Shock is a state of decreased perfusion and inadequate oxygen delivery to tissues. This results in end organ damage and is associated with high morbidity and mortality, regardless of etiology. Shock associated with an infection is known as septic shock. In the intensive care unit (ICU) population, septic shock is the most common admission to the ICU and, with a mortality rate approaching 30%, is one of the leading causes of death. Early antibiotics and early intravenous fluid administration are a key component of resuscitation, but over-resuscitation with fluids is associated with adverse outcomes, including death. Therefore, methods to determine the ideal amount of fluid to administer are required.

Increasingly, the use of a bedside ultrasound, known as point of care ultrasound (PoCUS), can be used to determine the cause of shock and to help guide the ideal amount of fluid administration. One method for determining the appropriate amount of fluid to administer is the measurement of the size of the inferior vena cava (IVC). The IVC is a large vein that travels through the abdomen and delivers blood back to the heart. In a breathing patient, the size of the IVC varies with the breathing cycle. During inspiration the contraction of the diaphragm results in the generation of a negative pressure inside the chest cavity. This pressure change results in a decrease in size of the IVC during inspiration; contrary, during expiration, the IVC will be maximally distended. One of the best methods for determining fluid responsiveness using IVC is known as the IVC collapsibility index, which is defined as the \[(Maximum diameter IVC-Minimum diameter) / Maximum diameter IVC\] x 100. If the index is greater than 41% (range, 40%-42%), the LR ranged from 3.5 (95% CI, 1.1-15) to 9.3 (95% CI, 0.88-51) that the patient would respond to fluid.

One limitation to the use of PoCUS to assess the IVC is that the size of the vessel is affected by pressure inside the chest cavity. For example, patients who have obstructive sleep apnea (OSA) are often treated using a device called a continuous positive airway pressure (CPAP) machine. CPAP works by applying a constant amount of air pressure to prevent the soft tissues in the neck from collapsing and obstructing the airway. This constant pressure results in an increase of the chest cavity pressure which has been shown to increase the IVC diameter and a decreased IVC collapsibility index. This could result in an error in not administering fluid when the patient would benefit from it.

A patient population where this error may occur are those admitted to hospital with pneumonia. Pneumonia is the most common presentation of septic shock in critical care. Often times these patients present with septic shock as well as respiratory failure, which is treated with supplemental oxygen. Increasingly, high flow nasal cannulas (HFNC) have been used in the initial management of respiratory failure. Studies in the critically ill populations have demonstrated that, compared to conventional oxygen therapy, HFNC provides superior oxygenation and decreases the need for intubation. HFNC is a device that is able to generate very high flows (up to 60 L/min) at an adjustable FiO2 such that even at very high peak inspiratory flows (i.e. breathing in quickly), FiO2 remains consistent. These high flows generate a CPAP effect, with an approximately linear relationship of 1 cmH2O for every 10 L/min of flow. Therefore, it is possible that patients who are placed on HFNC may have an artificially enlarged IVC measurement by ultrasound.

Considering the increasing scope and utilization of PoCUS as well as HFNC in the treatment of respiratory failure, it is important to establish what effect HFNC has on POCUS IVC measurements. To date, only one study has examined the effect of HFNC on the size of IVC. This study was conducted in heart failure patients where the patients were all volume overloaded (distended IVC). The study placed patients in heart failure on HFNC on flows of 20 and 40L/min which resulted in a decrease in the IVC collapsibility index. However, no study has examined the effect of HFNC non-volume overloaded patients at 60L /min.

Our hypothesis is that the application of HFNC will increase in the size of the IVC and decrease the IVC collapsibility index in healthy patients.