The major hemodynamic abnormality in hypovolemic shock is decreased venous

return (preload) to the heart, resulting in a decrease in CO. ḊO2

to the tissues is

reduced from this and the loss of oxygen-carrying hemoglobin (Hgb). The

physiologic response of the body to a sudden decrease in volume (preload) is to

activate the hypothalamic–pituitary–adrenal axis and autonomic nervous system to

release catecholamines (epinephrine, norepinephrine). The subsequent increase in

HR and contractility help maintain CO. The peripheral vasoconstriction caused by

the sympathomimetic response helps maintain arterial pressure. In addition, fluid

shifts from the interstitial spaces into the vasculature to increase preload. These

responses are effective at maintaining BP in patients with a loss of up to

approximately 30% of the total blood volume. N.G.’s increased HR and signs of

peripheral vasoconstriction are consistent with these compensatory changes. Her

SBP is still low, however, and she has signs of decreased perfusion to her brain,

manifested by confusion and disorientation. Given the severity of her condition, if

intravascular losses are not rapidly replaced, myocardial dysfunction may ensue and

lead to irreversible shock.

The goals of resuscitation of patients in hypovolemic shock are the correction of

inadequate tissue perfusion and oxygenation, and limiting secondary insults. HR, BP,

and urine output have been traditional markers for the adequacy of resuscitation, but

reliance on these end points alone is acceptable only in the initial management of

hemorrhagic shock. One concern is that patients may persist in a state of compensated

shock even after these parameters are normalized.

14 Ongoing deficiencies in ḊO2

to

vital organs may progress, and if left untreated, organ dysfunction and death may

result. Measurement of base (bicarbonate) deficit and lactate levels can be used to

assess the global adequacy of perfusion by tracking trends, ensuring that the levels

are decreasing. Metabolic acidosis can signal that resuscitation is incomplete despite

normal vital signs.

TREATMENT

Choice of Fluid in Hypovolemic Shock

CASE 17-1, QUESTION 2: Is an IV saline solution adequate to compensate for N.G.’s blood loss? What

other fluid options are available to resuscitate this patient?

Once an adequate airway is established and initial vital signs are obtained, the

most important therapeutic intervention in hypovolemic shock is the infusion of IV

fluids. Initially, crystalloids or colloids are used to restore blood volume as blood

products are limited, costly, and transfusion of these products carries a risk.

15 Blood

products may not be immediately available and are frequently unnecessary to manage

mild shock (less than 20% blood loss).

Crystalloids Versus Colloids

Resuscitative crystalloids are isotonic solutions that contain either saline (0.9%

sodium chloride; “normal saline” [NS]) or a saline equivalent (lactated Ringer’s

[LR] solution) (Table 17-4) . Colloidal solutions contain high molecular weight

molecules that are derived from natural products, such as proteins (albumin),

carbohydrates (dextrans, starches), and animal collagen (gelatin), and largely remain

in the intravascular space, thereby contributing to colloid oncotic pressure (COP)

(Table 17-5). Healthy semipermeable capillary membranes are relatively

impermeable to these large molecules.

The choice of a crystalloid versus a colloid solution to restore blood volume in

hemorrhagic shock is controversial. The controversy primarily involves the ultimate

distribution of these fluids in the extracellular compartment, which, in turn, depends

on their composition. Isotonic solutions (NS or LR) freely distribute within the

extracellular fluid compartment, which is divided between the interstitial and

intravascular spaces at a ratio of 3:1. This distribution is determined by the net

forces of COP and hydrostatic pressure, both inside and outside the capillary

vascular space. Consequently, large volumes of crystalloid fluid are required to

expand the intravascular space during resuscitation. In contrast, intact capillary

membranes are relatively impermeable to colloids and, therefore, colloids

effectively expand the intravascular space with little interstitial loss. Comparatively

smaller volumes of colloids than of crystalloids are required for resuscitation, and

their duration of action is longer. It is thought that 3 to 4 times as much volume of

crystalloid compared to colloid is necessary to provide the same degree of volume

expansion.

Table 17-4

Composition and Properties of Crystalloids

Solution

Sodium

(mEq/L)

Chloride

(mEq/L)

Potassium

(mEq/L)

Calcium

(mEq/L)

Magnesium

(mEq/L)

Lactate

(mEq/L)

Tonicity

Relative

to Plasma

Osmolarity

(mOsm/L)

5%

Dextrose

0 0 0 0 0 0 Hypotonic 253

0.9%

Sodium

chloride

154 154 0 0 0 0 Isotonic 308

PlasmaLyte

(Baxter)

140 103 10 5 3 8 Isotonic 312

Lactated

Ringer’s

130 109 4 3 0 28 Isotonic 273

7.5%

Sodium

chloride

1,283 1,283 0 0 0 0 Hypertonic 2,567

p. 353

p. 354

Table 17-5

Composition and Properties of Colloids

Solution

Colloid

Type

MWw

(KDaltons) DS

Sodium

(mEq/L)

Chloride

(mEq/L)

Potassium

(mEq/L)

Calcium

(mEq/L)

Glucose

(mg/L)

Osmolarity

(mOsm/L)

Albumin Bloodderived

67 130–160 ≤2 300

Hespan

6%

Hetastarch 450 0.7 154 154 309

Hextend

6%a

Hetastarch 450 0.7 143 124 3 5 90 307

Voluven Tetrastarch 130 0.4 154 154 308

Gentran

40

Dextran 40 40 154 154 50 308

Gentran

70

Dextran 70 70 154 154 50 308

aHextend also contains magnesium 0.9 mEq/L and lactate 28 mEq/L

MWw, weight-averaged molecular weight (number of molecules at each weight multiplied by the particle weight

divided by the total weight of all the molecules); MWn, arithmetic mean of all particle molecular weight; DS,

proportion of substituted to non-substituted glucose moieties (higher DS is more resistant to hydrolysis)

Proponents of crystalloids argue that both intravascular and interstitial fluids are

depleted in hypovolemic shock because of the rapid shifts between the extracellular

compartments. Volume replacement of both fluid spaces is best accomplished by

using crystalloids. In addition, loss of capillary integrity in shock can cause the leak

of larger molecules (including colloidal molecules) into the interstitium. This

increase in the interstitial oncotic pressure would favor fluid movement out of the

vascular space into the tissues, with resultant edema. Crystalloids do not produce

allergic or hypersensitivity reactions like many of the colloidal agents. Colloids can

also cause coagulopathies, have been associated with an increased incidence of acute

kidney injury, and are much more expensive than crystalloids.

Proponents of colloids maintain that resuscitation with these solutions more

rapidly and effectively restores intravascular volume after acute hemorrhage. For a

given infusion volume, colloidal solutions (e.g., albumin) will expand the

intravascular space 2 to 4 times more than crystalloids, and the intravascular effects

persist longer. Traditionally, it has been contended that the larger volumes of

crystalloids necessary to restore the vascular space will further dilute the plasma

proteins, resulting in a decreased COP and promotion of pulmonary edema.

However, clinical studies comparing colloids with crystalloids have failed to show

any differences in the development of pulmonary edema. This is likely because of the

relatively high alveolar capillary permeability to albumin causing a decreased

transcapillary COP gradient. Research suggests that certain subgroups may be at

greater risk for the development of pulmonary edema, but considerable variance

remains because of differences in physiologic end points, criteria for assessing

pulmonary edema, and the extent of shock.

Numerous meta-analyses have compared resuscitation with crystalloids or

colloids in an effort to find a consensus among divergent clinical trial results. Two

recent meta-analyses failed to show a mortality benefit for resuscitation with colloids

compared to crystalloids in patients with sepsis, trauma, burns, or following

surgery.

16,17 Patients receiving hydroxyethyl starch were at an increased risk of death

and acute kidney injury. It is important to recognize that study inclusion criteria

(heterogeneity), differences in fluid management, and dosages provide several

limitations to these meta-analyses. Conversely, a more recent randomized, controlled

trial found no difference in 28-day mortality or the need for renal replacement

therapy in patients with hypovolemic shock who received crystalloid- or colloidbased resuscitation.

18 Any mortality difference between colloids and crystalloids at

90 days remains uncertain as trials have shown conflicting results.

18–21

A Cochrane Database meta-analysis found that volume resuscitation with albumin

in hypovolemic critically ill patients did not reduce mortality compared to

crystalloids and was possibly associated with an increased risk of death in patients

with burns or hypoproteinemia.

22 This meta-analysis was heavily influenced by the

SAFE trial, the largest randomized, prospective trial to evaluate albumin versus NS

solution resuscitation in the critically ill.

23 The primary end point of the SAFE trial

was 28-day mortality, which showed no difference between albumin and NS

solution. No statistical differences were identified in any of the predetermined

subgroups (trauma, adult respiratory distress syndrome [ARDS], severe sepsis). A

trend toward increased mortality in the trauma patients who received albumin led to

a post hoc follow-up study in patients who had traumatic brain injury.

24 A higher

mortality rate was seen in patients with severe traumatic brain injury (Glasgow

Coma Scale 3–8), who received albumin versus those receiving saline for fluid

resuscitation.

More certain differences between colloids and crystalloids are availability and

cost. Crystalloid solutions are readily available and remain 20 to 100 times cheaper

than colloidal solutions; therefore, treatment costs can be significantly different and

must be considered when choosing between therapies with similar outcomes.

25 Given

the lack of evidence for a significant clinical difference between crystalloids and

colloids and the greater expense of using albumin, the guidelines for the use of

resuscitation fluids developed by the University Hospital Consortium, a nonprofit

alliance of US academic medical centers, remain unchanged.

26 Use of either NS or

LR solution would be appropriate for N.G. Colloidal products are not needed at this

time; instead, they should be reserved for persistent hypotension despite an

appropriate crystalloid challenge.

26 Given the severity of N.G.’s hemorrhagic shock,

blood transfusion is indicated and should be transfused as soon as available.