Introduction

·         The Sellick maneuver is the application of pressure on the cricoid ring to prevent aspiration. Cricoid pressure should be initiated during preparation for intubation from the time sedation is administered or bag-mask ventilation is initiated until the airway is demonstrated to be secured.

·         West describes four zones of perfusion in an upright lung. Beginning at the apices, they are:

Zone 1: Alveolar pressure (PAlv) exceeds pulmonary artery pressure (Ppa) and pulmonary venous pressure (Ppv), leading to ventilation without perfusion (alveolar dead space) (PAlv > Ppa > Ppv). Zone 1 is essentially nonexistent in healthy, normovolemic patients.

Zone 2: Pulmonary arterial pressure exceeds alveolar pressure, but alveolar pressure still exceeds venous pressure. (Ppa > PAlv > Ppv). Blood flow in zone 2 is determined by arterial-alveolar pressure differences, which steadily increase down the zone.

Zone 3: Pulmonary venous pressure now exceeds alveolar pressure, and flow is determined by the arterial-venous pressure difference (Ppa > Ppv >PAlv).

Zone 4: Interstitial pressure (Pinterstitium) is greater than venous and alveolar pressures; thus flow is determined by the arterial-interstitial pressure difference (Ppa > Pinterstitium > Ppv > PAlv). Zone 4 should also be minimal in a healthy patient.

A change from upright to supine position increases pulmonary blood volume by 25-30%, thus increasing the size of larger numbered West zones

·         CPR: Assessing the environment to see if it is safe to approach is the first priority when considering providing aid to an unconscious patient. You do not want to become a casualty yourself! CPR related infections are extremely rare, although Tuberculosis, HIV and Neisseria meningitidis have all been recorded. Once it has been confirmed that the patient is not breathing you must get help or alert the emergency services, even if this means leaving the patient (this is especially important in a pre hospital environment). However, if the patient is an infant or child, a victim of trauma, a near drowning or if drug or alcohol intoxication is likely, then one minute of CPR should be performed before going for help. The correct ratio of compressions to ventilations is 15:2 regardless of the number of rescuers present.

·         CAUSES OF HYPOXEMIA AND THEIR RESPECTIVE A-a GRADIENTS

1. Low fractional concentration of inspired O2: normal A-a gradient
2. Hypoventilation: normal A-a gradient
3. Ventilation/perfusion mismatch: elevated A-a gradient
4. Right-to-left shunting: elevated A-a gradient
5. Diffusion abnormality: elevated A-a gradient

·         In 1981, Stewart proposed a radically different model by suggesting that three independent variables (none of which is HCO3) determine the pH by primarily changing the degree of water dissociation into hydrogen ions. These variables are the strong ion difference, albumin concentration (the most abundant nonbicarbonate buffer in plasma), and the PaCO2. Based on the laws of mass action, the conservation of mass, and the conservation of charge, Stewart derived several complex mathematical equations based on these variables to describe acid-base balance. This model has been most useful in interpreting complex acid-base disorders in patients with severe hypoalbuminemia and in explaining hyperchloremic metabolic acidosis caused by normal saline infusions. Because Stewart's approach was initially viewed as complex and radical, his work was highly criticized, which limited its acceptance, but it is gaining popularity.

·         The base deficit (BD) is the number of mEq/L of base (or acid) needed to titrate a serum pH back to normal at 37°C while the PaCO2 is held constant at 40 mmHg, thus eliminating the respiratory component. Therefore, the BD represents only the metabolic component of an acid-base disorder. The ABG analyzer derives the BD from a nomogram based on the measurements of pH, HCO3, and the nonbicarbonate buffer hemoglobin. Although the BD is determined in part by the nonbicarbonate buffer hemoglobin, it is criticized because it is a derived from a nomogram, and assumes normal values for other important nonbicarbonate buffers such as albumin. Thus, in a hypoalbuminemic patient, the BD should be used with caution as it may conceal an underlying metabolic acidosis.

·         FLUIDS AND VOLUME REGULATION

1.       Estimating volume status requires gathering as much clinical information as possible because any single variable may be misleading. Always look for supporting information.

2.       Replace intraoperative fluid losses with isotonic fluids.

3.       Normal saline, when administered in large quantities, produces a hyperchloremic metabolic acidosis; the associated base deficit may lead the provider to conclude incorrectly that the patient continues to be hypovolemic.

4.       Hypotension is a late finding in acute hypovolemia because sympathetic tone will increase vascular tone to maintain cardiac output.

·         ELECTROLYTES

1.       Rapid correction of electrolyte disturbances may be as dangerous as the underlying electrolyte disturbance.

2.       Electrolyte disturbances cannot be corrected without treating the underlying cause.

3.       Acute hyponatremia may be observed during or after TURP or hysterotomies and manifests as confusion, seizures, or cardiac collapse. The treatment is supportive (airway and circulation); loop diuretics may also be necessary.

4.       Acute hyperkalemia is life-threatening and associated with ventricular tachycardia and fibrillation. It should always be suspected when cardiac collapse follows succinylcholine administration or in any patient with chronic renal disease.

5.       When other causes have been ruled out, persistent and refractory hypotension in trauma or other critically ill patients may be due to hypocalcemia or hypomagnesemia.

·         Hyperchloremia is associated with massive resuscitation with normal saline and with metabolic acidosis due to dilution of sodium bicarbonate, and should be part of the differential diagnosis of metabolic acidosis in this setting. Besides trauma, it has been noted during aortic, gynecologic, and cardiopulmonary bypass surgeries and during the management of sepsis.

·         COAGULATION

1. An outpatient with a bleeding diathesis can usually be identified through history (including medications) and physical examination. Preoperative coagulation studies in asymptomatic patients are of little value.
2. The most common intraoperative bleeding diathesis is dilutional thrombocytopenia.
3. The primary treatment for DIC is to treat the underlying medical condition.
4. Thromboelastography is a dynamic test of clotting and can be as useful as all other clotting tests combined.

·         There are five parameters of the TEG tracing: R, k, alpha angle, MA, and MA60.

R: Period of time from the initiation of the test to initial fibrin formation

k: Time from the beginning of clot formation until the amplitude of TEG reaches 20 mm, representing the dynamics of clot formation

Alpha angle: Angle between the line in the middle of the TEG tracing and the line tangential to the developing body of the tracing, representing the kinetics of fibrin crosslinking

MA (maximum amplitude): Reflects the strength of the clot, which is dependent on the number and function of platelets and their interaction with fibrin

MA60: Measures the rate of amplitude reduction 60 min after MA, representing the stability of the clot

·         PULMONARY FUNCTION TESTING

1. Abnormal PFTs identify patients who will benefit from aggressive perioperative pulmonary therapy and in whom surgery should be avoided entirely. This is especially the case when pulmonary resections are planned.
2. Forced vital capacity, forced expiratory volume in 1 second, FEV1/FVC ratio, and flow between 25% and 75% of the FVC (MMF25-75) are the most clinically helpful indices obtained from spirometry.
3. No single PFT result absolutely contraindicates surgery. Factors such as physical examination, arterial blood gases, and coexisting medical problems also must be considered in determining suitability for surgery.