Medical brain notes 7

  Vascular access and fluid management We tend to forget that we humans (and many of our animal relatives) are mostly water. When we think about it, we must marvel how the body stores the bulk of this water in cells and the interstitial, extracellular fluid, where much of the water is tied up in gel. Suspended in this interstitial lake is the vascular compartment, comparatively puny in volume but most important because of its rapid transport of fluids, nutrients, and waste throughout the system, and its continuous and efficient exchange of water with the interstitial compartment (Fig.  3.1 ). Clinically, we can see dehydration in sunken eyeballs, wrinkling skin and dry lips, or the excess of fluids in edema and swollen eyes; we can even hear it should water collect in the alveoli.

  Vascular access During anesthesia, and whenever the oral route is unavailable, we give fluids par-enterally. As long as we need to give only physiologic solutions, we can administer them subcutaneously; however, the uptake and distribution of such a depot of fluids takes time. Much preferred and much faster is the intravenous route. Thus, vascular access assumes a critically important role in the peri-operative care of patients. The vascular bed also offers an ideal route for many drugs that need to be distributed throughout the body. Finally, intravascular pressures provide infor-mation on cardiovascular function. Thus, vascular access has become a skill, and fluid management a science, mastered by anesthesiologists. Our skin is a wonderful organ. It wraps us securely into an elastic, fairly tough, self-repairing, protective envelope. When we break this envelope, we expose the patient to considerable risks. In addition to hazards associated with the actual placement of needles a...

  Peripheral venous cannulation Let’s go through the steps involved: (i)     Explain the need  for vascular access and obtain consent from the patient.Parents can be of great help in preparing a child for an i.v.   (ii)     Topicalize  If there is sufficient time (30–45 minutes), a topical anesthetic suchas EMLA (eutectic mixture of local anesthetics) can be applied to the intended site. In our practice, this is only worthwhile for small children.   (iii)    Acquire equipment (Table  3.1 )  Weusually select the largest catheter appro-priate for the selected vein.   (iv)   Don clean gloves  They need not be sterile. From now on, you are dealing withthe patient’s blood, and you should expose neither yourself nor the patient to the possibility of infection.   (v) Select the site  This involves more than just looking for the most visible vein.We often use the back of the hand because veins ar...

  Central venous catheterization The complication rate of central venous catheterization (Table  3.2 ) is much higher than for peripheral i.v.s, thus the first question should be whether central venous cannulation is truly necessary (Table  3.3 ). When placed emergently, for instance in a trauma patient, these catheters should be replaced within 48 hours to reduce the risk of infection. The next question deals with access site. The three most common insertion sites are as follows: ·             Femoral  Probably technically the easiest (remember, from lateral to medial,NAVEL – nerve, artery, vein, empty space, lympathics) and quickest, with the lowest rate of serious complications (though highest rate of minor complica-tions), but these catheters are more difficult to keep clean and therefore more likely to be a source of infection. ·             Subclavian (...

  IJ catheter placement technique Once we have confirmed the need for central venous catheterization, obtained the patient’s consent, and collected all equipment, we work as follows: (i)        Optimally position the patient: Trendelenburg’s position 2  (head-down, to increase the size of the vein and prevent air embolism), with the head turned about 45 degrees to the opposite side.   (ii)        Prepare: gown, sterile gloves, cap, mask, with catheter tray open and in easy reach.   (iii)       Prepare the site: we prefer chlorhexidine, but an iodine solution that has dried can be substituted depending on the patient’s allergies.   (iv)      Identify the insertion point: while there are many possible sites along the vessel, we advocate a mid to high approach, minimizing the possibility of pneumothorax. One technique: place the third finger of the left hand in ...

  Confirmation of intravenous location Several techniques can help to confirm that the needle is not in an artery – usually the carotid. While pulsatility and a bright red color are good hints, they are not foolproof. ·             If ultrasound guidance was used to place the catheter (as is routine at our insti-tution), use it to confirm position as well. ·             Attach a length of sterile clear tubing to the needle hub and lower the end, allowing it to fill several centimeters with blood, then raise above the patient’s heart level. A rising column of pulsating blood confirms arterial location, while a column that reflects the central venous pressure is a more welcome finding.

  Pulmonary artery catheterization In addition to the risks of central venous catheterization listed above, pulmonary artery (PA) catheterization has caused catastrophic pulmonary artery rupture and  comes with the increased risk of arrhythmias, complete heart block (particularly if the patient has a pre-existing left bundle branch block), pulmonary embolism, and cardiac valve damage.   Thus, this invasive technique requires rigorous justifi-cation. Do you really need to have PA pressure, PA occlusion pressure (PAOP, also known as pulmonary capillary wedge pressure, PCWP), or cardiac output? And how will it affect your management?   After placing an introducer (a special large-bore central venous catheter) via the central venous access technique above, a PAC is sterilely inserted through the introducer.   (i)     Prepare the catheter: flush and cap the PAC ports. Test the balloon for symmet-ric inflation and passive deflation on release of the syr...

  Fluid management As mentioned at the start of this chapter, we are mostly water, actually salt water with some other chemicals thrown in for good measure. The intravascular compartment, replete with cells and proteins, differs from the rest of the body. In fact the blood volume also differs with age and sex (Table  3.5 ). We may lose fluid in a number of ways, from the obvious – hemorrhage, urine, vomiting – to the less obvious – sweat, evaporation from exposed viscera or trachea, transudation between compartments. While fluid escapes from anywhere, replacement occurs only through the intravascular compartment. Fluid types Many types of fluids are available for intravascular administration (Table  3.6 ). ·             Crystalloid ·             Hypotonic solutions  With an osmolality less than that of serum (285–295 ·       ...

  Fluid requirements We calculate the intra-operative fluid requirement as follows:   ·             Maintenance  The 4–2–1 rule (Table 3.7 )provides a guide for hourly isotonicfluid requirements.   For a 70 kg man, this would amount to 40  +  20  +  50  =  110 mL/h.   ·             Fasting replacement  We apply the 4–2–1 rule for the duration of fasting andreplace 50% over the first hour, then 25% over each of the next two hours. ·             Insensible losses  2 mL/kg/h.   ·             Urine output  Replaced mL for mL.   ·             “ Third space ”  losses  Transfer of fluid to this sequestered, extravascular space occurs w...