Critical Care Lab Values and Interpretation

Overview of Laboratory Values for Critical Care Respiratory Therapists

• Context of Monitoring: Lab values are a critical component of internship logs and daily clinical practice. While Arterial Blood Gases (ABGs) are foundational and should be mastered by this stage, several other diagnostic tests are essential for comprehensive patient assessment, especially in the context of disease-specific management and mechanical ventilation.

• Holistic Assessment: Understanding non-respiratory labs allows the therapist to determine why a patient is on a ventilator and what factors contribute to their clinical trajectory. A good clinician looks beyond primary respiratory data to understand the physiological connections between metabolic states, organ function, and pulmonary stability.

Complete Blood Count (CBC)

• White Blood Cell (WBC) Count:     * Normal Range: $4,000$ to $11,000\,\text{per mm}^3$ (or $4$ to 11\,\text{k/\mu L}).     * Clinical Significance: Serves as a primary indicator of infection or systemic inflammation.     * High WBC (Leukocytosis): Usually indicates an active infection (e.g., pneumonia). Respiratory therapists care about high WBCs because the associated fever increases the metabolic rate, causing tachycardia and potentially increasing blood pressure, which affects ventilator management.     * Low WBC (Leukopenia): Defined as a count less than $4,000$. This indicates an immunocompromised state, common in chemotherapy, HIV, leukemia, or lymphoma patients.     * Reverse Isolation: Patients with very low WBC counts are placed on reverse isolation (gown, mask) to protect them from healthcare-acquired infections.

• WBC with Differential (DIF):     * Eosinophils: This specific type of white blood cell is relevant to the respiratory therapist in asthma cases. Elevated eosinophils suggest an allergic asthmatic component (eosinophilic triggered asthma), where the body reacts to triggers like trees or grasses.

• Red Blood Cell (RBC) Count and Components:     * Hemoglobin ($Hb$): Measures the oxygen-carrying capacity of the blood.         * Normal Range: $12$ to $17\,g/dL$.         * Gender Variation: Typically $12$ to $14\,g/dL$ in females and $14$ to $16\,g/dL$ in males. A male with a hemoglobin of $12\,g/dL$ is generally not a cause for panic.     * Hematocrit ($Hct$): The percentage of red blood cells relative to total plasma and other blood components.         * Rule of Three Formula: A quick estimation for blood gases is $\text{Hemoglobin} \times 3 = \text{Approximate Hematocrit}$. For example, a hemoglobin of $7\,g/dL$ results in a hematocrit of approximately $21\%$.         * Significance in Respiratory Care: High oxygenation settings on a ventilator are ineffective if the patient lacks sufficient red blood cells to transport that oxygen to the tissues.

Blood Transfusions and Clotting Factors

• Transfusion Protocols:     * Indications: Hemorrhaging, trauma (e.g., Motor Vehicle Accidents - MVAs, Gunshot Wounds - GSWs), or post-operative monitoring (e.g., open-heart surgery).     * Blood Temperature: Units from the lab are cold. Giving cold blood rapidly can harm the patient. Rapid infusers are used to squeeze the blood in under pressure while warming it to $37^{\circ}\text{C}$.     * MTP (Massive Transfusion Protocol): A specialized alert for extreme blood loss scenarios. This is common in trauma and Obstetrics (OB) for maternal hemorrhages during C-sections.     * Autologous Donation: In open-heart surgeries, surgeons may collect the patient's own blood and re-infuse it, ensuring perfect compatibility.

• Blood Typing:     * Types: A, B, O, and AB. The Rh factor determines positive or negative status.     * Universal Donor: O-negative is given in emergencies when the blood type is unknown.     * Rarest Type: AB-positive is noted as particularly rare.     * Logistics: Hospitals have limited supplies; blood typically lasts only $4$ to $5$ days in refrigeration before it must be replenished.

• Platelets:     * Normal Range: $150,000$ to $400,000\,\text{per mm}^3$.     * Function: Responsible for the clotting mechanism. Low platelets increase the risk of rapid bleeding out.

• Disseminated Intravascular Coagulopathy (DIC):     * A disorder where blood fails to clot. Blood is infused as fast as it exits through wounds. It is highly fatal and often leads survivors directly into Acute Respiratory Distress Syndrome (ARDS).

Electrolytes (BMP and CMP)

• The Basic Metabolic Panel (BMP): Focuses on salt, sugar, and renal function. The Comprehensive Metabolic Panel (CMP) adds liver function and protein levels.

• Sodium ($Na^+$):     * Normal Range: $135$ to $145\,mEq/L$.     * Function: Regulates muscle activity and fluid shifts.

• Potassium ($K^+$):     * Normal Range: $3.5$ to $5.0\,mEq/L$.     * Clinical Impact: Extremes lead to cardiac arrest. High potassium is the primary agent used in lethal injections to stop the heart. Low levels cause muscle spasms, cramping, and diaphragm dysfunction.     * RT Intervention: Continuous albuterol can be used to treat hyperkalemia (high potassium), as it drives potassium into the cells. However, this must be avoided if the patient is already tachycardic (e.g., heart rate of $125\,bpm$).

• Chloride ($Cl^-$):     * Normal Range: $98$ to $106\,mEq/L$.     * Function: Works with sodium for muscle and cellular balance.

Renal Function and Fluid Balance

• BUN (Blood Urea Nitrogen):     * Normal Range: $7$ to $20\,mg/dL$.

• Creatinine:     * Normal Range: $0.6$ to $1.3\,mg/dL$.

• Significance: Elevated BUN and creatinine indicate kidney dysfunction or failure. Dehydration can cause slight elevations (e.g., $23\,mg/dL$). Kidney failure impacts acid-base balance and fluid retention.

• Fluid Balance (I's and O's): If a patient is fluid-positive (e.g., net gain of $1,000$ to $1,400\,mL$ daily for multiple days), they may struggle to wean from the ventilator due to cardiac strain or pulmonary edema.

• Metabolic Impacts: Severe vomiting results in the loss of stomach acid, causing metabolic alkalosis.

Glucose and Nutrition

• Glucose:     * Normal Range: $70$ to $110\,mg/dL$ (some hospitals use $80$ to $105\,mg/dL$).     * Hyperglycemia: Levels above $400\,mg/dL$ can lead to Diabetic Ketoacidosis (DKA).     * Hypoglycemia: Extremely dangerous; causes paleness, diaphoresis (sweating), and loss of consciousness. Treated with a bolus of D50 (dextrose).

• Total Parenteral Nutrition (TPN): A yellow intravenous infusion bag containing fats, carbs, and electrolytes. High carbohydrate/fat diets increase metabolic rate, which can impede ventilator weaning.

Specialized Markers and Coagulation

• Lactate (Lactic Acid):     * Normal Range: $0.5$ to $2.0\,mmol/L$.     * Significance: Measures tissue hypoxia, shock, and sepsis. High levels (e.g., $12\,mmol/L$) indicate anaerobic metabolism.

• Rhabdomyolysis (Rhabdo): Muscle breakdown often caused by heat or extreme exertion, leading to elevated acids and potential cardiac arrest.

• Coagulation Studies:     * PT (Prothrombin Time): $11$ to $13.5\,\text{seconds}$.     * INR: $0.8$ to $1.2$.     * PTT: $25$ to $35\,\text{seconds}$.     * RT Concerns: If these values are elevated, the patient will bleed more easily during an ABG puncture. Blood thinners include Heparin, Warfarin (Coumadin), Eliquis, and Aspirin/NSAIDs.

• Cardiac Markers:     * Troponin: Indicator of myocardial infarction (MI). High values (in the thousands) signal heart damage.     * BNP (B-type Natriuretic Peptide): Indicator of heart failure or Congestive Heart Failure (CHF). Useful for diagnosing pulmonary edema when bilateral crackles are present.

• Liver Function Tests (LFTs):     * Markers: AST, ALT, ALP, and Bilirubin.     * Clinical Presentation: Liver failure presents as jaundice (yellowing of the eyes and skin). Newborns with high bilirubin are treated with ultraviolet (UV) light to lower the levels.

Advanced ABG Interpretation and Ventilation Calculations

• Anion Gap:     * Normal: $8$ to $12$.     * Significance: Used to diagnose the cause of metabolic acidosis (e.g., DKA).

• Winters' Formula:     * Used to determine the target $PaCO_2$ for a patient with metabolic acidosis who is undergoing respiratory compensation.     * Formula: $\text{Target } PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$     * Example Calculation: If bicarbonate ($HCO_3^-$) is $12$.         1. $12 \times 1.5 = 18$         2. $18 + 8 = 26$         3. Target Range: $24$ to $28\,mmHg$.     * Clinical Application: If the patient's $PaCO_2$ is not within the target window, the respiratory rate on the ventilator should be adjusted (usually increased) to prevent a life-threatening drop in pH. A pH below $6.8$ or $6.9$ is generally non-survivable.

Questions & Discussion

• Question (Student): Is D-I-C just in obstetrics?

• Response: No, DIC can occur in multiple surgical events or traumas, though it is frequently seen in OB emergencies.

• Question (Student): Will a kidney stone elevate BUN and creatinine?

• Response: Yes, research confirmed that kidney stones can indeed cause these numbers to elevate.

• Dialogue (WBC/Infection): A student noted a case where albuterol couldn't be given to lower potassium because the patient was already tachycardic. The speaker confirmed that albuterol increases heart rate and is contraindicated in such scenarios.

• Dialogue (Blood Draws): A student shared an experience in the ER where a patient had two fistulas (surgical arterial-venous connections) and required a Doppler ultrasound for a blood draw. The speaker reminded the class that fistulas are sensitive areas typically avoided for standard ABGs..