Gram-positive/negative Health Dictionary

The unit of weight in the metric system, equal to a little over 15·4 grains. For the purposes of weighing food, 30 grams are usually taken as being approximately equal to an

ounce. (See APPENDIX 6: MEASUREMENTS IN MEDICINE.)... gram

Stain developed in 1884 by Hans Christian Gram, whereby Gram positive bacteria stain purple while Gram negative bacteria stain red.... gram stain

A method for treating babies who suffer from alveolar collapse in the lung as a result of HYALINE MEMBRANE DISEASE (see also RESPIRATORY DISTRESS SYNDROME).... continuous positive airways pressure

A negative test result for a condition that is, in fact, present.... false negative

A positive test result for a condition that is not, in fact, present.... false positive

Bacteria can be stained with an iodine-based chemical dye called Gram’s stain (after the scientist who discovered the technique). Different bacteria react di?erently to exposure to the stain. Broadly, the bacterial specimens are stained ?rst with gentian violet, then with Gram’s stain, and ?nally counterstained with a red dye after a decolorising process. Bacteria that retain the gentian stain are called gram-positive; those that lose it but absorb the red stain are called gram-negative. Some species of staphylococcus, streptococcus and clostridium are gram-positive, whereas salmonella and Vibrio cholerae are gram-negative.... gram’s stain

The simplest form of intermittent positive-pressure ventilation is mouth-to-mouth resuscitation (see APPENDIX 1: BASIC FIRST AID) where an individual blows his or her own expired gases into the lungs of a non-breathing person via the mouth or nose. Similarly gas may be blown into the lungs via a face mask (or down an endotracheal tube) and a self-in?ating bag or an anaesthetic circuit containing a bag which is in?ated by the ?ow of fresh gas from an anaesthetic machine, gas cylinder, or piped supply. In all these examples expiration is passive.

For more prolonged arti?cial ventilation it is usual to use a specially designed machine or ventilator to perform the task. The ventilators used in operating theatres when patients are anaesthetised and paralysed are relatively simple devices.They often consist of bellows which ?ll with fresh gas and which are then mechanically emptied (by means of a weight, piston, or compressed gas) via a circuit or tubes attached to an endotracheal tube into the patient’s lungs. Adjustments can be made to the volume of fresh gas given with each breath and to the length of inspiration and expiration. Expiration is usually passive back to the atmosphere of the room via a scavenging system to avoid pollution.

In intensive-care units, where patients are not usually paralysed, the ventilators are more complex. They have electronic controls which allow the user to programme a variety of pressure waveforms for inspiration and expiration. There are also programmes that allow the patient to breathe between ventilated breaths or to trigger ventilated breaths, or inhibit ventilation when the patient is breathing.

Indications for arti?cial ventilation are when patients are unable to achieve adequate respiratory function even if they can still breathe on their own. This may be due to injury or disease of the central nervous, cardiovascular, or respiratory systems, or to drug overdose. Arti?cial ventilation is performed to allow time for healing and recovery. Sometimes the patient is able to breathe but it is considered advisable to control ventilation – for example, in severe head injury. Some operations require the patient to be paralysed for better or safer surgical access and this may require ventilation. With lung operations or very unwell patients, ventilation is also indicated.

Arti?cial ventilation usually bypasses the physiological mechanisms for humidi?cation of inspired air, so care must be taken to humidify inspired gases. It is important to monitor the e?cacy of ventilation – for example, by using blood gas measurement, pulse oximetry, and tidal carbon dioxide, and airways pressures.

Arti?cial ventilation is not without its hazards. The use of positive pressure raises the mean intrathoracic pressure. This can decrease venous return to the heart and cause a fall in CARDIAC OUTPUT and blood pressure. Positive-pressure ventilation may also cause PNEUMOTHORAX, but this is rare. While patients are ventilated, they are unable to breathe and so accidental disconnection from the ventilator may cause HYPOXIA and death.

Negative-pressure ventilation is seldom used nowadays. The chest or whole body, apart from the head, is placed inside an airtight box. A vacuum lowers the pressure within the box, causing the chest to expand. Air is drawn into the lungs through the mouth and nose. At the end of inspiration the vacuum is stopped, the pressure in the box returns to atmospheric, and the patient exhales passively. This is the principle of the ‘iron lung’ which saved many lives during the polio epidemics of the 1950s. These machines are cumbersome and make access to the patient di?cult. In addition, complex manipulation of ventilation is impossible.

Jet ventilation is a relatively modern form of ventilation which utilises very small tidal volumes (see LUNGS) from a high-pressure source at high frequencies (20–200/min). First developed by physiologists to produce low stable intrathoracic pressures whilst studying CAROTID BODY re?exes, it is sometimes now used in intensive-therapy units for patients who do not achieve adequate gas exchange with conventional ventilation. Its advantages are lower intrathoracic pressures (and therefore less risk of pneumothorax and impaired venous return) and better gas mixing within the lungs.... intermittent positive pressure (ipp)

See “healthy ageing”.... positive ageing

A state of health beyond an asymptomatic state. It usually includes the quality of life and the potential of the human condition. It may also include self-fulfilment, vitality for living and creativity. It is concerned with thriving rather than merely coping. See also “health”.... positive health

The probability that a person with a negative test is free of the disease and is not a false negative.... predictive value negative

The probability that a person with a reactive test has the disease and is not a false reaction.... predictive value positive

see BiPAP.... bi-level positive airways pressure

a physiological loop for the control of hormone production by a gland. High levels of a circulating hormone act to reduce production of the releasing factors triggering its own production, i.e. they have a negative *feedback on these trigger factors. As circulating levels of the hormone fall, the negative feedback is reduced and the releasing factor starts to be produced again, allowing the hormone level to rise again.... negative feedback loop

(in psychiatry) symptoms of schizophrenia characterized by a deficiency in or absence of some aspect of functioning, such as social withdrawal, loss of initiative, and blunted affect. Compare positive symptoms.... negative symptoms

adj. see normative.... positive

see noninvasive ventilation.... positive-pressure ventilation

(in psychiatry) symptoms of schizophrenia characterized by a distortion of some aspect of functioning, such as delusions, hallucinations, or disordered speech. Compare negative symptoms.... positive symptoms

an arthritis in which rheumatoid factor or anticitrullinated protein antibodies (ACPA) are not present in the serum. See also spondyloarthropathy.... sero-negative arthritis

see BiPAP.... variable positive airways pressure