Dr Krishna Reddy Nallamalla
An exponential rise in cases during the ongoing second wave of Covid is leading to a shortage of critical resources like beds, nurses, doctors, drugs, and oxygen. The fear of shortage is leading to hoarding of these resources and ultimately leading to a vicious cycle of panic. India is making all efforts on a war footing to generate and supply enough oxygen by leveraging all avenues including international help. In this context, it is important to know certain facts.
Understanding Oxygen metabolism
Normally oxygen enters the blood through a thin membranous barrier in the lungs. Oxygen is carried, bound to the hemoglobin in the blood. When oxygen saturates all the binding sites of hemoglobin present in the blood, we call it 100 percent saturated. Oxygen is released in the tissues, as tissues consume it to generate energy. Normally, tissues extract 25 percent of the bound oxygen at a normal heart pumping rate of 5 liters per minute to meet resting requirements. As per binding characteristics, oxygen can be released up to 50 percent. Hence, tissues can extract adequate oxygen even when bound oxygen falls to 75% at the normal state of requirements. To meet the oxygen needs of the body, the heart can increase its pumping byuptosix times to 30 liters per minute. If the oxygen supply is not sufficient, tissues use a backup system (called anaerobic metabolism) that does not require oxygen, to generate energy. If the energy generated is still not sufficient, some tissues go into hibernation by shutting off all non-essential functions. The above is to explain that the body has a highly complex and adaptive mechanism to survive despite inadequate supply of oxygen.
Why do oxygen levels fall in Covid?
As the virus infects and damages thecells lining the air sacs, the barrier to oxygen exchange increases. In addition, the inflammatory response of the body to tackle the virus also causes fluid leakage, thus further increasing the barrier. As more lung tissue gets involved, less oxygen enters into the blood. In response to the decreasing oxygen, the brain makes a person breath faster. Patients start becoming short of breath as the work or exertion of breathing increases. As air sacs get filled with fluids, venous blood (coming from the tissues) that is low in oxygen gets shunted without any addition of oxygen. All of the above factors lower the arterial blood oxygenlevels over time.
When does a person needs supplemental oxygen?
An infected person needs more energy and thus more oxygen to fight the virus. The need arises when oxygen levels start falling below 90 percent saturation levels. However, as a precaution, when saturations are consistently hovering around 93 percent or less, plans should be made to provide oxygen. If people start breathing laboriously or faster (>30 breaths per minute), they need oxygen support even if the saturations read above 90 percent.
How is oxygen administered?
Oxygen administration is started through a nasal cannula at 2 to 4 liters per minute. The flow rate is adjusted as per the saturation levels and respiratory rate. The target is to maintain saturation above 90 percent. There are special devices to give oxygen at higher flows, going up to 60 liters per minute. Oxygen can also be administered through a variety of masks (mask with bag, venture mask, polymask etc.,). Masks allow us to adjust the concentration of oxygen from 24 percent to 100 percent. As the lungs get flooded with fluids, we will need to apply pressure to push oxygen through the barrier membranes. Instruments such as Continuous or Biphasic Positive Airway Pressure (CPAP/ BiPAP) help us deliver oxygen at a higher pressure. They require a close-fitting mask over the mouth and nose to prevent leakage of air. The devices enable us to avoid invasive ventilation given through a tube inserted through nose or mouth into the trachea. If we are unable to maintain oxygen levels despite ventilation, blood can be oxygenated externally using special machines (heart-lung machines or Extra Corporeal Membrane Oxygenator, called ECMO) until lungs start recovering from the severe injury.
Can oxygen be taken at home?
While for some of the chronic heart and lung conditions oxygen is prescribed to be taken at home, for acute lung conditions it is recommended to get admitted to a hospital once oxygen levels fall below 93 percent. Given the prevailing bed shortages, oxygen can be administered at home using either gas cylinders or liquid oxygen cylinders or oxygen concentrators. Standard size cylinder contains ~1360 liters of oxygen at 2000 psi. It generally lasts ~ 11 hours for a flow rate of 2 liters per minute. It lasts for a shorter duration if the flow rate is increased. Hence, the flow rate should be kept at minimum just enough to maintain the saturation at around 90 percent. Small portable cylinders containing 300 liters of oxygen last for 2 hours only at 2 liters per minute. Liquid oxygen cylinders have a capacity of 30 to 40-liters. Each liter of liquid oxygen generates 860 liters of oxygen gas. Hence, each cylinder can last from 8 to 10 days depending on the flow rate. Oxygen concentrators generate oxygen by removing nitrogen using a molecular sieve. However, they need continuous power supply. Patients with Covid are strongly recommended to get hospitalized when they need oxygen. For planning purposes, early warning signs can be detected by monitoring the oxygen saturation after 6 minutes of walking. If oxygen saturation that is normal at rest, falls after walking for 6-minutes, this gives an early warning signal for hospitalization.
How to plan in advance?
It is advisable to gather information about various hospitals designated for Covid beforehand. Since the demand-supply gap is most acute in cities and for popular hospitals, people should be prepared to go wherever a bed is available. It is a good idea to keep information about hospitals in suburban locations. They may not be as crowded as city hospitals. It is not advisable to hoard oxygen at home. It only adds to the shortage crisis. It is a good idea to keep information on agencies renting oxygen concentrators, if a bed is not available. Very frequent monitoring of oxygen saturation is not needed. It only adds to the anxiety and panic.
Dr Krishna Reddy Nallamalla
Country Director, ACCESS Health International