Volume assessment

1

Why do we administer fluids? What is the most important reason perioperatively?

Although there are many reasons to administer fluids to patients (correct electrolyte imbalances, administer drugs, nutrients, antibiotics, etc.), the ultimate goal of perioperative fluid administration is to increase cardiac output in an effort to better perfuse end-organs (e.g., heart, brain, kidneys). This goal can be realized when a fluid bolus increases venous return, thereby yielding a more optimal position on the Frank-Starling curve, resulting in increased stroke volume.

The assessment of volume status (particularly among critically ill patients) and the determination that intravascular volume expansion will ultimately lead to beneficial effects on end-organs is complex. Importantly, inappropriate fluid administration has been shown to cause harm and increased mortality. However, avoiding fluid administration in patients who are hypovolemic also causes harm. Therefore the administration of fluids, like any medication, should be scrutinized for expected benefit and likelihood of potential adverse effects.

2

What are some complications of fluid imbalance?

The risks of fluid imbalance (i.e., hypovolemia and hypervolemia) follow a “U”-shape curve, because both have associated complications, with the least complications occurring at the bottom of the curve when the patient is euvolemic. Complications of hypervolemia include acute renal failure, peripheral edema, delayed ambulation, pulmonary edema, poor wound healing, and ileus. Complications of hypovolemia include acute renal failure, tachycardia, demand cardiac ischemia (i.e., type 2 myocardial infarction), hypotension, end-organ hypoperfusion and mesenteric ischemia.

3

What is early goal-directed therapy? How does it differ from goal-directed fluid therapy?

Early goal-directed therapy (EGDT) is a protocolized resuscitation strategy to specified end-points in the management of sepsis. In particular, titrating crystalloid, blood products, and vasoactive agents to static physiological indices, such as central venous pressure (CVP) of 8 to 12 mm Hg, urine output (UOP) greater than 0.5 mL/kg/h, mean arterial pressure (MAP) greater than 65 mm Hg and mixed venous oxygen saturation (SmvO ₂ ) over 65% amongst other end points, including early administration of antibiotics. The original study by Rivers, et al. in 2001 showed significantly improved survival with this approach compared with what was then standard therapy, which did not include the early administration of antibiotics. Following this study, there were major concerns with EGDT, including the rigid, protocol-driven nature of the interventions and the fact that resuscitation goals were not tailored to the unique physiological needs of each patient (e.g., titrating fluid boluses to treat sepsis in a patient with systolic heart failure and chronic kidney disease to a UOP > 0.5 mL/kg/h). In particular, this protocolized approach was often associated with problems related to iatrogenic hypervolemia, as many patients required significant amounts of volume to achieve these static physiologicalal endpoints.

Three large multicenter randomized controlled trials have since reexamined EGDT and found no reduction in all-cause mortality for patients with sepsis compared with current standards of care. Further studies to elucidate the impressive survival benefit realized in the original Rivers study found that the only independent factors in reducing mortality in sepsis were: (1) early recognition of sepsis, and (2) early antibiotic administration. These factors were included in the original Rivers EGDT protocol and were not the standard of care at that time. However, interestingly, early sepsis recognition and antibiotic administration was the standard of care in the latter EGDT trials, which is likely the reason these trials showed no benefit with EGDT. Because volume resuscitation to static physiologicalal endpoints (i.e., CVP) may lead to the aforementioned complications, recent surviving sepsis guidelines now recommend using dynamic measurements to guide volume resuscitation (often referred to as goal-directed fluid therapy [ GDFT ]).

4

Enhanced recovery after surgery protocols frequently recommend GDFT (among other interventions) whenever feasible. What are the clinical benefits of GDFT in the perioperative setting, and how is it instituted?

One goal widely proposed in the perioperative setting is “zero fluid balance” at the end of surgery. This implies that the patient’s volume status after surgery is the same as it was before surgery, provided the patient was originally euvolemic. Interventions to maintain this perioperative target have been shown to prevent ileus and promote earlier hospital discharge. Judicious fluid administration results in less bowel edema, which, coupled with early oral (PO) intake postoperatively, likely facilitates early return of bowel function.

To institute GDFT perioperatively, patients should be euvolemic at the beginning of a surgery (i.e., clear liquids should ideally be encouraged up to 2 hours preoperatively), and dynamic indices of preload should be used whenever feasible. Fluid administration with this approach emphasizes the judicious use of maintenance fluids (e.g., < 3 mL/kg/h) and to only administered fluid boluses based on clinical assessment of hypovolemia using dynamic physiologicalal measurements. Collectively, this approach is often used using enhanced recovery after surgery (ERAS) protocols, which notably emphasize volume maintenance and resuscitation using dynamic parameters.

5

What methods are commonly used to assess volume status?

There are numerous methods used to assess volume status. In general, these can be categorized as the following:

• Physical examination: crackles on lung auscultation, orthostatic vital signs, lower extremity edema, jugular venous distention, abnormalities in heart rate, cold-extremities, UOP, dry mucous membranes, delayed capillary refill, etc.

• Imaging studies: chest radiograph, lung ultrasound, or computed tomography (CT) chest showing evidence of pulmonary edema. Findings on chest radiograph include cephalization, kerley lines, or pleural effusions

• Static parameters: static physiological measurements: CVP, pulmonary capillary wedge pressure/pulmonary artery occlusion pressure, SmvO ₂ , left ventricular end-diastolic diameter using echocardiography, extravascular lung water (EVLW), or inferior vena cava (IVC) diameter using ultrasound

• Dynamic parameters: dynamic physiologicalal responses (based on the Frank-Starling law) to provocative tests which affect preload. Measuring dynamic parameters generally requires the use of an arterial-line, esophageal Doppler, pulmonary artery catheter, echocardiography, or pulse contour analysis device to analyze changes in blood pressure (BP), stroke volume, or cardiac output in response to preload alteration. Preload can be altered by producing changes in venous return (generally from positive pressure ventilation, passive leg raises [PLRs], or a small fluid challenges). Dynamic indices are significantly more accurate compared with all other modalities in predicting volume responsiveness

6

Discuss the Frank-Starling law and its role in using dynamic modalities to assess hypovolemia?

The Frank-Starling law correlates stroke volume with preload or end-diastolic volume. As the ventricular volume increases during diastole, the surface area overlap between actin-myosin fibers increases, allowing more cross-bridges to form, which will increase the force of contraction. Patients who are intravascularly hypovolemic are on the “steep” portion of this curve, and any changes in preload will result in a large change in stroke volume. As the end-diastolic volume or preload increases, stroke volume also increases, until optimal actin-myosin overlap occurs and the stroke volume plateaus. Interestingly, if the end-diastolic volume continues to enlarge beyond a certain volume, the actin-myosin overlap may actually spread too far apart and no longer be able to form cross-bridges, leading to decreased contractility and decreased stroke volume ( Fig. 25.1 ).

Dynamic indices rely on this law to assess volume responsiveness, under the premise that the primary reason to give fluid is to increase stroke volume. The primary goal with using dynamic indices is to optimize the patient’s position on the Frank-Starling curve (i.e., so they are on the “flat” rather than the “steep” portion of this curve). Various other approaches may be used to assess the relationship between preload and stroke volume, including PLR, empiric 250 mL fluid challenges, or hemodynamic changes observed from positive pressure ventilation.

7

Describe the symptoms, physical examination, and imaging study findings that suggest hypervolemia. How sensitive and specific are these? How do they compare with dynamic modalities in the assessment of volume status?

History, physical examination findings, and imaging studies ( Table 25.1 ) are often used to assess for hypervolemia. Although these modalities in isolation are relatively specific, they are not sensitive and likely have high interobserver reliability because of their qualitative nature. However, using multiple findings together may increase their accuracy, as is often used in clinical practice. For example, a patient with a known history of systolic heart failure, presenting with shortness of breath, and physical examination findings of jugular venous distension, rales, and peripheral edema, is likely hypervolemic.

Whereas history, physical examination, or imaging studies are not as accurate as dynamic modalities, they have a role in clinical practice. Because dynamic tests often require relatively invasive monitoring devices, traditional methods may be helpful in acute situations when rapid assessment and treatment is necessary. Further, it is important to realize that dynamic modalities are meant to predict volume responsiveness and therefore assess hypovolemia, not hypervolemia. However, in a complicated patient presenting with multiple comorbidities, dynamic tests may be helpful to “rule-out” hypovolemia as an etiology for hemodynamic instability.

8

Can a chest radiograph be used to assess volume status?

The chest x-ray (CXR) has moderate positive predictive value for hypervolemia when the following findings are demonstrated: dilated upper lobe vasculature, cardiomegaly, interstitial edema, pleural effusion, and Kerley B-lines. However, these findings have poor sensitivity in detecting hypervolemia, and radiographic findings frequently lag the clinical manifestations of pulmonary edema. Therefore the CXR should be interpreted with caution, as the absence of the earlier findings does not preclude hypervolemia. Further, there are no radiographic findings reliably predictive of intravascular hypovolemia.

9

How accurate are orthostatic vital signs in the assessment of volume status?

Not very. Studies evaluating orthostatic measurements and postural dizziness revealed only a 22% sensitivity in identifying hypovolemia in patients who experienced 500 mL to 1 L of blood loss. Further, other factors, such as deconditioning, autonomic dysfunction, and other logistical barriers (e.g., intubation and sedation) may impair its clinical accuracy or utility. In other words, the very patient population deemed to benefit the most from volume assessment (intensive care unit [ICU] patients) often have pragmatic challenges or contraindications to the widespread use of this modality.

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