The peak oxygen
consumption (VO2) during exercise appears to provide the most
objective assessment of functional capacity in patients with heart failure
and may be the best predictor of when to list an individual patient for cardiac
transplantation.
·
Exercise tolerance
is determined by three factors: pulmonary gas
exchange, cardiac performance, and skeletal muscle metabolism. Exercise
capacity can be quantified clinically by measurement of oxygen uptake (VO2),
carbon dioxide production (VCO2), and minute ventilation.
·
After initial dissection, the patient is fully heparinized. The
perfusion-sensitive organs (kidneys and liver) are removed before cardiectomy. The donor heart is excised via
median sternotomy. After excision, the donor heart is placed in a plastic bag containing ice-cold saline
and transported in an ice-filled cooler. Optimal myocardial function
after transplantation is achieved when the donor heart ischemic time is less
than 4 hours.
·
Post-CPB LV
dysfunction may be a result of a prolonged donor heart ischemic
time, inadequate myocardial perfusion, intracoronary embolization
of intracavitary air, or surgical manipulation. The
incidence of post-CPB LV dysfunction is greater in donors requiring prolonged,
high-dose inotropic support before organ harvest.
Post-CPB
RV failure is a significant cause of early morbidity and mortality,
accounting for nearly 20% of early deaths. Therefore, prevention, diagnosis,
and aggressive treatment of RV dysfunction after CPB are essential. Acute RV
failure after cardiac transplantation may be due to preexistent pulmonary HTN
in the recipient, transient pulmonary vasospasm, tricuspid or pulmonic valve insufficiency secondary to early
postoperative RV dilation, and donor-recipient size mismatch. Additional
factors that may contribute to postoperative RV dysfunction include a prolonged
donor heart ischemic time, inadequate myocardial protection, and surgical
manipulation of the heart.
·
In contrast to nonselective vasodilators such as
nitroglycerin and sodium nitroprusside, which produce
systemic hypotension, inhaled NO (20-80 ppm) selectively reduces PVR in the ventilated
area of the lung, improving ventilation-perfusion (V/Q) mismatch. NO has little
systemic effect because it is inactivated by hemoglobin and has a 5- to
10-second half-life.
·
three stages of
liver transplantation:
The
preanhepatic stage (stage 1) begins with
the surgical incision and dissection and mobilization of the patient's diseased
liver. During this stage, the surgeons identify the hepatic artery, portal
vein, and the inferior vena cava, above and below the liver.
The
anhepatic stage (stage 2) isolates the
liver from the circulation and commences with the occlusion of the hepatic
artery and portal vein. Occlusion of the inferior vena cava above and below the
liver is performed so that the liver can be removed. During the anhepatic stage, the donated liver is reinserted into the
circulation by anastomoses to the patient's vena
cava, portal vein, and hepatic artery. The anhepatic
stage concludes with removal of the vascular clamps resulting in reperfusion of
the donor liver graft.
Stage
3 starts during reperfusion and extends to the conclusion of the
operation. It mainly encompasses the process of biliary
reconstruction and assessment of graft function.
·
Reperfusion syndrome
during stage 2 of liver transplantation: is characterized by either
a decrease of 30% or more in mean arterial pressure (from baseline) for greater
than 1 minute and occurring within the first 5 minutes of reperfusion, or a
mean arterial pressure less than 60 mmHg under the same circumstances.
Following unclamping, approximately 30% of patients will exhibit profound
cardiovascular collapse on reperfusion irrespective of attentive management
during stage 2. The bradycardia, myocardial
depression, and systemic vasodilation noted during
reperfusion are secondary to rapid increases in serum potassium, decreases in
temperature, acute acidosis, and release of vasoactive
substances by the grafted liver. These vasoactive
mediators include prostaglandins, kallikrein,
platelet-activating factor, and leukotrienes.
Increased age and larger donor organs also are considered risk factors.
Generally, treatment with calcium
and/or epinephrine improves cardiovascular function. Fluid administration
should be judicious because it can aggravate the already increased filling
pressures (secondary to myocardial depression), resulting in impaired hepatic
perfusion. Although the hemodynamic changes generally subside within 10-15
minutes, pulmonary hypertension, elevated central venous pressure (CVP), and
hypotension may persist.
·
two basic types of
oxygenators:
Bubble
oxygenators work by bubbling oxygen (O2) through the patient's blood
and then defoaming the blood to minimize air microemboli.
In
membrane oxygenators, O2 and CO2 diffuse across a semipermeable membrane. Membrane units are generally
preferable owing to a decreased risk of gas microemboli
and less damage to blood elements.
·
There are differing opinions as to whether blood
gases should be corrected for the temperature during CPB because the solubility
of gases decreases with hypothermia. All blood gases are analyzed at 37%. In pH-stat measurements, the obtained value is
corrected on a nomogram and the reported values refer
to the partial pressure at the hypothermic temperatures. More commonly blood
gases are reported uncorrected for temperature, a method referred to as alpha-stat blood gas management. Probably in
terms of outcome, there is little difference in how blood gases are
interpreted.
·
CARDIOPULMONARY BYPASS
- There is no "best" anesthetic technique during CPB. Patients with a decreased ejection fraction will not tolerate propofol infusions or volatile anesthesia as well as patients with preserved stroke volume and will probably require an opioid-based technique.
- Patients should be completely anticoagulated before initiating CPB or face the risk of massive intravascular clot formation.
- The CPB reservoir should never be allowed to empty during CPB as massive air embolism is a consequence.
- Factors involved in myocardial preservation include cardioplegia, hypothermia, preventing the heart from becoming warm from an adjacent structure, and ventricular venting and consequences of inadequate myocardial preservation include decreased cardiac output, ischemia, dysrhythmias, and failure to come off pump.
- Always consider inadequate surgical technique for the patient failing to come off pump.
- Neurologic complications, in particular neurocognitive deficits, are surprisingly common after CPB.
·
There are two basic
techniques for lung isolation:
Double-lumen
endotracheal tube (DLT),
in which a bifurcated tube with both an endotracheal
and endobronchial lumen can be used to isolate,
selectively ventilate, or collapse the right or left lung independently
according to the operative approach. This is the most common technique.
Blockade of a bronchus to allow lung collapse
distal to the occluding bronchial blocker.
·
three methods of
bronchial blockade:
Univent
tube (Vitaid Airway Management,
Williamsville, NY), also known as a torque control blocker
Wire-guided
Endobronchial Blocker WEB (Cook Critical
Care, Bloomington, IN), also known as an Arndt blocker
Fogarty
embolectomy catheter (Baxter Healthcare
Corporation, Irvine, CA)
·
SOMATOSENSORY-EVOKED
POTENTIALS AND SPINAL SURGERY
- SSEPs are used when spinal cord or brain parenchyma is at risk for ischemia during surgery.
- Volatile anesthetics have the most profound effects on SSEP waveforms.
- An anesthetic technique that minimizes volatile anesthetic exposure is best-an opiate-based technique with low-dose (< 1 MAC) volatile or a total intravenous anesthetic (TIVA).
- During distraction of the spinal column in scoliosis surgery (or other critical parts of surgery), minimize interventions that will lower mean arterial blood pressure or deepen anesthetic levels acutely to allow differentiation of changes in SSEP waveforms from anesthetic effect.
·
SSEPs
are the electrophysiologic responses of the nervous
system to the application of a discrete stimulus at a peripheral nerve anywhere
in the body. They reflect the ability of a specific neural pathway to conduct
an electrical signal from the periphery to the cerebral cortex.
·
Deliberate
hypotension is the intentional reduction of systemic perfusion
pressure. The major indication for using a deliberate hypotensive
technique is to reduce intraoperative blood loss and
to produce a relatively bloodless surgical site. Deliberate hypotension
decreases blood loss and thereby reduces the need for blood transfusion. This
technique has been used in neurosurgical, orthopedic, vascular, and major
craniofacial and other plastic procedures. Deliberate hypotension also may be
used to help manage patients who refuse blood transfusions (e.g., for religious
reasons).
·
DELIBERATE
HYPOTENSION
1. Deliberate
hypotension is a technique that can limit blood loss and improve operative
conditions but appropriate patient selection is necessary.
2. Three signs of cyanide toxicity from SNP
infusion include (1) the need for doses > 10 µg/kg/min, (2) tachyphylaxis occurring within 60 minutes, or (3)
resistance to SNP.
3. Treatment of cyanide toxicity includes
stopping the infusion of SNP; delivery of 100% oxygen; administration of amyl
nitrite by inhalation for 30 seconds every 2 minutes; administration of sodium
nitrite, 10 mg/kg intravenously, followed by an infusion of 5 mg/kg over 30
minutes; and administration of sodium thiosulfate,
150 mg/kg, not to exceed 12.5 gm, immediately after sodium nitrite.
·
Postoperative
visual loss (POVL), ranging from vision deficits to blindness, are
catastrophic complications following surgery and anesthesia. Ischemic optic
atrophy appears to be the final mechanism though numerous patient-related and intraoperative events have been implicated. Spine surgery
and cardiac surgery are the most common procedures associated with POVL.
·
ANESTHESIA FOR
MINIMALLY INVASIVE SURGERY
- PaCO2 increases during CO2 pneumoperitoneum in laparoscopy because of CO2 absorption and ventilation-perfusion mismatch; if the patient is spontaneously breathing, ventilatory depression may also contribute to hypercapnia.
- Hemodynamic changes during laparoscopy include increased systemic vascular resistance, mean arterial pressure, and left ventricular wall stress; cardiac output decreases initially, then gradually increases back to baseline.
- Pulmonary changes during laparoscopy include increases in peak inspiratory pressure, intrathoracic pressure, and respiratory resistance and decreases in vital capacity, functional residual capacity, and pulmonary compliance.
·
Only nonelectrolyte solutions can be used for irrigation during
TURP. Electrolyte solutions are avoided to minimize the dispersion
of current throughout the bladder when electrocautery
is used. Dissemination of electrocautery current
would be uncomfortable for the patient and dangerous to both patient and surgeon.
After completion of surgery and before the patient is moved to the postanesthesia care unit, bladder irrigation should be
changed to normal saline. Because fluid absorption from continuous bladder
irrigation may continue in the postoperative period, eliminating nonelectrolyte solutions reduces the risk of postoperative hyponatremia.
·
TRANSURETHRAL
RESECTION OF THE PROSTATE
- Hyponatremia secondary to TURP syndrome may present with restlessness, mental confusion, nausea, vomiting, dizziness, headache, unresponsiveness, transient visual changes, hypertension, hypotension, heart rate changes, cardiac arrhythmias, pulmonary edema, or cyanosis.
- Prompt treatment of TURP syndrome is essential and includes terminating the procedure, changing the bladder irrigant to normal saline, infusing normal saline, evaluating serum sodium and other chemistries, administering diuretics, and considering the use of hypertonic saline in unstable patients.
- The hyponatremia of TURP syndrome is due to fluid excess and not a loss of sodium.
- The preferred anesthetic technique is subarachnoid block so that mental status can be followed as an early indicator of hyponatremia.
·
Laser light has
three defining characteristics:
Coherence: All waves are in
phase, both in time and in space
Collimation: The waves
travel in parallel directions
Monochromaticity:
All waves have the same wavelength
·
ELECTROCONVULSIVE
THERAPY
- Methohexital should be considered the drug of choice for the induction of anesthesia for ECT.
- ECT causes pronounced sympathetic activity, which may result in myocardial ischemia or even infarction in patients with coronary artery disease.
- To perform ECT safely it is necessary to complete a preoperative history and physical examination, use standard monitors, have readily available equipment and medications appropriate for full cardiopulmonary resuscitation, utilize an induction agent (e.g., methohexital) and muscle relaxant (e.g., succinylcholine), and have a beta blocker readily available (e.g., esmolol).
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