

Anytime venous air embolism is suspected, the surgical procedure must be quickly terminated. If one is inserting a central venous catheter, one may attempt aspiration. The catheter should ideally be placed just 2 cm below the junction of the right atrium and SVC.

Treatment / Management

Treatment of venous gas embolism is largely supportive. The first intervention should be to discontinue any gasses such as nitrous oxide, which may be flowing into the patient. One hundred percent O2 supplementation may be used to correct hypoxia/hypoxemia as well as decrease the size of the bubble due to a diffusion gradient allowing nitrogen to leave the gas embolus. For severe venous gas embolism causing hemodynamic instability, cardiac massage may be beneficial. Cardiac massage would move air out of the pulmonary outflow vasculature and into the smaller vessels, thus improving blood flow. If a catheter is already in place, such as a subclavian vein catheter, it is possible to advance the catheter into the heart and "suck out" the gas emboli.

Positioning in the left lateral decubitus may be beneficial during this procedure so as to trap the emboli in the right heart for easier removal. However, there is no data supporting the emergent placement of an intravenous catheter for venous gas embolism management if one is not already in place.[8][9][10]

Inotropic agents such as dobutamine and ephedrine may be used if the right heart function is compromised. This will increase preload to the left heart resulting in improved cardiac output. 

While hyperbaric oxygen therapy is commonly used in arterial gas embolism, its use in venous gas embolism is not routine. Most patients with small venous gas embolisms have few, if any, symptoms and do not require treatment. Generally, with smaller venous gas embolisms, the embolus will eventually reach the lungs, become trapped, and diminish in size as the patient breathes. In cases of severe hemodynamic instability, hyperbaric oxygen therapy may be considered, but there is no data clearly demonstrating benefit.

If air embolism is suspected after chest trauma and the arrest does not respond to CPR, one may need to perform an immediate thoracotomy and clamp the hilum. However, this is a major undertaking with enormous morbidity and low survival rates. It should never be performed by a non-thoracic surgeon. 

Differential Diagnosis

• Acute myocardial infarction
• Heart failure
• Tension pneumothorax
• Septic shock
• Ischemic stroke

Prognosis

For most patients with a minute amount of air in the venous system, there are no complications. However, anytime air enters the CNS, the prognosis is grave. Three factors that determine outcomes include the patient's mental status, neurological deficits, and age. There is always the potential of life-threatening complications with venous air embolism, especially if the air remains lodged into a vessel. Mortality rates of 30-80% have been reported following venous air embolism after chest trauma. Surgeons who perform procedures with the patient in the fowler position or insufflate the body with carbon dioxide should remain extra vigilant as many cases of air embolism continue to be reported.

Deterrence and Patient Education

The best way to manage venous air embolism is to prevent it in the first place. Some preventive measures include the following:

• Avoid hyperventilating patients in the upright or seated position
• Modify the position during surgery so that the head is lower than the legs. This creates positive pressure in the sigmoid and transverse sinuses.
• If the patient has a patent foramen ovale, avoid seated positions for surgery.
• Limit the amount of positive pressure during mechanical ventilation
• Avoid inserting central lines in hypovolemic patients
• Always ensure that the hub of the catheters is closed
• Check connections to central lines frequently
• Tell the patient not to take a deep breathe when inserting a central line

Pearls and Other Issues

Trendelenburg position has been advocated in the past, but when studied, has been shown to be ineffective and potentially harmful.

Enhancing Healthcare Team Outcomes

The diagnosis and management of venous gas embolism are usually with an interprofessional team that consists of an anesthesiologist, internist, hyperbaric chamber specialist, cardiologist, ICU nurses,  and neurologist. The treatment of venous gas embolism is largely supportive. Besides providing oxygen and managing hemodynamic instability, observation for complications is important. 

While hyperbaric oxygen therapy is commonly used in arterial gas embolism, its use in venous gas embolism is not routine. Most patients with small venous gas embolisms have few, if any, symptoms and do not require treatment. Generally, with smaller venous gas embolisms, the embolus will eventually reach the lungs, become trapped, and diminish in size as the patient breathes. In cases of severe hemodynamic instability, hyperbaric oxygen therapy may be considered, but there is no data clearly demonstrating benefit.

During central venous line insertion, the nurse should educate the patient not to take a deep breath for a few seconds. The nurse should ensure that the hub is on the catheter at all times. In addition, the connections to the central line must be checked frequently. Anytime IV infusions are administered, the nurse must make sure that the line is free of air bubbles. Attention to detail is vital if one wants to prevent venous air embolism. Anytime venous air embolism is suspected, the nurse should stop all infusions and immediately call the clinician in charge.

The outcomes of patients with venous gas embolism depend on the amount and presence of neurological symptoms. Patients without symptoms have a good prognosis, but those who develop neurological symptoms may have residual deficits even after treatment. When large volumes of gas are involved, death is common. [11] [Level 5]