A glimpse into the procedure saving dozens of patients with severe Covid-19
The word bypass carries many meanings in medicine. When a person suffers from heart disease, often doctors harvest a vein from the leg and sutured it around clogged coronary arteries creating a route around the blockage. Similarly, when people with advanced diabetes arrive at dialysis centers, they submit themselves to a process by which blood is diverted away from damaged kidneys into a dialysis machine where it can be separated, and its waste products removed before being returned to the patient’s ciruclation. Even in utero the fetal cardiovascular system has valves in place to strategically bypass organs that would otherwise act as a oxygen drain, robbing the developing fetus of crucial oxygen from the mother. Ironically, this system of fetal circulation bypasses the very organ in the womb that will one day be the sole source of oxygen in life: the lungs.
The major cause of mortality in Covid-19 results from the symptoms that arise when the lungs are severely damaged. Deep inside the lungs are millions of delicate air sacs where air is exchanged between the outside environment and the blood stream. The thin structure of lung air sacs — called alveoli — coupled with the massive surface area of the lungs is what makes gas exchange so efficient. When air comes in, oxygen floods into the blood, while CO2 escapes out.
The thin barrier that divides air from blood, often only one or two cells thick along with a small amount of connective tissue, is known as the interstitium. This thin barrier is one of the primary targets of SARS-CoV-2. As the virus gains access to the interstitium, the immune system is activated. Swelling is one of the most common responses to immune activity. Swelling is benign and harmless in many cases of immune or allergic response, but when the uber-thin interstitium of the lungs swells, it may actually expand to two or three times its normal width, dramatically decreasing gas exchange.
As the immune response further escalates, fluid from the interstium leaks out into the alveoli and fills the space where air normal goes. This fluid, laden with viruses and other opportunistic pathogens is quickly followed by immune cells and soon the once delicate air-filled alveoli become thick, congested fluid filled masses. This is pneumonia. Many Covid-19 patients rapidly progress to a state where gas exchange is so compromised that they go into acute respiratory distress syndrome (ARDS). This is where doctors can employ extracorporeal membrane oxygenation — known simply as ECMO.
ECMO follows the principle of bypass by relieving the lungs of the duty of gas exchange. Following similar principles as kidney dialysis, blood is pulled from the veinous circulation — where oxygen concentrations are lowest — and sent through a pump system that shuttles blood into an oxygenator that oxygenates the blood, while removing CO2 waste. Once blood is fully oxygenated, it is sent through a heat exchanger that brings it back to body temperature and returned to the body’s circulation. This newly oxygenated blood is then delivered to tissues by the heart.
ECMO was developed in the 1950’s as a life support technique. In modern times it is common to see hospitals with devoted areas for ECMO care of critically ill patients suffering from acute heart or lung failure. ECMO also plays a critical role in emergency rooms. Patients suffering from cardiac arrest often undergo cardiopulmonary resuscitation (CPR) to artificially restore both blood oxygenation — through breaths — and circulation — through chest compressions.
CPR in itself is a crude practice. It relies on techniques that are susceptible to human error. Patient head position, timing and depth of breath delivery, timing and location of chest compressions are all variables that lead to sucessful or non-successful CPR. Even chest compressions deep and forceful enough to compress the heart are enough to crack the ribs of patients. Despite how it is portrayed on TV medical dramas, CPR is a violent procedure with poor rates of success. ECMO can offer a better — albeit a more invasive — option in life support.
Recognizing the value of ECMO in cases of emergency, the University of Minnesota went one step farther and took ECMO out of the hospitals and onto the streets by creating their own mobile EMCO unit — the first of its kind in the nation — to respond to medical emergencies that involve cardiac or pulmonary arrest.
ECMO Versus Mechanical Ventilators
Mechanical ventilators have received a lot of attention since the onset of the Covid-19 pandemic. These machines, which artificially breath for critically ill patients, are often the last resort for those Covid-19 patients in severe respiratory distress. Placing a patient on a ventilator is considered to a serious and often ominous sign that they are teetering on the edge of survival. Ventilators and ECMO serve the same end function — to maintain the delivery of oxygen-rich blood to tissues — but do so in different ways.
When we breath, we the lungs are working to balance two key aspects of respiration: ventilation — the delivery of air to the lung tissue — and perfusion — the delivery of blood to lung tissue. When air and blood are brought into close proximity, gas exchange can occur. Gas exchange itself is a passive process where gases move from areas of high concentration to areas of low concentration. This process — called diffusion — requires no added energy or mechanical force. Ventilation on the other hand is an active process, requiring the work of muscles in the body to move the lungs like a bellows. The rhythmic movement of muscles like the diaphragm and those that surround the rib cage draw air in and out allowing for constant gas delivery to and from the lungs.
In severe cases of Covid-19, the lungs not only lose the ability to exchange gases as describe above, but can become so damaged by the virus that they lose the ability to ventilate. This loss of lung function — the active movement of lungs by the chest wall — requires a ventilator that forces are into the lungs. If enough viable lung tissue exists, ventilation may be sufficient to facilitate the gas exchange needed to sustain life. This procedure requires that patients be anesthetized and intubated — the placement of a tube directly into the lower respiratory tract — for the ventilator to function. ECMO, on the other hand can be performed at the bed side with simple sedation, assuming the patient’s lungs are ventilating properly on their own. Patients who have lost the ability to ventilate would need to be placed on a ventilator before they were able to undergo ECMO.
The Downside of ECMO
For all of its value, ECMO comes with its own risks. If nothing else, the procedure is invasive, requiring the placement of tubes into large vessels in the neck or leg. This carries risks of infection at the incision site that may enter the blood stream. Infections at the site of IV or central lines in the hospital is a common occurance. There is also an increased risk of blood clotting as a result of the procedure, which often requires patients be put on a blood thinner while undergoing ECMO. As we have seen from various reports, Covid-19 patients are already at an increased risk of blood clots, making this precaution ever more important when deciding to use ECMO for severe Covid-19 cases.
Even with ECMO in place, the body is still in the midst of fighting the Covid-19 infection. Because ECMO in no way protects the lungs from the damage of the viral infection, it is purely a life-sustaining procedure and not a treatment for the underlying infection.
Advancing knowledge of human physiology allows health care providers to bypass the normal functioning of the body in times of crisis. The strategic overriding of body systems can allow for controlled implementation of life-saving intervention. While many race to find treatments, or develop vaccines against Covid-19, the need for efficient life support techniques is needed in hospital ICUs to avoid rising mortality from the disease. ECMO offers a simple solution to preserving one of the cornerstones of human life — respiration.