Arterial Blood Gases (ABG)
ABG interpretation is essential for understanding a patient’s acid-base balance.
Normal ABG Values
- pH: 7.35 (acidic) – 7.45 (alkaline)
- CO2: 35 – 45 (acidic when high)
- HCO3 (Bicarbonate): 22 – 26 (alkaline when high)
HCO3
- A bicarbonate that is regulated and excreted by the kidneys.
How to Read an ABG – ROME Mnemonic
- RO = Respiratory Opposite:
- pH high, CO2 low = Respiratory Alkalosis (hyperventilating).
- pH low, CO2 high = Respiratory Acidosis (not breathing enough).
- ME = Metabolic Equal:
- pH high, HCO3 high = Metabolic Alkalosis (e.g., low K+, vomiting).
- pH low, HCO3 low = Metabolic Acidosis (e.g., lactic acid, cardiac arrest).
Respiratory Acidosis
- What happens: CO2 builds up; the patient is not breathing effectively.
- ABG: Low pH, High CO2.
- Treatment: Increase ventilation (ventilate the patient), place an oral airway, reverse sedatives.
Respiratory Alkalosis
- What happens: The patient is over-breathing/hyperventilating.
- ABG: High pH, Low CO2.
- Treatment: Slow down respiratory rate on the ventilator, sedation.
Metabolic Acidosis
- Causes:
- Lactic acid buildup from ischemic tissue (most critical).
- Cardiac arrest, sepsis, ketoacidosis, renal failure (requiring dialysis).
- What occurs: An excessive production of metabolic acid in the body.
- Treatment: Administer Sodium Bicarbonate, support liver function.
Metabolic Alkalosis
- Causes:
- Excessive loss of K+ from vomiting.
- Excessive gastric suctioning.
- K+ wasting diuretics (e.g., Lasix, loop diuretics, hydrochlorothiazide).
- Too much IV Sodium Bicarbonate.
- Treatment: Address the underlying cause (e.g., antiemetics for vomiting, slow GI suction, change diuretics).
ABG Compensation (for exam)
- Compensation means the pH is within normal limits.
- ABG: pH is within normal limits, but both CO2 and HCO3 are outside normal limits.
Temperature Regulation
Maintaining normothermia is vital for patient outcomes.
Normal Temperature & Hypothermia
- Normal: 98.6°F / 37°C.
- Hypothermia: Below 96.8°F / 36°C.
Why Core Temperatures are Preferred
- Core temperatures are 3-5 degrees warmer than peripheral temperatures. A patient’s core temperature can drop significantly without being detected peripherally, especially during anesthesia when vasodilation occurs.
Warming Devices
- Use FDA-approved warming devices according to manufacturer instructions.
- Intermittent use is safest.
- Do not pre-warm the OR bed with the patient warmer; this can heat metal parts and cause burns.
OR Temperature and Humidity
- Temperature: 68-75°F (per TJC).
- Humidity: 20-60%.
- Room temperature is the least effective way to maintain normothermia. Room warming has little impact unless it exceeds 78-80°F. The room temperature can exceed 75°F for a specific patient/procedure, but must be returned to range afterward.
Irrigation Fluids/Blood Warming
- Warm to 98.6-104°F (37-40°C).
Avoiding Post-Op Shivering from Hypothermia
- Shivering increases O2 consumption by 400%, leading to:
- Myocardial ischemia and arrhythmias (less O2 to heart).
- Surgical site infections (SSI) (less O2 to the wound).
- Acidosis (mitochondria work harder to produce heat, generating lactic acid).
- Bleeding (hypothermia and acidosis impair clotting).
- WEIRD EXAM QUESTION: If choices for hypothermia complications don’t fit, choose “decreased bleeding” or “circulatory arrest” (rare, but induced hypothermia can be used to slow bleeding in certain complex cardiac procedures).
Surgical Specialty Considerations
Specific considerations apply to various surgical procedures.
Head and Neck Procedures
- Keep a trach tray nearby in case of airway swelling.
Jaw Wired Shut Considerations
- Wire cutters must accompany the patient to PACU and home (for emergent intubation).
- Provide a picture with directions for wire cutting to the patient for EMS.
Ear Surgery Anticipatory Issues
- Dizziness
- Nausea/Vomiting
Trach Patient Discharge
- Send the patient home with the obturator (for tube changes).
Cast Removal
- Remove casts outside the OR to avoid dust and flakes.
Wet Cast Handling
- Use normal exam gloves.
- Handle with palms only to avoid creating indentations.
- Leave open to air to dry (no immediate sling).
- Elevate above heart level to minimize swelling.
Bone Cement (Methyl Methacrylate)
- Vapor Avoidance: Avoid inhaling vapors, which can cause respiratory and eye irritation. Vapors can also melt contact lenses (wear eyewear).
- Pregnancy: Crosses the placental barrier, so pregnant individuals should avoid exposure.
- Anesthesia Notification: Inform anesthesia when placing bone cement in the canal, as it can cause vasodilation and a drop in blood pressure.
- Cold Room Effect: A cold room will cause the bone cement to set up slower.
Flap Procedures
- Blood supply is the greatest concern.
- Protect the flap from vasoconstriction (which leads to flap failure).
- Monitor circulation with a Doppler.
- Protect the flap from shearing or pressure.
- Keep the flap warm to promote capillary dilation.
Long Bone Procedures
- Highest risk: Fat Embolism (use the “Deflate” button on tourniquets).
- Complications: Fat emboli, DVT, PE.
Preventing Deep Vein Thrombosis (DVT)
- SCDs (Sequential Compression Devices): Ensure they are on and functioning before induction.
- Anticoagulants: Coumadin/Heparin.
- Early ambulation.
Pulmonary Embolism (PE)
- Signs: SOB, chest pain, pink frothy sputum (differentiates from MI).
- Action: Get monitors, oxygen, IV fluids.
Virchow’s Triad (DVT Formation)
- Venous Stasis (not moving).
- Vascular Injury (cutting).
- Hypercoagulability (factors that trigger clots).
Fluid Imbalance
Managing fluid balance is critical, especially in vulnerable patient populations.
Patients Prone to Fluid Imbalance
- Burn patients
- CHF patients
- Pediatric patients
- Neuro patients (ICP, spinal cord injury)
- Liposuction patients
- Diabetes Insipidus patients
- Renal patients
Burn Patients and Fluid Imbalance
- Day 1 & 2: Fluid shifts to interstitial tissue (3rd spacing), leading to swelling, puffiness, and weepy wounds. Intravascularly, the patient is dehydrated.
- Day 3: Fluid returns to the vascular system, resulting in fluid volume overload (FVO) and hemodilution.
CHF Patients and Fluid Imbalance
- Already prone to FVO due to dilated vessels and edema.
- Anesthesia causes further vasodilation.
- Instruct patients NOT to take diuretics before surgery (they are already intravascularly dehydrated).
- Will likely receive more IV fluid intraoperatively due to initial intravascular dehydration.
Neuro Patients and Fluid Imbalance
- Give just enough fluid.
- Too much IV fluid will increase ICP.
- Too little fluid will decrease cerebral perfusion.
Spinal Cord Injury Patients and Fluid Imbalance
- Loss of vasomotor tone below the injury site.
- BP will drop due to lack of tone, not necessarily fluid loss.
Liposuction Patients and Fluid Imbalance
- Removing fat can lead to the body attempting to fill the new space with fluid (3rd spacing).
- The surgeon must monitor suctioned amounts to prevent excessive fluid removal.
Tumescent Technique (Liposuction)
- Involves injecting fluid (Lidocaine for numbness + Epinephrine to decrease bruising and fluid shift) between tissue planes to separate them.
Diabetes Insipidus (DI)
- Cause: Pituitary gland or hypothalamus surgery/trauma, leading to a decrease in Antidiuretic Hormone (ADH).
- Result: Uncontrolled diuresis (1L+ urine/hour).
- Treatment:
- Fluid replacement ml for ml.
- Medications: DDAVP (synthetic ADH) or Vasopressin (less common, increases BP due to vasoconstriction).
Renal Patients and Fluid Imbalance
- Most important question: “When was your most recent dialysis?”
- Importance: Dialysis can leave a patient dehydrated. During induction, BP drops from vasodilation can be life-threatening if they lack compensatory volume. Renal patients are also generally prone to FVO.
Signs of Fluid Volume Overload (FVO) vs. Hypovolemia
| Fluid Volume Overload (FVO) | Hypovolemia |
| Edema | Dry mucous membranes |
| Dyspnea | Decreased urine output |
| Rales/Wet crackles | Dizziness/Fainting |
| Weight gain (kg overnight) | High HR (compensatory) |
| High BP, High CVP | Low BP |
| Bulging fontanelle (infants) | Sunken fontanelle (infants) |
Electrolyte Imbalance
Electrolytes play crucial roles in bodily functions.
Normal Sodium (Na+) Value & Job
- Normal: 135-145 mEq/L.
- Job: Regulate fluid volume (“Sodium sucks” – pulls H2O wherever it goes).
Hyponatremia (Low Sodium)
- What happens: FVO; Na+ is diluted. This is why NaCl is used for irrigation.
- Concerns with Hysteroscopy and TURPs:
- Air embolism.
- I&O: Irrigation fluids can be absorbed, leading to hemodilution (low Na+).
- Hyponatremia can cause fluid shifts into tissues, including the brain, leading to neuro concerns.
- Monitor I&O and watch for discrepancies. In TURP/TURBTs, continuous irrigation prevents clots and aids visualization.
- Concern with Glycine or Sorbitol in TURP/TURBT: Using these instead of NaCl can lead to FVO with free water, as they metabolize into free water.
- Body’s Response to FVO & Hyponatremia:
- Senses diluted Na+ and tries to concentrate it by urinating it off.
- Fluid shifts into tissues, including the cerebrum, causing neuro symptoms (N/V, irritability, headache, blurry vision).
- Symptoms: Nausea/Vomiting, irritability, headache, blurry vision, edema, cerebral edema, muscle cramps/twitching (Na is not a “P” electrolyte, so muscle movement goes opposite to its level).
- Treatment: Treated as FVO: fluid restriction, diuretics. If neuro symptoms, hypertonic saline and a monitored bed.
Hypernatremia (High Sodium)
- What happens: Dehydration/hypovolemia; Na+ is concentrated. The body pulls fluid from other areas into the vascular system.
- Symptoms: Thirst, muscle weakness (Na is not a “P” electrolyte, so muscle movement goes opposite to its level), heat stroke (body pulling CSF into vascular space), seizures/coma (due to CSF depletion and potential brain herniation).
- Treatment: Treated as dehydration: give fluids.
Electrolyte Buddies (H+, K+, and Glucose)
- H+ and K+ are often linked: Losing acid (e.g., from vomiting, diarrhea) can lead to K+ loss.
- In DKA (Diabetic Ketoacidosis), Glucose, H+, and K+ are all high.
Normal Potassium (K+) Value
- Normal: 3.5-5.0 mEq/L.
Potassium (K+)
Hypokalemia (Low Potassium)
- Normal Range: 3.5 – 5.0 mEq/L
- Factors Contributing to Low Potassium:
- Diuretics that cause potassium loss (e.g., loop diuretics like Lasix, hydrochlorothiazide).
- Excessive fluid loss from the GI tract due to vomiting, diarrhea, or bowel preparation.
- Overuse of laxatives.
- Alkalosis (as potassium shifts into cells during high pH).
- Manifestations of Low Potassium: These indicate a general slowing of bodily functions.
- Hypotension (low blood pressure) – reflecting overall lethargy.
- Abdominal bloating and absent bowel sounds.
- Muscle weakness or paralysis.
- Presence of a U wave on an EKG.
- Mnemonic: “Potassium starts with P, so muscles mimic its level.” (Low K+ leads to weak/paralyzed muscles).
- Intervention: Administer potassium, either orally or intravenously.
Hyperkalemia (High Potassium)
- Causes of Elevated Potassium:
- Excessive intravenous potassium administration. (Treated with Kayexalate).
- Release of intracellular potassium into the bloodstream due to:
- Depolarizing agents: Like those used in malignant hyperthermia, or succinylcholine administered to patients with underlying muscle disorders, where there’s no “stop gate” for depolarization. (Treated with D50 and insulin to shift potassium back into cells).
- Relationship of H+, K+, and Glucose: These three are interconnected. When one increases, the others tend to increase, and vice versa.
- Diabetic Ketoacidosis (DKA): Elevated H+, K+, and glucose.
- Burns: Initial shift of H+, K+, and glucose downward, followed by an upward shift.
- Addison’s Disease (an “acidic” condition): Elevated H+, K+, and glucose.
- Manifestations of High Potassium:
- Gastrointestinal cramping. (As potassium starts with “P,” muscle actions mirror the potassium level, so high K+ leads to increased muscle activity like cramping).
- Elevated T wave on Lead II of an EKG.
- Hypertension (high blood pressure).
- Potentially severe spastic paralysis and cardiac arrest if levels are significantly high.
- Treatment Strategies:
- If due to IV potassium overdose: Kayexalate is the sole treatment. It can be given orally or as an enema and works by absorbing potassium from the GI tract, leading to significant GI upset.
- For other causes:
- D50 (50% Dextrose) + Insulin: Insulin facilitates the movement of potassium back into cells, and D50 prevents hypoglycemia.
- Correcting Acidosis: Eliminating excess hydrogen ions (H+) will also cause potassium levels to decrease.
Calcium (Ca2+)
Normal Calcium Values:
- Serum Calcium (on electrolyte panel): 8.5 – 10.5 mg/dL
- Ionized Calcium (free, unbound Ca2+): 4.5 – 5.6 mg/dL
Understanding Calcium and Albumin:
- Calcium binds to albumin. If serum calcium is low and albumin is also low on the Comprehensive Metabolic Panel (CMP), the total calcium level may be misleading. In such cases, always check the ionized calcium level (which represents the physiologically active, unbound calcium) for an accurate assessment.
Hypocalcemia (Low Calcium)
- Causes of Low Calcium:
- Blood Transfusions: Citrate used in blood bags binds with calcium, reducing available calcium.
- Hypoparathyroidism:
- Can occur after parathyroidectomy or thyroid surgery that disturbs the parathyroid glands.
- The parathyroid gland regulates calcium and phosphorus (which have an inverse relationship, like a “teeter-totter”). Low parathyroid hormone leads to low calcium and high phosphorus.
- Diuretics that cause calcium loss (e.g., loop diuretics like Lasix, hydrochlorothiazide).
- Electrolyte Concerns with Parathyroidectomy: Expect low calcium and high phosphorus.
- Exam Hint: When evaluating electrolyte imbalances, look for scenario clues rather than just similar symptoms. For example, if a patient has muscle twitching/cramping AND a recent parathyroidectomy, the most likely cause is hypocalcemia due to impaired calcium excretion.
- Manifestations of Low Calcium:
- Twitching and cramping (As calcium doesn’t start with “P,” muscle movement is opposite to its level, so low Ca+ leads to increased muscle activity).
- Laryngospasm.
- Arrhythmias.
- Chvostek’s sign (facial twitching when tapping the cheek).
- Trousseau’s sign (hand cramp induced by a blood pressure cuff).
- Intervention: Administer calcium orally or intravenously.
Hypercalcemia (High Calcium)
- Causes of High Calcium:
- Total Parenteral Nutrition (TPN): Most common cause if TPN provides too much calcium.
- Hyperparathyroidism: A parathyroid tumor can lead to excessive hormone secretion and high calcium levels, with correspondingly low phosphorus levels.
- Bone Cancer / Multiple Myeloma: Release calcium into the vascular system. (Treated with Mithramycin).
- Sarcoidosis (in lungs): Can cause increased gastrointestinal absorption of dietary calcium.
- Manifestations of High Calcium:
- Neuromuscular depression (As calcium doesn’t start with “P,” muscle movement is opposite to its level, so high Ca+ leads to decreased muscle activity).
- Arrhythmias.
- Treatment Strategies:
- Mithramycin: Used if hypercalcemia is due to bone tumors/cancer.
- Phosphate Replacement: Most common treatment. Increasing phosphate levels will pull down calcium levels due to their inverse relationship.
Importance of Calcium:
- Clotting Cascade: Calcium is Factor IV in the clotting cascade.
- Muscle Contraction: Essential for myocardial muscle contraction and maintaining vascular tone. Low calcium can make patients appear like CHF patients.
Phosphorus (Phos)
- Normal Values: 1.0 – 2.0 mEq/L
- Key Concept: Phosphorus is on a “teeter-totter” with Calcium.
- High calcium information implies low phosphorus information.
- Low calcium information implies high phosphorus information.
- Mnemonic: “Phosphorus starts with P, so muscles mimic its level.”
Magnesium (Mg)
- Normal Values: 1.5 – 2.5 mEq/L
Hypomagnesemia (Low Magnesium)
- Most Common Cause: Malnutrition.
- Other Causes: Alcoholism, pancreatitis, and diuretics that cause potassium, calcium, and then magnesium wasting.
- Manifestations:
- Muscle spasms, twitching. (As magnesium doesn’t start with “P,” muscle movement is opposite to its level, so low Mg+ leads to increased muscle activity).
Hypermagnesemia (High Magnesium)
- Most Common Cause: Therapeutic administration to stop premature labor or manage preeclampsia. (Normal therapeutic range for these conditions is 6-8 mEq/L).
- Watch for:
- Rapid Onset Pulmonary Edema: This is the only “deal-breaker.” It occurs because respiratory muscles are depressed (magnesium doesn’t start with “P,” so muscles are opposite to its level, meaning high Mg+ leads to decreased muscle activity). If this occurs, stop magnesium infusion immediately, administer Calcium Gluconate for reversal, and deliver the baby.
- Hypotension: Episodes of low blood pressure (manage as needed).
- Poor Deep Tendon Reflexes (DTRs): The baby may be floppy with a low APGAR score.
- Why is Magnesium Given in Preeclampsia/Premature Labor? To halt labor and provide time to administer steroids to aid fetal lung development.
- Treatment for V-Fib and Torsade’s de Pointes: Magnesium is the treatment of choice.
Cellular Metabolism
Waste Products of Mitochondrial Energy Production
When mitochondria produce energy, they generate:
- Heat
- Carbon Dioxide (CO2)
- Hydrogen Ions (H+) (acid)
Complete Blood Count (CBC) and Coagulation
The CBC and coagulation tests provide critical information about a patient’s blood components and clotting ability.
Components of a CBC
- Red Blood Cells (RBCs) (contain hemoglobin).
- Hemoglobin (Hgb) (carries oxygen).
- Hematocrit (HCT) (indicates anemia).
- Platelets (involved in clotting).
- White Blood Cells (WBCs).
- PT/PTT (time to clot).
Red Blood Cells (RBCs)
- Normal Range:
- Men: 4.3 – 5.9 x 10^6 cells/uL
- Women: 3.5 – 5.5 x 10^6 cells/uL
- Function: RBCs contain hemoglobin, which carries oxygen.
Hemoglobin (Hgb)
- Normal Range:
- Men: 13.2 – 17.5 g/dL
- Women: 11.5 – 16 g/dL
- Function: Hemoglobin is inside RBCs and is responsible for oxygen transport.
Hematocrit (HCT)
- Normal Range:
- Men: 42 – 52%
- Women: 37 – 47%
- Indication: A low HCT indicates anemia.
- Ideally, H&H levels should be checked a couple of weeks pre-op. If low, iron supplements can be given preoperatively to increase HCT. This is particularly relevant for patients with conditions like rheumatoid arthritis (RA).
Platelets
- Normal Range: 150,000 – 450,000 cells/uL
- Action for Low Levels: If platelet count is low, check PT/PTT & INR. If those coagulation tests are within acceptable limits, surgery can typically proceed.
White Blood Cells (WBCs)
- Normal Value: 5,000 – 10,000 cells/uL
- High WBCs indicate: Infection, autoimmune disease (e.g., lupus), or leukemia (may require bone marrow biopsy).
- Low WBCs indicate: Prolonged deep bone infection, bone marrow suppression (e.g., from chemotherapy/radiation).
Prothrombin Time (PT)
- Normal Value: 11 – 12.5 seconds
- Evaluates: The extrinsic and common pathways of coagulation.
- Extrinsic pathway: Involves Factor VII (7) (relates to clotting outside the body).
- Common pathway: Involves Factors X, V, II, I (10, 5, 2, 1 – think “all bills in your wallet except a 20”).
- Indications for Testing:
- Bleeding or clotting disorders.
- Liver disease/elevated liver enzymes (liver produces Vitamin K-dependent clotting factors, so PT indicates liver function).
- Warfarin/Coumadin therapy (used to prevent clots in patients with heart valves/stents). Check PT and INR for monitoring.
Activated Partial Thromboplastin Time (aPTT or PTT)
- Normal Value: 30 – 40 seconds
- Evaluates: The intrinsic and common pathways of coagulation.
- Intrinsic pathway: Involves Factors XII, XI, IX, VIII (NOT Factor 7) (relates to issues inside the body).
- Common pathway: Involves Factors X, V, II, I.
- Indications for Testing:
- Bleeding/clotting disorders.
- Heparin therapy (used to prevent clots for intrinsic issues).
- Hemophilia: (Factor VIII deficiency, an intrinsic pathway issue). Prophylactic Factor VIII is typically given even if PTT is normal.
- Early DIC: PTT may initially be shortened (<30 sec) as clotting factors are being rapidly consumed. Perform serial PTTs during trauma surgery to monitor trends, as eventually, PTT will lengthen once factors are depleted.