Key Feature 5: In a dehydrated patient: Identify the precipitating illness or cause, especially looking for non-gastro-intestinal, including drug-related, causes
Skill: Clinical Reasoning
Phase: Hypothesis generation

​Today I encountered a 16 year old female with a history of primary lymphagiectasia (aka lymphagiwhattheheckisthis) who presented to the Children's Emergency Department with sudden onset of incessant vomiting that awoke her suddenly from sleep 3 hours prior to my assessment. On examination she appeared moderately dehydrated (mildly tachycardic with a prolonged cap refill). When I told my preceptor the story, he wasn't convinced that she was dehydrated to a moderate degree, knowing that over such a short timeline of vomiting it would be unlikely to be more than mildly dehydrated. We went in to reassess the patient together, and to his surprise, he agreed that she also appeared moderately dehydrated on assessment. We proceeded to order IV fluids given that she was not able to keep oral fluids down at this point, and then we quickly did some research on her rare disease, to see if her presentation was possibly a complication of it. We learned that one of the complications of this disease was edema and third spacing. Aha! Although we hadn't narrowed down a specific reason for her abdominal pain and vomiting yet, we had a plausible reason for why she was already moderately dehydrated. More than just fluid loss from vomiting, this patient was likely having some third spacing going on. 

When encountering a patient who is hypovolemic, what is the DDx for the hypovolemia? The UpToDate article, Clinical assessment and diagnosis of hypovolemia (dehydration) in children (2018), provides a breakdown:

1. Hemorrhage (A medical adage, "Hypovolemia is from blood loss unless proven otherwise.")
2. Third space loss (Ex: from edema or ascites, which both have their own DDx. Hemorrhage can also occur into a third space, such as is the case with a retroperitoneal hemorrhage)
3. Other
   1. Gastrointestinal (ex: vomiting, diarrhea)
   2. Skin (ex: fever, burns)
   3. Urine (ex: glucosuria, diuretic therapy, diabetes insipidus)
   4. Prolonged inadequate oral fluid intake

By the end of postgraduate training, using a patient-centred approach and appropriate selectivity, a resident, considering the patient’s cultural and gender contexts, will be able to...

• Manage urgent and emergency medical conditions in various settings, recognizing the trend towards short stay hospital observation and outpatient management

Dehydration

Key Feature 1: When assessing the acutely ill patient, look for signs and symptoms of dehydration. (ex: look for dehydration in the patient with a debilitating pneumonia).
Skill: Clinical Reasoning
Phase: Hypothesis generation

Key Feature 2: In the dehydrated patient, assess the degree of dehydration using reliable indicators (ex: vital signs) as some patients' hydration status may be more difficult to assess (ex: elderly, very young, pregnant). 
Skill: Clinical Reasoning
Phase: Physical, Investigation

Key Feature 3a: In a dehydrated patient: Determine the appropriate volume of fluid for replacement of deficiency and ongoing needs.
Skill: Clinical Reasoning
Phase: Treatment, Diagnosis

Key Feature 3b: In a dehydrated patient: Use the appropriate route (oral if the patient is able; IV when necessary). 
Skill: Clinical Reasoning
Phase: Treatment

Key Feature 4: When treating severe dehydration, use objective measures (ex: lab values) to direct ongoing management. 
Skill: Clinical Reasoning
Phase: Investigation, Treatment

Key Feature 5: In a dehydrated patient: Treat the precipitating illness concurrently. 
Skill: Clinical Reasoning
Phase: Treatment

Key Feature 6: Treat the dehydrated pregnant patient aggressively, as there are additional risks of dehydration in pregnancy. 
Skill: Clinical Reasoning, Selectivity
Phase: Hypothesis generation, Treatment

Diarrhea

Key Feature 1a: In all patients with diarrhea: Determine hydration status.
Skill: Clinical Reasoning
Phase: Diagnosis

Key Feature 1b: In all patients with diarrhea: Treat dehydration appropriately.
Skill: Clinical Reasoning
Phase: Treatment

Pneumonia

Key Feature 2: In a patient with signs and symptoms of pneumonia, do not rule out the diagnosis on the basis of a normal chest X-ray film (ex: consider dehydration, neutropenia, human immunodeficiency virus [HIV] infection).
Skill: Clinical Reasoning, Selectivity
Phase: Diagnosis, Hypothesis generation

Key Feature 3: In a patient with a diagnosis of pneumonia, assess the risks for unusual pathogens (ex: a history of tuberculosis, exposure to birds, travel, HIV infection, aspiration).
Skill: Clinical Reasoning
Phase: History, Hypothesis generation
​
Today I encountered a 9 year female with congenital muscular dystrophy who was presenting with an acute cough, vomiting, and diarrhea. She looked unwell, and dehydrated, and on lung exam she had decreased breath sounds on one side of her lungs compared to the other. Chest xray revealed a middle lobe pneumonia. 

What are the signs and symptoms of dehydration? 

Late signs 

1. Altered level of consciousness
2. Hypotension
3. Decreased urine output 

​Early signs

1. Tachycardia
2. Tachypnea
3. Sunken anterior fontanelle
4. Lack of tears when crying
5. Dry mucous membranes
6. Cool distal extremities
7. Weak peripheral pulses
8. Cap refill >5s
9. Decreased skin turgor
10. Sudden weight loss 

Evaluating someone's volume status (i.e., whether they are hypovolemic, euvolemic, or hypervolemic) is a clinical assessment; there are no tests that can specifically tell you whether or not someone is dehydrated or fluid-overloaded. It's all about considering various clinical indicators of volume status, and different patient factors can make certain indicators more or less useful. For example, in the very young patient, the degree to which their anterior fontanelle is filled, from bulging to very sunken, can be a very useful indicator of volume status, but this is not useful once the anterior fontanelle is closed, which is usually the case by the age of 2. Whereas in the elderly patient with wrinkled skin, using skin turgor to assess degree of dehydration is pretty well useless. And when it comes to the patient who is pregnant, a high index of suspicion is needed because there are many physiologic adaptations to pregnancy that may mask early signs of dehydration. Furthermore, the ramifications could be as significant as dehydration inducing uterine cramping and preterm labour, or could manifest in long-term complications related to impairments in fetal growth and development; a lower threshold to  begin a rehydration regimen can have a significant benefit-to-risk ratio. Vital signs are a useful component of the evaluation of volume status as they are objective (including, in the pregnant patient, fetal heart rate +/- amniotic fluid volume assessment by ultrasound), but by themselves they may not be sufficient enough to determine volume status as abnormal vital signs can be a reflection of much more than volume status. For example, a dehydrated patient could be expected to be tachycardic, but tachycardia can also be caused by fever, or hyperthyroidism, or anxiety, or pain, etc. So the vital signs must be interpreted in the clinical context as well, much as the more subjective indicators of volume status need to be.

When a patient is determined to be dehydrated, it's time to get them rehydrated while treating the precipitating cause, which for the above patient case meant to give antibiotic therapy for the pneumonia.* The approach to rehydration will depend on how dehydrated you think the patient is, and is generally clinically assessed as being mild, moderate, or severe. For any degree of degree of dehydration, from mild to severe, rehydration will consist of a replacement phase (to replace fluid debt; normal saline [NS] is always used) followed by maintenance (to replace ongoing losses; type of solution here will depend on context). The approach will also depend on patient factors, such as whether they are a pediatric or adult patient, whether they have comorbidities such as kidney disease or diabetes, whether there are any ongoing sources of fluid loss (ex: diarrhea, vomiting, sweating), and if they have an electrolyte disturbance.

Note that most people walking around in the world are maintaining their fluids orally, and in general this is the preferred approach. In the severely dehydrated patient, however, oral replacement just won't suffice, and so intravenous fluid resuscitation is always indicated in this situation. In the moderately dehydrated patient, it is still very important to replace their fluid debt, but the situation is not as critical, at least not yet. If the patient is tolerating oral fluids and is well enough to drink lots of fluid, instead of automatically initiating intravenous fluid replacement, you may wish to consider a trial of oral rehydration first (if it would be feasible given the reason for the dehydration and the patient's ability to compensate). And then, if the patient is tolerating oral fluids, it's always best to try replacing fluid debt orally if the severity of the dehydration is nothing more than mild. It goes without saying (but I'll say it anyway for completion) that anyone who can tolerate oral fluids definitely does not need intravenous replacement (unless ongoing losses are so extreme that the patient just can't keep up). In this latter situation, the patient would be like everyone else who compensates for typical water loss in a day by taking in enough fluid in food and drinks to compensate for fluid lost in urine, stool, perspiration, and insensible losses (from the skin and respiratory tract). Sometimes this would be the only intervention that would be keeping a patient in hospital, and with patients/caregivers who you expect will be reliable to follow-up if things are not improving or worsening, it is a waste of health care resources and is usually less pleasant for the child who could otherwise be more comfortable in their own bed. Generally, patients who are assessed for urgent or emergent medical issues and who are deemed safe to follow-up as an outpatient may be discharged for outpatient management. This is preferred by most patients and healthcare providers, the latter of whom are also familiar with the real possible of secondary nosocomial illness secondary to hospital admission.

For the patient with mild to moderate dehydration who is a candidate for oral rehydration therapy, is any fluid okay? Well any fluid is better than no fluid, but some fluids are better than others. In particular, the World Health Organization recommends Oral Rehydration Solution, which is a specific mixture of water, glucose, and salt. Because water follows sodium, if we have sodium in the solution, then as this is taken up by the gastrointestinal tract, more water will also get absorbed. There are also receptors in the gut that take up more sodium when glucose is present (sodium-glucose transport proteins), and so by adding in glucose as well, the uptake of water is even greater. To make your own Oral Rehydration Solution, combine 1L of water with a half of a teaspoon (2.5 mL) of salt and 2 tablespoons (30 mL) of sugar. Alternatively, there are products that can be purchased other-the-counter from pharmacies.

For those in whom oral rehydration therapy will not suffice, IV fluids are indicated. Below are my approaches to rehydration in adult and pediatric patients, respectively, when IV fluids are required.

Adult

1. Replacement
   1. If severely dehydrated
      1. Give 1-2 L bolus of NS
      2. Continue to give additional boluses until euvolemic
      3. Consider blood transfusion if hypovolemia is secondary to blood loss
   2. If moderately dehydrated
      1. 50 mL/h in addition to calculated maintenance fluid
   3. If mildly dehydrated
      1. 30 mL/h in addition to calculated maintenance fluid
2. Maintenance
   1. Baseline investigations (reassess as clinically indicated)
      1. Serum potassium
      2. Serum sodium
      3. Serum glucose
      4. Serum creatinine
      5. Serum urea
      6. Consider the need to monitor ins and outs as well as patient weight
   2. Maintenance fluid requirement calculated using 4:2:1 rule
      1. Calculated as 4 mL/kg for the first 10 kg of body weight, followed by 2 mL/kg for the next 10 kg of body weight, followed by 1 mL/kg for every addition kg of body weight thereafter (ex: If a patient weighs 60 kg, their maintenance fluid requirement equals 100 mL)
   3. Default fluid choice: 1/2NS with KCl 20 mEq/L
   4. Add dextrose if indicated, written as: D51/2NS with KCl 20 mEq/L
      1. Contraindicated if patient is hypokalemic
   5. Adjust electrolyte concentrations as indicated:
      1. If serum sodium starts to fall or patient is hyponatremic, increase to NS instead of 1/2NS
      2. If serum sodium starts to rise or patient is hypernatremic, decrease to 1/4NS
      3. If serum potassium starts to fall or patient is hypokalemic, increase potassium dose
      4. If serum potassium starts to rise or patient is hyperkalemic, eliminate potassium
   6. Remember to consider the patient's context when making your choice of fluid type and infusion rate

Pediatric

1. Replacement
   1. If severely dehydrated
      1. Give 20 mL/kg bolus of NS
      2. Continue to give additional boluses until euvolemic
      3. Consider blood transfusion if hypovolemia is secondary to blood loss
   2. If moderately dehydrated
      1. Give 10 mL/kg bolus of NS over 30 min
      2. Alternatively, if a patient is going to be hospitalized and is expected to need intravenous therapy anyway, consider giving 20 to 40 mL/kg over two to four hours 
   3. If mildly dehydrated
      1. Give 10 mL/kg bolus of NS over 60 min
2. Maintenance
   1. Baseline investigations (reassess as clinically indicated)
      1. Serum potassium
      2. Serum sodium
      3. Serum glucose
      4. Serum creatinine
      5. Serum urea
      6. Consider the need to monitor ins and outs as well as patient weight
   2. Maintenance fluid requirement calculated using 4:2:1 rule
      1. Calculated as 4 mL/kg for the first 10 kg of body weight, followed by 2 mL/kg for the next 10 kg of body weight, followed by 1 mL/kg for every addition kg of body weight thereafter (ex: If a patient weighs 60 kg, their maintenance fluid requirement equals 100 mL). Generally speaking, the maintenance infusion rate is not to exceed 100 mL/h in a pediatric patient. 
   3. Default fluid choice: NS with KCl X mEq/L 
      1. KCl 10 mEq/L if <10 kg
      2. KCl 10-20 mEq/L if ≥ 10 kg
   4. Add dextrose if indicated, written as: D5NS
      1. Contraindicated if patient is hypokalemic
   5. Adjust electrolyte concentrations as indicated:
      1. If serum sodium starts to fall or patient is hyponatremic, increase to hypertonic saline instead of NS
      2. If serum sodium starts to rise or patient is hypernatremic, decrease to 1/2NS
      3. If serum potassium starts to fall or patient is hypokalemic, increase potassium dose
      4. If serum potassium starts to rise or patient is hyperkalemic, eliminate potassium
   6. Remember to consider the patient's context when making your choice of fluid type and infusion rate

All this talk of fluid replacement is getting me thirsty! 

*Note that while this patient presented with x-ray evidence of pneumonia, many dehydrated patients may have a falsely negative chest x-ray early on in the disease process, which may also be the case for patients who are immunocompromised. If suspected clinically and the patient has risk factors for being unable to mount a swift early immune response, consider the need for empiric treatment for suspicion of pneumonia to prevent patient decompensation/deterioration. It's also important to consider risk factors a patient may have for acquisition of unusual pathogens, as may also occur in the patient who is immunocompromised or who has had unusual exposures to animals or other environments through travel. In the elderly or neurologically/cognitively compromised patients, consider their risk of chemical pneumonitis from aspiration. These risk factors matter because they influence choice of antibiotic.