·
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
- Low fractional concentration of inspired O2: normal A-a gradient
- Hypoventilation: normal A-a gradient
- Ventilation/perfusion mismatch: elevated A-a gradient
- Right-to-left shunting: elevated A-a gradient
- 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
- 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.
- The most common intraoperative bleeding diathesis is dilutional thrombocytopenia.
- The primary treatment for DIC is to treat the underlying medical condition.
- 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
- 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.
- 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.
- 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.
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