The most commonly prescribed medication in the hospital is intravenous fluid. Hypotensive? Give fluid. Tachycardic? Give fluid. NPO? Give fluid. Fever? Give fluid. It has become less of a drug and more a ubiquitous background state for many patients. Why do we give fluids to these patients? In order to talk about fluid as a drug let us first look at how the body interacts with fluid.
When people talk about fluid in the body they talk about total body water and its individual components. It is important to know how much fluid is where, but it is more important to know how fluid stays in those spaces.
Table 1: Body Fluid Compartments: Where is the fluid?
Figure 1: Body Fluid Compartments
The amount of fluid in the intravascular space is only 3 liters, keep this in mind when giving patients fluids.
What keeps fluid in the intravascular space?
This is arguably the most important concept to understand when it comes to fluid management in the ICU. Understanding how to keep fluid in the intravascular space is one of the cornerstones of being a resuscitationist. The answer is Starling forces. Starling forces are the movement of fluid between the intravascular and interstitial space. Changes in Starling forces lead to complications that can range from hypovolemic shock, volume overload, and pulmonary edema with respiratory failure.
Starling forces are based on the summation of the pressures that want to drive fluid into the intravascular space and the pressures that want to drive fluid into the interstitial space. These pressures are osmotic pressure, oncotic pressure, and hydrostatic pressure.
Starling equation:
The Starling equation determines the rate of fluid either in or out of the intravascular space. This is done at the level of the capillaries. Jv is the rate at which it moves in volume per sec (m3/s). It is the difference between the hydrostatic forces minus the difference in oncotic forces.
Jv = LpS [(Pc - Pi] - σ(πc-πi)]
Jv = Rate of fluid entering or leaving the intravascular space
Lp = Hydraulic permeability coefficient
S = Surface area of membrane
Pc = Capillary hydrostatic pressure
Pi = Interstitial hydrostatic pressure (small)
πc = Capillary oncotic pressure
πi = Interstitial oncotic pressure (negligible)
σ = reflection coefficient for protein
By understanding this equation it is easy to see that in order to keep fluid in the intravascular space the capillary oncotic pressure must be increased or the capillary hydrostatic pressure decreased.
Osmotic pressure:
Osmotic pressure occurs in the intravascular and interstitial compartments and opposes each other. The osmolarity in the intravascular space wants to pull fluid in, the same is true for the osmolarity in the interstitial space, but it wants to pull fluid into the interstitial space.
Oncotic pressure:
The intravascular space has proteins that also help contribute to the osmotic pressure, although it can be looked at as its own thing since it is protein. There are significantly fewer proteins in the interstitial space and can be considered zero.
Hydrostatic pressure:
The 3rd force, hydrostatic pressure, is the pressure that wants to push fluid out of the space. The intravascular hydrostatic pressure wants to push fluid into the interstitial space and the interstitial hydrostatic pressure wants to push fluid into the intravascular space. The interstitial hydrostatic pressure is low and usually negligible.
I think of a balloon with a small hole in it. When it is only partially filled with fluid there would be little to no leak from the small hole. The balloon is not completed distended and there is pressure on the walls. When the balloon is filled completely, the pressure builds up, and the water will stream out of the hole until the volume decreases to a point where the pressure drops below a critical point. This is hydrostatic pressure. The osmotic and oncotic pressure draw in the fluid until it reaches a critical point and then the hydrostatic pressure pushes it back out.
Figure 2: Starling Forces
These are the forces that allow fluid to move in and out of the cell at the capillary level. The fluid is typically pushed out of the intravascular space at the capillary level and then comes back in at the venule level.
Figure 3:
The constant movement of fluid out of the intravascular space at the capillary level and back into the intravascular space at the venule level is well described in deranged physiology and I included the figure that shows this movement. I will not be going into this detail of the fluid movement, I will be focusing on how these forces can be manipulated in the clinical setting in order to resuscitate and treat critically ill patients.
Stay tuned for Part 2. . .
References:
1. Tobias A, Ballard BD, Mohiuddin SS. Physiology, Water Balance. [Updated 2020 Oct 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK541059/
2. Starling forces and fluid exchange in the microcirculation: Deranged Physiology. https://derangedphysiology.com/main/cicm-primary-exam/required-reading/cardiovascular-system/Chapter%20471/starling-forces-and-fluid-exchange-microcirculation. Accessed 7/12/21.